RFC: Decomposing deref(N) into deref(N) + nofree

TLDR: We should change the existing dereferenceability related attributes to imply point in time facts only, and re-infer stronger global dereferenceability facts where needed.

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If you prefer to read proposals in a browser, you can read this email here.

This proposal greatly benefited from multiple rounds of feedback from Johannes, Artur, and Nick. All remaining mistakes are my own.

Johannes deserves a lot of credit for driving previous iterations on this design. In particular, I want to note that we’ve basically returned to something Johannes first proposed several years ago, before we had specified the nofree attribute family.

The Basic Problem

We have a long standing semantic problem with the way we define dereferenceability facts which makes it difficult to express C++ references, or more generally, dereferenceability on objects which may be freed at some point in the program. The current structure does lend itself well to memory which can’t be freed. As discussed in detail a bit later, we want to seamlessly support both use cases.

The basic statement of the problem is that a piece of memory marked with deref(N) is assumed to remain dereferenceable indefinitely. For an object which can be freed, marking it as deref can enable unsound transformations in cases like the following:

o = deref(N) alