[analyzer] Temporaries

Clang Frontend Static Analyzer
#1

Handling C++ temporary object construction and destruction seems to be the biggest cause of false positives on C++ code at the moment. I'd be looking into this, even though for now i don't see the whole scale of problem.

== CFG, destructors, and ProgramPoints ==

We should probably enable `-analyzer-config cfg-temporary-dtors=true` by default soon. It is a fairly low-impact change because it only alters the CFG but the analyzer rarely actually uses the new nodes. Destructors for the most part are still evaluated conservatively, with improper object regions. So it causes almost no changes in the analyzer's positives for now, but it definitely opens up room for further improvements.

I'm observing a couple of problems with this mode at the moment, namely the rich CFG destructor element hierarchy is not currently complemented by an equally rich ProgramPoint hierarchy. This causes the analysis to merge nodes which are not equivalent, for example two implicit destructors of the same type (with the same function declaration) may sometimes cause the ExplodedGraph to coil up and stop analysis (lost coverage) because of having the same program state at the erroneously-same program point. Because situations when we can guarantee a change in the program state are pretty rare, we'd have to produce more program point kinds to handle this correctly.

CallEvent hierarchy is also very much affected in a similar manner - because apparently we're constructing program points by looking at CallEvents, so they'd need to carry all the information that's needed to construct the pre-call/post-call program point.

== Construction contexts ==

We are correctly modeling "this" object region during construction/destruction of variables with automatic storage duration, fields and base classes, and on operator new() since recently, as long as these aren't arrays of objects. It was not yet implemented for other cases such as temporaries, initializer lists, fields or C++17 base classes within aggregates, and pass-by-value from/to functions (the latter being a slightly different problem than temporaries).

First of all, by "not yet implemented" i mean that instead of constructing into (destroying) the correct object (in the correct memory region), we come up with a "temporary region", which looks exactly like a region of a valid C++ temporary but is only used for communicating that it is not the right region. Then we look at the region, see that it is a temporary, and avoid inlining constructors, because it would make little sense when the region is not correct. However, if we are to model temporaries, we need another way of communicating our failure to find the correct region, which is being addressed by ⚙ D42457 [analyzer] Don't communicate evaluation failures through memregion hierarchy.

Also in the cases when the correct region is used, it is being computed in a funky way: in order to figure out where do we construct the object, we walk forward in the CFG (or from child to parent in the AST) to find the next/parent statement that would accomodate the newly constructed object. This means that the CFG, while perfectly telling us what to do in what order (which we, unlike CodeGen, cannot easily take from AST because we cannot afford recursive evaluation of statements, mainly because of inlining), discards too much context to be helpful in understanding how to do it.

I tried to add the information about such "trigger statements" for constructors into CFG and it is extremely helpful and easy to both use and extend. This assumes adding a new sub-class of CFGElement for constructors, which maintain a "construction context" - for now it's just a trigger statement. For instance, in

class C { ... };
void foo() {
C c;
}

...the trigger for constructor C() would be DeclStmt `C c`, and once we know this we can immediately figure out that the construction target is the region of variable `c`. Construction trigger is not necessarily a statement - it may be a CXXCtorInitializer, which is an AST node kind of its own. Additionally, when constructing aggregates via initializer lists, we may have the same statement trigger multiple constructors, eg. in

class C { public: C(int); ~C(); };
struct S { C c1, c2, c3; };
void foo() {
S s { 1, 2, 3 };
}

... we have three different constructors (actually six different constructors if we include elidable copy constructors) for c1, c2, c3 (and lack of constructor for `s` because of the aggregate thing). It would be more natural to declare that the specific field or index would be a part of the CFG's construction context, as well as the intermediate InitListExpr, so even in these simple cases the construction context may get quite bulky. And this example is quite popular on actual code - probably the second worst cause of false positives after temporaries.

For now i have no specific plan on what would construction context for temporaries contain in the general case. I might not be able to get the data structures right from the start. In any case, it might be necessary to perform additional memory allocations for these CFG elements (for analyzer's CFG only, so it wouldn't affect compilation time or warnings).

I believe that getting the correct target region in as many cases as possible would be the main part of my work for the nearest future. And i want to move most this work to CFG, while letting the analyzer pick up the trigger statement from it and trust it as blindly as possible.

== Workflow ==

When working on operator new, i tried hard to maintain a reasonable history, making around 15 local rebases. It was not awesome because it was hard for the reviewers to understand the context of the new changes, and changes could have easily kicked in during rebases. A few lessons learned here would be to commit more aggressively, i.e. avoiding stockpiling a large history of patches (essentially a large branch), which in turn would be possible through trying harder to avoid unrelated hard-to-test core changes (as long as it doesn't require weird workarounds) that aren't covered by a flag (such as those evalCast fixes), in order to make sure reviewing take less responsibility. It's fine if some parts would later be fixed (assuming they would indeed be fixed), if it means making the turnaround faster and the tail of patches shorter - that's also the attitude i'd try to maintain when reviewing stuff myself.

#2

A bit of an update.

== Temporary destructors ==

Adding some initial support for temporary destructors seems pretty easy and straightforward at this point, given the awesome work done by Manuel Klimek on our CFG a few years ago.

1. We already have the awesome CFGTemporaryDtor elements, which have the backreference to the CXXBindTemporaryExpr for their temporaries.

2. In simple cases CXXBindTemporaryExprs have an immediate constructor within them, and therefore we can provide the CXXBindTemporaryExprs as the construction context's trigger statements, and therefore have a reliable CXXTempObjectRegion for constructors.

3. Then we already track the CXXBindTemporaryExprs for the active temporaries in the program state. We can attach any constructor information to them, such as the target region, if we need to (probably we can reconstruct it by looking at the expression and the location context, not sure if we want to).

4. So when we reach the CFGTemporaryDtor element, we can just lookup all the info we need, and perform the destruction properly.

5. There's a bit of a liveness problem, because it seems that our liveness analysis tends to throw away the temporary too early. I can easily hack this problem away by marking all regions that correspond to active temporaries as live. I'll see if there's a better solution.

== CXXDefaultArgExpr problems ==

There's a known problem with those. Consider:

void foo(const C &c = C()) {
}

void bar() {
foo();
}

Each call of foo() contains a CXXDefaultArgExpr for c. The default argument value C() is constructed before we enter foo() and destroyed after we leave foo(). However, c's initializer, "= C()", is *not part of the AST of bar()*. In particular, when foo() is called twice, the initializer for the two calls is the same, only CXXDefaultArgExprs are different. This screws a lot of invariants in the analyzer: program points start coinciding (causing the analysis to loop and cache out), Environment doesn't expect the same expression in the same location context have two different values (suppose calls are nested into each other), analysis taking wrong branches, and so on.

Luckily, default-arg expressions that aren't zero integers or null pointers are pretty rare. Still, we'd need to eventually think how to avoid any really bad practical problems with them.

#3

More explanations on how the analyzer keeps making its way around the C++ AST.

== Lifetime extension ==

This is a brain dump of how (and how much) lifetime extension of temporary objects is currently broken in the static analyzert. Spoilers: not too much, it seems, but annoying nevertheless.

Consider an example:

  1    class C \{
  2    public:
  3      C\(\) \{\}
  4      \~C\(\) \{\}
  5    \};
  6
  7    void foo\(\) \{
  8      const C &c = C\(\);
  9    \}

With the AST for the variable declaration:

   DeclStmt 0x7fa5ac85cba0 <line:8:3, col:19>
   \`\-VarDecl 0x7fa5ac85c878 <col:3, col:18> col:12 c 'const C &' cinit
     \`\-ExprWithCleanups 0x7fa5ac85cb30 <col:16, col:18> 'const C' lvalue
       \`\-MaterializeTemporaryExpr 0x7fa5ac85cb00 <col:16, col:18> 'const C' lvalue extended by Var 0x7fa5ac85c878 'c' 'const C &'
         \`\-ImplicitCastExpr 0x7fa5ac85cae8 <col:16, col:18> 'const C' <NoOp>
           \`\-CXXBindTemporaryExpr 0x7fa5ac85cac8 <col:16, col:18> 'C' \(CXXTemporary 0x7fa5ac85cac0\)
             \`\-CXXTemporaryObjectExpr 0x7fa5ac85ca88 <col:16, col:18> 'C' 'void \(\)'

*here goes a periodic reminder that CXXTemporaryObjectExpr is a sub-class of CXXConstructExpr*

Notice how MaterializeTemporaryExpr is the innermost expression (the first one in the order of execution) that is an lvalue. Essentially, you can think of it as the mythical "rvalue-to-lvalue" cast that takes in a C++ object rvalue and returns the this-value for that object. Because all C++ objects have a this-value that never changes throughout their lifetime, it is essentially their identity. Otherwise you can't call methods on them.

MaterializeTemporaryExpr also contains information about the lifetime extension process: we needed the this-value in order to bind it to variable 'c'. You see that in the AST.

In the analyzer, however, MaterializeTemporaryExpr does a different thing, as a temporary solution (no pun intended). It constructs a new temporary region out of thin air and binds the rvalue object to that temporary in the Store. The respective function in our code is called "createTemporaryRegionIfNeeded". It also has a separate purpose of converting rvalue sub-object adjustments into lvalue sub-object adjustments, which we wouldn't discuss this time.

Now that we learned how to inline temporary constructors and destructors, it essentially means that the this-value in the constructor and in the destructor would be different. Because first we construct the object into temporary region R1, then we take lazyCompoundVal{R1} to represent the value of CXXTemporaryObjectExpr, then we materialize lazyCompoundVal{R1} to R2, then we bind R2 to variable 'c', then we call the automatic(!) destructor for 'c' which contains R2. To be clear, the region store at the time of destruction would be:

c: R2,
R2: lazyCompoundVal{R1}.

It means that fields of the object would contain the correct values, there would be the correct number of destructors called (no temporary destructors, just one automatic destructor), but the identity of the object (this-value) would change in the process. Unless the object actually makes any decisions or does any manipulations that involve its this-value, the modeling should be correct. When the object starts to actively use its this-value in its inlined methods, the analysis would do wrong stuff. Additionally, it causes a mess in the checkers when they try to track objects by their this-values - i.e. IteratorChecker has a lot of additional code to work around the fact that the region for the object constantly changes.

From the above it is clear that MaterializeTemporaryExpr should not construct any new regions, at least not in this case. We already have the correct region, namely R1, which should be re-used.

It is tempting to take R1 directly from lazyCompoundVal{R1} - it already has memories about once being a value of R1. I'm not sure it's not going to work - it may work, at least i'm not ready to come up with a counterexample. But the meaning of LazyCompoundVal's parent region is different and coincides with what we want just accidentally. Namely, lazyCompoundVal{R1} is a value of an object that was bound to R1 in some particular moment of time in the past, without any explanation of when this moment of time was - but there's no indication if R1 is the region of the temporary we've just constructed, or a region of an unrelated object that used to have the same value back then. As we'd see later, MaterializeTemporaryExpr doesn't always contain a constructor within it - there are a lot of cases to cover, and if the trick doesn't work even once, it's going to be hard, so i'd probably not going to commit into maintaining this invariant. Though it might be plausible to modify add an SVal kind that does exactly what we mean here - i.e. a value that does indeed correspond to a specific C++ object identified by region. It might be a beautiful solution, but it may also fail miserably if tricky cornercases show up - so i'm not ready to commit to that. Also the correct way of dealing with such values (i.e. for how long are they relevant?) would be extremely difficult to explain to checker developers.

The more straightforward approach here is to include MaterializeTemporaryExpr (hereinafter MTE) into the construction context. It means, if a temporary that we're about to construct would be lifetime-extended later, we'd rather know about that during construction, and maintain a map in the program state from MTE to their respective temporary regions that were used for representing the respective construction targets. Upon encountering the respective MTE we'd simply retrieve the implicit temporary storage for the value from the program state and declare that temporary region to be the value of the MTE. This would mimic the approach we have with CXXBindTemporaryExprs (hereinafter BTE) and their respective regions that allows temporary destructors to work - but this time it's going to be about MaterializeTemporaryExprs and automatic destructors. I imagine that on the checker side this can potentially be exposed via some sort of State->getTemporaryStorage(Expr) call, but i believe that generally this process should be as transparent to the checkers as possible.

It sounds as if both of these maps could be eliminated by always making sure that the target temporary is constructed "with" the MTE (for lifetime-extended temproraries) or BTE (for temporaries that require destruction at the end of full-expression). In this case, with the help of construction context-assisted lookahead, we declare that the target of the construction is CXXTempObjectRegion(MTE, LC) or CXXTempObjectRegion(BTE, LC) respectively, rather than CXXTempObjectRegion(CXXConstructExpr). Then during evaluation of MTE or BTE we'd simply construct the same region with the expression we're currently evaluating, and it's automagically going to be the correct region. This approach, however, becomes confusing when we start dealing with elidable constructors (explained below). So for now i believe that it is quite irrelevant which expression is identifying the temporary region.

== Elidable constructors ==

While it doesn't sound like an immediately important task to implement copy elision in the analyzer, it may help with making some things easier. And it'd also make some reports fancier, as mentioned in https://reviews.llvm.org/D43144.

Elidable copy-constructors can be explained as a form of lifetime extension. Instead of copying the temporary, they keep using the original value of the temporary, which in some pretty twisted sense means that they are lifetime-extending it to be able to use it. For example, if we modify our example by replacing the lifetime-extending reference variable with a value-type variable:

  1    class C \{
  2    public:
  3      C\(\) \{\}
  4      \~C\(\) \{\}
  5    \};
  6
  7    void foo\(\) \{
  8      C c = C\(\);
  9    \}

...then we'd still have an MTE, even though lifetime extension would seem to be gone:

   DeclStmt 0x7fb8f005afb8 <line:8:3, col:12>
   \`\-VarDecl 0x7fb8f005ac50 <col:3, col:11> col:5 c 'C' cinit
     \`\-ExprWithCleanups 0x7fb8f005afa0 <col:9, col:11> 'C'
       \`\-CXXConstructExpr 0x7fb8f005af68 <col:9, col:11> 'C' 'void \(const C &\) noexcept' elidable
         \`\-MaterializeTemporaryExpr 0x7fb8f005af00 <col:9, col:11> 'const C' lvalue
           \`\-ImplicitCastExpr 0x7fb8f005aee8 <col:9, col:11> 'const C' <NoOp>
             \`\-CXXBindTemporaryExpr 0x7fb8f005aec8 <col:9, col:11> 'C' \(CXXTemporary 0x7fb8f005aec0\)
               \`\-CXXTemporaryObjectExpr 0x7fb8f005ae88 <col:9, col:11> 'C' 'void \(\)'

In this case the MTE is expressing the fact that the temporary constructed via CXXTemporaryObjectExpr can be "lifetime-extended" (by actually merging it with the stack variable) rather than copied, if the CXXConstructExpr surrounding it would be chosen to be elided. The AST doesn't make the elision choice for us - but is compatible with both choices. The MTE essentially overrides the necessity of immediate destruction provided by the BTE, and lets the surrounding AST decide upon the lifetime of the object.

Because the analyzer currently does not do copy elision, it will use the MTE only to compute the argument for the elidable copy-constructor, and then perform the copy-construction, and then destroy the original temporary at the end of the full-expression. Note, however, that in this case we need to properly support both the BTE (for the temporary destructor to work) and the MTE (for computing its value). We need to implement the MTE's ability to perform "rvalue-to-lvalue-cast" even if the temporary destruction is still necessary. For this reason, if we rely on constructing temporary regions with the correct BTEs or MTEs, at least one of these tasks becomes impossible to perform.

If we were to support copy elision, then the CXXTemporaryObjectExpr constructor would go directly into the variable region. For the purposes of modeling, it'd mean that only CXXTemporaryObjectExpr would actually need to be modeled. But this would require additional coding in the construction context to be able to realize that the target is the variable while modeling the CXXTemporaryObjectExpr.

For the sake of completeness, let's consider the ternary operator example:

  1    class C \{
  2    public:
  3      C\(int\) \{\}
  4      \~C\(\) \{\}
  5    \};
  6
  7    void foo\(int coin\) \{
  8      const C &c = coin ? C\(1\) : C\(2\);
  9    \}

The respective AST would be:

   DeclStmt 0x7fc1e20023e0 <line:8:3, col:34>
   \`\-VarDecl 0x7fc1e2001dc8 <col:3, col:33> col:12 c 'const C &' cinit
     \`\-ExprWithCleanups 0x7fc1e2002370 <col:16, col:33> 'const C' lvalue
       \`\-MaterializeTemporaryExpr 0x7fc1e2002340 <col:16, col:33> 'const C' lvalue extended by Var 0x7fc1e2001dc8 'c' 'const C &'
         \`\-ImplicitCastExpr 0x7fc1e2002328 <col:16, col:33> 'const C' <NoOp>
           \`\-ConditionalOperator 0x7fc1e20022f8 <col:16, col:33> 'C'
             >\-ImplicitCastExpr 0x7fc1e2002170 <col:16> 'bool' <IntegralToBoolean>
             > \`\-ImplicitCastExpr 0x7fc1e2002158 <col:16> 'int' <LValueToRValue>
             >   \`\-DeclRefExpr 0x7fc1e2001e28 <col:16> 'int' lvalue ParmVar 0x7fc1e2001c18 'coin' 'int'
             >\-CXXBindTemporaryExpr 0x7fc1e2002248 <col:23, col:26> 'C' \(CXXTemporary 0x7fc1e2002240\)
             > \`\-CXXConstructExpr 0x7fc1e2002208 <col:23, col:26> 'C' 'void \(const C &\) noexcept' elidable
             >   \`\-MaterializeTemporaryExpr 0x7fc1e20021a0 <col:23, col:26> 'const C' lvalue
             >     \`\-ImplicitCastExpr 0x7fc1e2002188 <col:23, col:26> 'const C' <NoOp>
             >       \`\-CXXFunctionalCastExpr 0x7fc1e2002078 <col:23, col:26> 'C' functional cast to class C <ConstructorConversion>
             >         \`\-CXXBindTemporaryExpr 0x7fc1e2002058 <col:23, col:26> 'C' \(CXXTemporary 0x7fc1e2002050\)
             >           \`\-CXXConstructExpr 0x7fc1e2002018 <col:23, col:26> 'C' 'void \(int\)'
             >             \`\-IntegerLiteral 0x7fc1e2001e60 <col:25> 'int' 1
             \`\-CXXBindTemporaryExpr 0x7fc1e20022d8 <col:30, col:33> 'C' \(CXXTemporary 0x7fc1e20022d0\)
               \`\-CXXConstructExpr 0x7fc1e2002298 <col:30, col:33> 'C' 'void \(const C &\) noexcept' elidable
                 \`\-MaterializeTemporaryExpr 0x7fc1e2002280 <col:30, col:33> 'const C' lvalue
                   \`\-ImplicitCastExpr 0x7fc1e2002268 <col:30, col:33> 'const C' <NoOp>
                     \`\-CXXFunctionalCastExpr 0x7fc1e2002130 <col:30, col:33> 'C' functional cast to class C <ConstructorConversion>
                       \`\-CXXBindTemporaryExpr 0x7fc1e2002110 <col:30, col:33> 'C' \(CXXTemporary 0x7fc1e2002108\)
                         \`\-CXXConstructExpr 0x7fc1e20020d0 <col:30, col:33> 'C' 'void \(int\)'
                           \`\-IntegerLiteral 0x7fc1e20020b0 <col:32> 'int' 2

Each branch contains two constructors: the temporary and the elidable copy. The temporaries are surrounded with their respective BTEs and copy-elision-kind MTEs, which indicates that they need to be either destroyed as temporaries, or, if copy elision is chosen, have their lifetime decided upon by the surrounding AST. The elidable copy constructors also, being temporaries, have their respective BTEs. Note, however, that there is only one MTE for both BTEs for the elidable constructors.

So after the conditional operator is resolved (which is the first thing we need to do, according to the CFG), we'd go ahead and perform the constructors, and their trigger would be their respective BTE in the non-elide case, and the single top-level MTE in the elide case. In the non-elide case, copy constructors would be triggered by the top-level MTE.

It means that, once again, copy elision would prevent us from handling both the BTE and the copy-elision-kind MTE in the single construction, allowing the "predictable target region" trick to work: when we need the temporary destructor, we construct directly into CXXTempObjectRegion of the BTE and it gets automatically picked up during destruction, and when we need the automatic destructor, we construct directly into CXXTempObjectRegion of the MTE and we can easily compute the value of the MTE. But when we don't do copy elision, we'd have to keep at least one of those in the program state map.

== Return by value ==

Returning C++ objects by value is actually very similar to constructing it. Consider:

  1    class C \{
  2    public:
  3      C\(\) \{\}
  4      \~C\(\) \{\}
  5    \};
  6
  7    C bar\(\) \{
  8      C c;
  9      return c;
 10    \}
 11
 12    void foo\(\) \{
 13      const C &c = bar\(\);
 14    \}

With the respective AST for DeclStmt in foo():

   DeclStmt 0x7fe62c84f8e8 <line:13:3, col:21>
   \`\-VarDecl 0x7fe62c84f6b8 <col:3, col:20> col:12 c 'const C &' cinit
     \`\-ExprWithCleanups 0x7fe62c84f878 <col:16, col:20> 'const C' lvalue
       \`\-MaterializeTemporaryExpr 0x7fe62c84f848 <col:16, col:20> 'const C' lvalue extended by Var 0x7fe62c84f6b8 'c' 'const C &'
         \`\-ImplicitCastExpr 0x7fe62c84f830 <col:16, col:20> 'const C' <NoOp>
           \`\-CXXBindTemporaryExpr 0x7fe62c84f810 <col:16, col:20> 'C' \(CXXTemporary 0x7fe62c84f808\)
             \`\-CallExpr 0x7fe62c84f7e0 <col:16, col:20> 'C'
               \`\-ImplicitCastExpr 0x7fe62c84f7c8 <col:16> 'C \(\*\)\(\)' <FunctionToPointerDecay>
                 \`\-DeclRefExpr 0x7fe62c84f770 <col:16> 'C \(\)' lvalue Function 0x7fe62c84f190 'bar' 'C \(\)'

And for the ReturnStmt in bar():

   ReturnStmt 0x7fe62c84f5b0 <line:9:3, col:10>
   \`\-CXXConstructExpr 0x7fe62c84f578 <col:10> 'C' 'void \(const C &\) noexcept' elidable
     \`\-ImplicitCastExpr 0x7fe62c84f518 <col:10> 'const C' lvalue <NoOp>
       \`\-DeclRefExpr 0x7fe62c84f4f0 <col:10> 'C' lvalue Var 0x7fe62c84f280 'c' 'C'

Since https://reviews.llvm.org/D42875 we can already realize that the constructor in bar(), assuming that we're inlining bar() during analysis, would be constructed into something that is a return value of bar(). This allows us, by looking that the StackFrameContext's call site, to figure out that it is being constructed into the CallExpr in foo(). Now if only we knew that that the call site is a lifetime-extended temporary, i.e. if only we had a pointer to the foo()'s MTE at the CallExpr's CFG element, we'd be able to find the correct target region for construction: the CXXTempObjectRegion for the MTE in the StackFrameContext of foo(). So i'm proposing to add some sort of construction context to not only constructors, but also to functions that return objects, and then during construction perform the lookup in three easy steps:

1. in the callee's CFG from constructor to return statement,
2. through the location from the return statement to the call site,
3. then through the caller's CFG from the call site to the MTE.

If the function is not inlined, we can still make use of the construction context to represent the return value as a LazyCompoundValue of the MTE's temporary. It would eliminate the need to replace the return value with another value while evaluating the MTE, and of course the need to re-bind the object to a different this-region.

So i believe that this is a good way to eliminate the need for the "createTemporaryRegionIfNeeded" thing in the function calls as well.

#4

While all three more or less work, a combination of the three - temporary destructors, broken lifetime extension, and copy elision - causes massive false positives at the moment. Consider:

  1    \#include <memory>
  2
  3    void use\(const char \*\);
  4
  5    void foo\(\) \{
  6      char \*p = new char\[10\];
  7      std::unique\_ptr<char \[\]> x = std::unique\_ptr<char\[\]>\(p\);
  8      use\(p\);
  9    \}

This causes a use-after free warning, even though everything we ever wanted was inlined. Here's the AST for line 7:

DeclStmt 0x7fddb999d8a8 <line:7:3, col:58>
`-VarDecl 0x7fddba8ad738 <col:3, col:57> col:28 x 'std::unique_ptr<char >':'std::__1::unique_ptr<char , std::__1::default_delete<char > >' cinit
`-ExprWithCleanups 0x7fddb999d890 <col:32, col:57> 'std::unique_ptr<char >':'std::__1::unique_ptr<char , std::__1::default_delete<char > >'
`-CXXConstructExpr 0x7fddb999d858 <col:32, col:57> 'std::unique_ptr<char >':'std::__1::unique_ptr<char , std::__1::default_delete<char > >' 'void (std::__1::unique_ptr<char , std::__1::default_delete<char > > &&) noexcept' elidable
`-MaterializeTemporaryExpr 0x7fddb999d840 <col:32, col:57> 'std::unique_ptr<char >':'std::__1::unique_ptr<char , std::__1::default_delete<char > >' xvalue
`-CXXFunctionalCastExpr 0x7fddb999b7f0 <col:32, col:57> 'std::unique_ptr<char >':'std::__1::unique_ptr<char , std::__1::default_delete<char > >' functional cast to std::unique_ptr<char > <ConstructorConversion>
`-CXXBindTemporaryExpr 0x7fddb999b7d0 <col:32, col:57> 'std::unique_ptr<char >':'std::__1::unique_ptr<char , std::__1::default_delete<char > >' (CXXTemporary 0x7fddb999b7c8)
`-CXXConstructExpr 0x7fddb999b6a0 <col:32, col:57> 'std::unique_ptr<char >':'std::__1::unique_ptr<char , std::__1::default_delete<char > >' 'void (char *, typename enable_if<__same_or_less_cv_qualified<char *, pointer>::value, __nat>::type) noexcept'
>-ImplicitCastExpr 0x7fddb999ad88 <col:56> 'char *' <LValueToRValue>
> `-DeclRefExpr 0x7fddba8ad958 <col:56> 'char *' lvalue Var 0x7fddba8ad1d0 'p' 'char *'
`-CXXDefaultArgExpr 0x7fddb999b680 <<invalid sloc>> 'typename enable_if<__same_or_less_cv_qualified<char *, pointer>::value, __nat>::type':'std::__1::unique_ptr<char , std::__1::default_delete<char > >::__nat'

So we're constructing a temporary unique_ptr, binding the temporary for subsequent destruction, functional-casting it (no-op), materializing it for elidable move-construction, doing elidable move-construction into variable 'x', then destroying the original temporary.

During move-construction, we're erasing our pointer from the temporary and transfer it into 'x'.

However, due to the previous MaterializeTemporaryExpr, which "is" "the" "lifetime extension" ("through elidable move"), we have incorrectly created a copy of the temporary. So during move-construction we've erased our pointer in the copy of the temporary, but not in the original temporary.

The destructor, however, is deleting the original correct temporary, not the erroneous copy! And the original temporary still owns the pointer.

It's not possible for MaterializeTemporaryExpr to inform the destructor that the address has changed because our MaterializeTemporaryExpr doesn't know the old address that needs to be replaced.

Sounds like it's time to do something about lifetime extension.

#5

Hello Artem,

Thank you for working on this and sharing the status.
Do we expect coverage changes after allowing destructors to be inlined?

25.01.2018 20:08, Artem Dergachev via cfe-dev пишет:

#6

For now i'm not seeing much coverage increase, and i'm not aiming for that, though i've already seen a couple of neat temporary-specific new true positives.

We avoid the gtest noreturn destructor sink mentioned in https://reviews.llvm.org/D42779 but there's not much of these in regular code.

Apart from that, i'm expecting a typical skew in coverage due to more aggressive inlining, but that's it.

For now it's hard to understand coverage change because there are quite a few new false positives hanging around, similar to http://lists.llvm.org/pipermail/cfe-dev/2018-February/056909.html

#7

A bit of an update. Previous messages for easier navigation through this thread:

http://lists.llvm.org/pipermail/cfe-dev/2018-January/056691.html
http://lists.llvm.org/pipermail/cfe-dev/2018-February/056813.html
http://lists.llvm.org/pipermail/cfe-dev/2018-February/056898.html
http://lists.llvm.org/pipermail/cfe-dev/2018-February/056909.html
http://lists.llvm.org/pipermail/cfe-dev/2018-February/056929.html

== Overall status ==

Previous changes had minimal effects on the false positives i observed. However, once return-by-value and construction-conversion support has landed, improved modeling has finally yielded significant results, fixing at least 150 false positives (around 40% of all positives) on a few sub-projects of WebKit (incl. WebCore, JavaScriptCore).

Large chunks of the remaining false positives are related to brace initialization constructors which i'll try to support at least in simple cases (eg. a single layer of braces).

Many of the remaining reports are hard to understand - it seems that trackNullOrUndefValue isn't always doing a good job. Other reports are over-saturated with inlined temporary destructors: path pruning may be working incorrectly.

On other projects i've seen interesting bug reports against code that works incorrectly when copy elision isn't happening (of a poorly implemented class that performs memory management). We're capable of finding such bugs because we don't model copy elision yet. Such code is not portable, but many users that use just one compiler can afford relying on its implementation details, and copy elision is usually supported by most major compilers. These reports would probably go away because i'd like to model copy elision anyway because it's generally a good thing: it improves analyzer performance and reduces path lengths. And we'd still catch these bugs when non-elidable copies will be made.

These results are with temporary destructor inlining turned on. I'm holding off on destructor inlining for now because i'm seeing false positives on other projects, even though the reference counting suppression is working fairly well.

Lack of default argument support is still causing a small but noticeable coverage drop. The CFG for this syntax seems usable but i guess full support would require introducing a new LocationContext sub-class. Binary conditional operators are still not supported (even in CFG), but it doesn't seem to cause many problems.

== C++17 problems ==

> Copy elision - cppreference.com
> (since C++17)
> Under the following circumstances, the compilers are REQUIRED to omit the copy- and move- construction of class objects even if the copy/move constructor and the destructor have observable side-effects. They need not be present or accessible, as the language rules ensure that no copy/move operation takes place, even conceptually:
> In initialization, if the initializer expression is a prvalue and the cv-unqualified version of the source type is the same class as the class of the destination, the initializer expression is used to initialize the destination object...
> In a function call, if the operand of a return statement is a prvalue and the return type of the function is the same as the type of that prvalue...

This is going to be fun to support. Specifically, this means that neither initialization 'C c = C();' nor return-statement 'return C()' (where 'C()' may as well be replaced with a function call of type 'C') contains an elidable copy in the AST anymore, but only a single constructor of variable 'c' (or the returned-to object) - probably with an intermediate CXXBindTemporaryExpr if the class has a non-trivial destructor. A few consequences of this change:

* For now the CFG itself is broken in this case because it includes a temporary destructor that shouldn't be there.

* The construction context for a function that returns an object by value may now be not only a temporary object context, but also a simple variable context or a returned value context - we'll need to handle these cases.

* Liveness does not work correctly - previously it was relying on CXXBindTemporaryExpr or MaterializeTemporaryExpr but now these aren't guaranteed to even exist, so by the time we reach 'c' from 'C()' we may already destroy bindings to the not-yet-alive variable 'c' we've added in the constructor. This is especially obvious for the conditional operator case, i.e. 'C c = x ? C(1) : C(2);' - in this case in C++17 there is just one constructor happening on every particular path.

* Returned values may now chain together - the target object of a constructor that's called as part of the return statement may be multiple stack frames above the constructor call, which we'd need to unwind recursively.

For now i'll disable temporary- and returned-value- construction contexts in C++17.

#8

== Copy elision ==

= Why? =

I've underestimated the importance of implementing copy elision at first.

Eliding copies changes observable behavior of the program if the copy/move constructor has side effects. Whether to elide creation of a temporary object in certain operations is up to implementation, but most implementations currently support and prefer copy elision, so if we don't do the same in the analyzer we'd be analyzing a different program, and users who don't care about portability may become grumpy because they are sure that copy elision is happening.

Additionally, copy elision makes paths longer - including the number of diagnostic pieces - which makes reports harder to understand and exhausts the analyzer's budget faster.

= How? =

Consider an example:

  1    class C \{
  2    public:
  3      C\(int, int\);
  4      C\(const C &amp;\);
  5      \~C\(\);
  6    \};
  7
  8    void foo\(\) \{
  9      C c = C\(123, 456\);
 10    \}

The respective AST:

   DeclStmt 0x7fe1d3001ed8 &lt;line:9:3, col:20&gt;
   \`\-VarDecl 0x7fe1d3001d08 &lt;col:3, col:19&gt; col:5 c &#39;C&#39; cinit
     \`\-ExprWithCleanups 0x7fe1d3001ec0 &lt;col:9, col:19&gt; &#39;C&#39;
       \`\-CXXConstructExpr 0x7fe1d3001e88 &lt;col:9, col:19&gt; &#39;C&#39; &#39;void \(const C &amp;\)&#39; elidable
         \`\-MaterializeTemporaryExpr 0x7fe1d3001e70 &lt;col:9, col:19&gt; &#39;const C&#39; lvalue
           \`\-ImplicitCastExpr 0x7fe1d3001e58 &lt;col:9, col:19&gt; &#39;const C&#39; &lt;NoOp&gt;
             \`\-CXXBindTemporaryExpr 0x7fe1d3001e38 &lt;col:9, col:19&gt; &#39;C&#39; \(CXXTemporary 0x7fe1d3001e30\)
               \`\-CXXTemporaryObjectExpr 0x7fe1d3001db8 &lt;col:9, col:19&gt; &#39;C&#39; &#39;void \(int, int\)&#39;
                 &gt;\-IntegerLiteral 0x7fe1d3001d78 &lt;col:11&gt; &#39;int&#39; 123
                 \`\-IntegerLiteral 0x7fe1d3001d98 &lt;col:16&gt; &#39;int&#39; 456

And the CFG:

[B1]
1: 123
2: 456
3: C([B1.1], [B1.2]) (CXXConstructExpr, [B1.4], [B1.6], class C)
4: [B1.3] (BindTemporary)
5: [B1.4] (ImplicitCastExpr, NoOp, const class C)
6: [B1.5] (MaterializeTemporaryExpr)
7: [B1.6] (CXXConstructExpr, [B1.8], class C)
8: C c = C(123, 456);
9: ~C() (Temporary object destructor)
10: [B1.8].~C() (Implicit destructor)

This explains pretty well what we should do if we don't do copy elision: we construct a temporary on B1.3 that's destroyed on B1.9 after being materialized into an lvalue that's referenced by the copy-constructor parameter; the copy-constructor will copy the object into the actual variable.

Now, if we are to elide the copy constructor, the original constructor C(123, 456) should construct the variable directly, and all statements in between (B1.4, B1.5, B1.6, B1.7) and the destructor (B1.9) turn into no-op.

I can imagine two ways of implementing such behavior:

(i). Completely remove elided elements B1.4, B1.5, B1.6, B1.7, B1.9 from the CFG.
(ii). Keep CFG roughly the same but model elided elements as no-op in the analyzer.

I believe that approach (ii) is ultimately better, even though (i) sounds very tempting.

Approach (i) assumes that the modified CFG would kinda look like this:

[B1]
1: 123
2: 456
3: C([B1.1], [B1.2]) (CXXConstructExpr, [B1.4], class C)
4: C c = C(123, 456);
5: [B1.4].~C() (Implicit destructor)

Our CFGConstructor element [B1.3] will essentially be frankensteined from *inner* CXXTemporaryObjectExpr 0x7fe1d3001db8 (it's printed as CXXConstructExpr in those CFG dumps) and the construction context of the *outer* copy-constructor CXXConstructExpr 0x7fe1d3001e88.

This CFG looks pretty good on its own. It's almost indistinguishable from the CFG for 'C c(123, 456)' and contains all that's needed to properly initialize 'c'.

However the more i think about it, the more it seems to violate the principle of least astonishment. For instance, getSVal(DeclStmt->getSingleDecl()->getInit(), LC) would yield UnknownVal because the constructor that has initialized the variable is different from the one that's present in the initializer of the variable. The code to obtain the "actual" initializer expression may be very complicated because initializers can be quite complex (imagine a ?: operator; the code for extracting the actual initializer will essentially duplicate the code for finding construction contexts).

Essentially, all of the expressions between the DeclStmt and the CXXTemporaryObjectExpr will never be set in the Environment and would be unknown. Any user (imagine a checker) who will try to understand what is going on in the current program point by exploring values of expressions will fail here.

We are going to evaluate our DeclStmt without evaluating its immediate sub-expressions. Why would the sub-sub-sub-sub-sub-expression that we need to evaluate the DeclStmt be still kept alive in the Environment?

If we try to make checkers' life easier by setting values in the Environment anyway for expressions that aren't present in the CFG, how would liveness analysis know when to garbage-collect them given this analysis is also CFG-based?

In fact I'm quite scared to imagine how our liveness analysis would react when he realizes that there are a lot of expressions right in the middle of a feasible path that are completely omitted from the CFG. I guess we are omitting some dead paths from the CFG, but omitting expressions in the middle of a statement will almost certainly make liveness analysis more complicated.

Long story short, approach (i) violates a large amount of very basic and intuitive invariants that we've so far maintained. Which is scary and will make further development harder, even if we manage to make it work.

Approach (ii) is much more in the spirit of C++ standard [class.copy.elision]:

> "the implementation treats the source and target of the omitted copy/move operation as simply two different ways of referring to the same object".

Essentially, copy elision is an aliasing relationship between the stack variable 'c' and the temporary object within its initializer. This makes perfect sense in the CodeGen where both of these are just some stack addresses. In analyzer terms, we declare that the CXXTempObjectRegion for our CXXTemporaryObjectExpr and the VarRegion for our variable 'c' are the same region. Because we don't have aliasing / renaming support in the analyzer, we take the route of picking our regions correctly from the start (that's, in some sense, the whole point of this construction context hassle! - avoiding the need to rename regions later - which would have probably been the right thing to do even if we could rename regions). It leaves us with the following values for the expressions in the Environment:

CXXTemporaryObjectExpr -> lazyCompoundVal{VarRegion 'c', Store after construction}
CXXBindTemporaryExpr -> lazyCompoundVal{VarRegion 'c', Store after construction}
MaterializeTemporaryExpr -> c
CXXConstructExpr -> lazyCompoundVal{VarRegion 'c', Store after construction}

Only CXXTemporaryObjectExpr will be actually modeled (hopefully by inlining the constructor), with VarRegion 'c' acting as this-argument.

This leaves us with CXXConstructExpr that constructs into its own source argument, which is exactly what the standard expects from us, even if it's a bit amusing. It would not construct anything, of course, just bind the value. It is questionable whether we need a checker callback for the elided constructor (and if we do, which ones specifically).

CFG needs to be extended to add more context to the CXXTemporaryObjectExpr in order to execute approach (ii) correctly. We still need access to the CXXConstructExpr's construction context in order to perform the very first construction, so i guess that the construction context for CXXTemporaryObjectExpr would be a combination of both. Note that there may be more than one elided copy constructor (eg. there are two if the initializer is a ?: operator - and, well, there may be multiple nested ?: operators).

Additionally, when we model elided expressions, we need to know that they are elided by looking at them. We may add this information to their CFG elements (this, again, looks tempting, but requires more CFG element kinds) or mark expressions as elided in the program state (which we can easily do because they are already mentioned in one of the construction contexts).

#9

== Errata ==

In the previous message i wrote:

> copy elision makes paths longer

I meant lack of copy elision><

== C++17, Functional casts, Liveness ==

I wanted to share this example to document a certain bug we seem to currently have with our liveness analysis, in order to get back to it later and also to justify how careful we should be.

A naive attempt to support C++17 mandatory copy elision for variable and constructor initializers (return values are a bit more tricky) would be to model C++17-specific construction contexts added in ⚙ D44597 [CFG] [analyzer] Add C++17-specific variable and return value construction contexts. and https://reviews.llvm.org/D44763 similarly to their "simple" counterparts.

This approach fails, however, in a fairly funny manner. Consider an example:

class C1 {
   int x;
public:
   C1(int x): x(x) {}
   int getX() const { return x; }
   ~C1();
};

class C2 {
   int x;
   int y;
public:
   C2(int x, int y): x(x), y(y) {}
   int getX() const { return x; }
   int getY() const { return y; }
   ~C2();
};

void foo(int coin) {
   C1 c1 = coin ? C1(1) : C1(2);
   c1.getX();
   C2 c2 = coin ? C2(3, 4) : C2(5, 6);
   c2.getX();
}

In this example C1 is a wrapper around a single integer and C2 is a wrapper around a pair of integers; there are no other differences between these two classes. However, if we attempt to treat CXX17ElidedCopyConstructionContext and SimpleVariableConstructionContext similarly in ExprEngine::getRegionForConstructedObject(), we suddenly get the following warning:

$ clang -cc1 -analyze -analyzer-checker core test.cpp -std=c++17
test.cpp:14:22: warning: Undefined or garbage value returned to caller
int getX() const { return x; }
^~~~~~~~
1 warning generated.

The warning appears for C2 but not for C1. The warning is a false positive that appears because construction results were removed from the Store because variable 'c2' is not yet live when the construction happens. However, variable 'c1' is live when its construction happens.

It is, emm, uhm, moderately easy to notice that the AST for constructors of C1 and C2 on ternary operator branches are completely different: C1(1) is a functional cast from integer 1 to class C1, while C2 is a normal temporary object expression.

Indeed, here's the AST for c1:

     >-DeclStmt 0x7ff92c002570 <line:20:3, col:31>
     > `-VarDecl 0x7ff92c002100 <col:3, col:30> col:6 used c1 'C1' cinit
     >   `-ExprWithCleanups 0x7ff92c002558 <col:11, col:30> 'C1'
     >     `-ConditionalOperator 0x7ff92c002528 <col:11, col:30> 'C1'
     >       >-ImplicitCastExpr 0x7ff92c002510 <col:11> 'bool' <IntegralToBoolean>
     >       > `-ImplicitCastExpr 0x7ff92c0024f8 <col:11> 'int' <LValueToRValue>
     >       >   `-DeclRefExpr 0x7ff92c002160 <col:11> 'int' lvalue ParmVar 0x7ff92c001f80 'coin' 'int'
     >       >-CXXFunctionalCastExpr 0x7ff92c002418 <col:18, col:22> 'C1' functional cast to class C1 <ConstructorConversion>
     >       > `-CXXBindTemporaryExpr 0x7ff92c0023f8 <col:18, col:22> 'C1' (CXXTemporary 0x7ff92c0023f0)
     >       >   `-CXXConstructExpr 0x7ff92c002388 <col:18, col:22> 'C1' 'void (int)'
     >       >     `-IntegerLiteral 0x7ff92c002198 <col:21> 'int' 1
     >       `-CXXFunctionalCastExpr 0x7ff92c0024d0 <col:26, col:30> 'C1' functional cast to class C1 <ConstructorConversion>
     >         `-CXXBindTemporaryExpr 0x7ff92c0024b0 <col:26, col:30> 'C1' (CXXTemporary 0x7ff92c0024a8)
     >           `-CXXConstructExpr 0x7ff92c002470 <col:26, col:30> 'C1' 'void (int)'
     >             `-IntegerLiteral 0x7ff92c002450 <col:29> 'int' 2

... and here's the AST for c2:

     >-DeclStmt 0x7ff92c002b10 <line:22:3, col:37>
     > `-VarDecl 0x7ff92c0026c8 <col:3, col:36> col:6 used c2 'C2' cinit
     >   `-ExprWithCleanups 0x7ff92c002af8 <col:11, col:36> 'C2'
     >     `-ConditionalOperator 0x7ff92c002ac8 <col:11, col:36> 'C2'
     >       >-ImplicitCastExpr 0x7ff92c002ab0 <col:11> 'bool' <IntegralToBoolean>
     >       > `-ImplicitCastExpr 0x7ff92c002a98 <col:11> 'int' <LValueToRValue>
     >       >   `-DeclRefExpr 0x7ff92c002728 <col:11> 'int' lvalue ParmVar 0x7ff92c001f80 'coin' 'int'
     >       >-CXXBindTemporaryExpr 0x7ff92c0029b8 <col:18, col:25> 'C2' (CXXTemporary 0x7ff92c0029b0)
     >       > `-CXXTemporaryObjectExpr 0x7ff92c002968 <col:18, col:25> 'C2' 'void (int, int)'
     >       >   >-IntegerLiteral 0x7ff92c002760 <col:21> 'int' 3
     >       >   `-IntegerLiteral 0x7ff92c002780 <col:24> 'int' 4
     >       `-CXXBindTemporaryExpr 0x7ff92c002a78 <col:29, col:36> 'C2' (CXXTemporary 0x7ff92c002a70)
     >         `-CXXTemporaryObjectExpr 0x7ff92c002a28 <col:29, col:36> 'C2' 'void (int, int)'
     >           >-IntegerLiteral 0x7ff92c0029e8 <col:32> 'int' 5
     >           `-IntegerLiteral 0x7ff92c002a08 <col:35> 'int' 6

Note that the AST for c1 has MORE expressions (which presumably means more chances to garbage-collect), but in fact it is c2 that is getting garbage-collected.

This looks to me like a bug - or at least an inconsistency - in liveness analysis, and the long-term fix that will allow us to finally support C++17 would probably be to fix this inconsistency.

The fact that liveness analysis has never given us any problems so far - apart from not supporting some new features we requested from it - is very surprising. Our liveness analysis must be very conservative if the first problem we've seen with it in, like, years is in a C++17 feature that was not even planned back when liveness analysis was written. If liveness analysis is so conservative, maybe we could speed up the analyzer dramatically by making it eliminate more expressions and variables during removeDead()?

#10

Hi Artem,

I can say that approach 2 is much more clear for me. At least, from the user's point of view.

22.03.2018 06:45, Artem Dergachev via cfe-dev пишет:

#11

More explanations on how the analyzer keeps making its way around the C++
AST.

== Lifetime extension ==

This is a brain dump of how (and how much) lifetime extension of temporary
objects is currently broken in the static analyzert. Spoilers: not too
much, it seems, but annoying nevertheless.

Consider an example:

     1 class C {
     2 public:
     3 C() {}
     4 ~C() {}
     5 };
     6
     7 void foo() {
     8 const C &c = C();
     9 }

With the AST for the variable declaration:

      DeclStmt 0x7fa5ac85cba0 <line:8:3, col:19>
      `-VarDecl 0x7fa5ac85c878 <col:3, col:18> col:12 c 'const C &' cinit
        `-ExprWithCleanups 0x7fa5ac85cb30 <col:16, col:18> 'const C' lvalue
          `-MaterializeTemporaryExpr 0x7fa5ac85cb00 <col:16, col:18>
'const C' lvalue extended by Var 0x7fa5ac85c878 'c' 'const C &'
            `-ImplicitCastExpr 0x7fa5ac85cae8 <col:16, col:18> 'const C'
<NoOp>
              `-CXXBindTemporaryExpr 0x7fa5ac85cac8 <col:16, col:18> 'C'
(CXXTemporary 0x7fa5ac85cac0)
                `-CXXTemporaryObjectExpr 0x7fa5ac85ca88 <col:16, col:18>
'C' 'void ()'

*here goes a periodic reminder that CXXTemporaryObjectExpr is a sub-class
of CXXConstructExpr*

Notice how MaterializeTemporaryExpr is the innermost expression (the first
one in the order of execution) that is an lvalue. Essentially, you can
think of it as the mythical "rvalue-to-lvalue" cast that takes in a C++
object rvalue and returns the this-value for that object. Because all C++
objects have a this-value that never changes throughout their lifetime, it
is essentially their identity. Otherwise you can't call methods on them.

MaterializeTemporaryExpr also contains information about the lifetime
extension process: we needed the this-value in order to bind it to variable
'c'. You see that in the AST.

In the analyzer, however, MaterializeTemporaryExpr does a different thing,
as a temporary solution (no pun intended). It constructs a new temporary
region out of thin air and binds the rvalue object to that temporary in the
Store. The respective function in our code is called
"createTemporaryRegionIfNeeded". It also has a separate purpose of
converting rvalue sub-object adjustments into lvalue sub-object
adjustments, which we wouldn't discuss this time.

Now that we learned how to inline temporary constructors and destructors,
it essentially means that the this-value in the constructor and in the
destructor would be different. Because first we construct the object into
temporary region R1, then we take lazyCompoundVal{R1} to represent the
value of CXXTemporaryObjectExpr, then we materialize lazyCompoundVal{R1} to
R2, then we bind R2 to variable 'c', then we call the automatic(!)
destructor for 'c' which contains R2. To be clear, the region store at the
time of destruction would be:

  c: R2,
  R2: lazyCompoundVal{R1}.

It means that fields of the object would contain the correct values, there
would be the correct number of destructors called (no temporary
destructors, just one automatic destructor), but the identity of the object
(this-value) would change in the process. Unless the object actually makes
any decisions or does any manipulations that involve its this-value, the
modeling should be correct. When the object starts to actively use its
this-value in its inlined methods, the analysis would do wrong stuff.
Additionally, it causes a mess in the checkers when they try to track
objects by their this-values - i.e. IteratorChecker has a lot of additional
code to work around the fact that the region for the object constantly
changes.

From the above it is clear that MaterializeTemporaryExpr should not
construct any new regions, at least not in this case. We already have the
correct region, namely R1, which should be re-used.

Hi Artem!

We found a strange false positive that might be related to what you
describe above but not sure though. Could you take a look?
It looks like we are seeing a null pointer dereference error, and the null
value comes from the destructor which was invoked on a temporary.
If this is not the case, the path diagnostic might be misleading.

Here is the finding:
http://cc.elte.hu:15010/Default/#run=Xerces_Xerces-C_3_2_1_unexplored_first&checker-name=unix.MismatchedDeallocator&checker-name=cplusplus.NewDeleteLeaks&checker-name=core.uninitialized.UndefReturn&checker-name=core.UndefinedBinaryOperatorResult&checker-name=core.NullDereference&checker-name=core.NonNullParamChecker&checker-name=core.DivideZero&checker-name=core.CallAndMessage&tab=allReports&reportHash=2f8548f84329734e674f15d844caeb38&report=6764&subtab=2f8548f84329734e674f15d844caeb38

Regards,
Gábor

#12

Hmm, i don’t fully understand it yet. It seems to me that RefHash3KeysIdPoolEnumerator drains the whole pool upon destruction (, so it indeed can’t really be safely passed around or copied (i.e. elidable copies are not moves). So i’d easily believe that this code only works because all copies are elided, i.e. the code is not portable (at least not until C++17) but usually works because most compilers are clever enough. We’ll probably get rid of such positives when we implement copy elision. In case i got it all wrong, do you accidentally have a reproducer for me to have a closer look? If CTU stuff is hard to provide a reproducer for, you should be able to write a small code snippet directly into RefHash3KeysIdPool.c to reproduce the problem. I’ll also accept a trimmed (as in -trim-egraph) exploded graph dot file even if it’s huge. P.S. The “Calling…” icon is hilarious :smiley:

#13

More explanations on how the analyzer keeps making its way around the C++
AST.

== Lifetime extension ==

This is a brain dump of how (and how much) lifetime extension of
temporary objects is currently broken in the static analyzert. Spoilers:
not too much, it seems, but annoying nevertheless.

Consider an example:

     1 class C {
     2 public:
     3 C() {}
     4 ~C() {}
     5 };
     6
     7 void foo() {
     8 const C &c = C();
     9 }

With the AST for the variable declaration:

      DeclStmt 0x7fa5ac85cba0 <line:8:3, col:19>
      `-VarDecl 0x7fa5ac85c878 <col:3, col:18> col:12 c 'const C &' cinit
        `-ExprWithCleanups 0x7fa5ac85cb30 <col:16, col:18> 'const C'
lvalue
          `-MaterializeTemporaryExpr 0x7fa5ac85cb00 <col:16, col:18>
'const C' lvalue extended by Var 0x7fa5ac85c878 'c' 'const C &'
            `-ImplicitCastExpr 0x7fa5ac85cae8 <col:16, col:18> 'const C'
<NoOp>
              `-CXXBindTemporaryExpr 0x7fa5ac85cac8 <col:16, col:18> 'C'
(CXXTemporary 0x7fa5ac85cac0)
                `-CXXTemporaryObjectExpr 0x7fa5ac85ca88 <col:16, col:18>
'C' 'void ()'

*here goes a periodic reminder that CXXTemporaryObjectExpr is a sub-class
of CXXConstructExpr*

Notice how MaterializeTemporaryExpr is the innermost expression (the
first one in the order of execution) that is an lvalue. Essentially, you
can think of it as the mythical "rvalue-to-lvalue" cast that takes in a C++
object rvalue and returns the this-value for that object. Because all C++
objects have a this-value that never changes throughout their lifetime, it
is essentially their identity. Otherwise you can't call methods on them.

MaterializeTemporaryExpr also contains information about the lifetime
extension process: we needed the this-value in order to bind it to variable
'c'. You see that in the AST.

In the analyzer, however, MaterializeTemporaryExpr does a different
thing, as a temporary solution (no pun intended). It constructs a new
temporary region out of thin air and binds the rvalue object to that
temporary in the Store. The respective function in our code is called
"createTemporaryRegionIfNeeded". It also has a separate purpose of
converting rvalue sub-object adjustments into lvalue sub-object
adjustments, which we wouldn't discuss this time.

Now that we learned how to inline temporary constructors and destructors,
it essentially means that the this-value in the constructor and in the
destructor would be different. Because first we construct the object into
temporary region R1, then we take lazyCompoundVal{R1} to represent the
value of CXXTemporaryObjectExpr, then we materialize lazyCompoundVal{R1} to
R2, then we bind R2 to variable 'c', then we call the automatic(!)
destructor for 'c' which contains R2. To be clear, the region store at the
time of destruction would be:

  c: R2,
  R2: lazyCompoundVal{R1}.

It means that fields of the object would contain the correct values,
there would be the correct number of destructors called (no temporary
destructors, just one automatic destructor), but the identity of the object
(this-value) would change in the process. Unless the object actually makes
any decisions or does any manipulations that involve its this-value, the
modeling should be correct. When the object starts to actively use its
this-value in its inlined methods, the analysis would do wrong stuff.
Additionally, it causes a mess in the checkers when they try to track
objects by their this-values - i.e. IteratorChecker has a lot of additional
code to work around the fact that the region for the object constantly
changes.

From the above it is clear that MaterializeTemporaryExpr should not
construct any new regions, at least not in this case. We already have the
correct region, namely R1, which should be re-used.

Hi Artem!

We found a strange false positive that might be related to what you
describe above but not sure though. Could you take a look?
It looks like we are seeing a null pointer dereference error, and the null
value comes from the destructor which was invoked on a temporary.
If this is not the case, the path diagnostic might be misleading.

Here is the finding: http://cc.elte.hu:15010/Default/#run=Xerces_Xerces-C_
3_2_1_unexplored_first&checker-name=unix.MismatchedDeallocator&checker-
name=cplusplus.NewDeleteLeaks&checker-name=core.uninitialized.UndefReturn&
checker-name=core.UndefinedBinaryOperatorResult&checker-name=core.
NullDereference&checker-name=core.NonNullParamChecker&
checker-name=core.DivideZero&checker-name=core.
CallAndMessage&tab=allReports&reportHash=2f8548f84329734e674f15d844caeb
38&report=6764&subtab=2f8548f84329734e674f15d844caeb38

Regards,
Gábor

Hmm, i don't fully understand it yet. It seems to me that
RefHash3KeysIdPoolEnumerator drains the whole pool upon destruction (fIdPtrs
is not a member of the temporary, it's an external entity), so it indeed
can't really be safely passed around or copied (i.e. elidable copies are
not moves).

Oh, I think you are right! Somehow I assumed the there is a deep copy going
on but after looking at the copy constructor, it looks like that is not the
case. Sorry for bothering :slight_smile:

#14

A bit of an update. Previous messages for easier navigation through this
thread:

http://lists.llvm.org/pipermail/cfe-dev/2018-January/056691.html
http://lists.llvm.org/pipermail/cfe-dev/2018-February/056813.html
http://lists.llvm.org/pipermail/cfe-dev/2018-February/056898.html
http://lists.llvm.org/pipermail/cfe-dev/2018-February/056909.html
http://lists.llvm.org/pipermail/cfe-dev/2018-February/056929.html
http://lists.llvm.org/pipermail/cfe-dev/2018-March/057255.html
http://lists.llvm.org/pipermail/cfe-dev/2018-March/057357.html
http://lists.llvm.org/pipermail/cfe-dev/2018-April/057641.html

== Story so far ==

Our AST for C++ constructors is rather unusual: the this-argument of the construct-expression is not its sub-expression but is instead scattered around its parent expressions. For example, constructor within 'A a(1, 2, 3)' has AST

DeclStmt
`- VarDecl 'a'
`- CXXConstructExpr
>- 1
>- 2
`- 3

from which it can be seen that it constructs variable 'a', but there's no information about variable 'a' within CXXConstructExpr itself or its children. This lead to problems within the analyzer that was unable to figure out what object is constructed by looking at the CFG statements that go in order of execution (first CXXConstructExpr, then DeclStmt). Additionally, this leads to problems with liveness analysis, because the constructor ends up putting stuff into a memory region that isn't yet live.

In order to deal with that, i've augmented the CFGElement for the construct-expression to provide a ConstructionContext object that captures all necessary parent statements. Whenever CFG provides a ConstructionContext, it is easy for the analyzer to work correctly. This helped us support many new cases of object construction that weren't previously supported, including temporary object construction and construction into operator new(). This helped us suppress huge amounts of false positives, noticeably we can now produce reasonable results on WebKit which is a relatively exotic C++ codebase. Still, many more cases need to be handled.

== Plans ==

I'll probably focus on adding support for copy elision first. A few mails ago i explained how important this feature is, and it is probably also the biggest self-contained chunk of work that remains in this realm so far. I'll start with a bit of refactoring because some technical debt remains - see below :slight_smile:

== Simple variables and liveness ==

So far i paid relatively little attention to how the simple case

A a(1, 2, 3);

works. It kidna worked, so i expected it to not need fixing apart from the simple refactoring to switch from CFG lookahead to the ConstructionContext. However when i looked at https://bugs.llvm.org/show_bug.cgi?id=37270 it turned out to be much worse than i expected.

The CFG for that would look like this:

CXXConstructExpr (prvalue expression of type A, construction context points to the DeclStmt)
... (sometimes some irrelevant implicit stuff in between)
DeclStmt 'a'

Now let's see. Assume there's a construction context.

(a) The way i expected it to have worked:

1. CXXConstructExpr is modeled and produces bindings to 'a'. It evaluates to LazyCompoundVal of 'a'.
2. DeclStmt doesn't need to be modeled because bindings are already there.

(b) The way it might as well have worked:

1. CXXConstructExpr is modeled and produces bindings to 'a'. It evaluates to LazyCompoundVal of 'a'.
2. DeclStmt is modeled by binding the value of its initializer, which is a LazyCompoundVal of 'a', to 'a'.

(c) The way it actually turned out to work:

1. CXXConstructExpr is modeled and produces bindings to 'a'. It evaluates to MemRegionVal '&a' (wat?).
2. DeclStmt is modeled by binding the LazyCompoundVal of 'a' that is loaded from its initializer '&a' to 'a' (like (b).2 just with a load).

On (c).1 things get really weird because CXXConstructExpr is a prvalue expression of object type, and therefore must be modeled as a NonLoc. The load on (c).2 doesn't really correspond to any meaningful AST or semantics here.

In order to regain some sanity, i tried to see what was wrong with (a) and (b). Approach (b) is actually relatively easy to implement, however it has a downside of producing a default LazyCompoundVal binding in the Store instead of many direct bindings normally produced by an inlined constructor. Apart from the performance overhead of looking through an extra LazyCompoundVal every time we load from the variable later, we'll have analysis precision suffer because RegionStore is bad at modeling binding extents. There is a variety of tests in our test suite that would degrade because of that.

So it's indeed useful to try to preserve the bindings as they were bound by the inlined constructor.

Now, would (a) actually work? NO!!! Because, well, all of a sudden we find out that now bindings to 'a' are dead in sense of liveness analysis and are removed from the Store. And the fact that they're still present within the LazyCompoundVal won't help us on (a).2 because we promised to not bind the LazyCompoundVal. Yet it worked just fine in (c) because the bindings were kept alive because region '&a' was kept alive because it was incorrectly bound to the CXXConstructExpr in the Environment!

Here comes the moment of truth. Construction contexts for operator new and temporaries all had their own program state traits to prevent the memory region from dying until we leave the construction site. Apparently, these construction context kinds *were not special*. Essentially, every kind of construction context needs a path-sensitive program state trait in order to work.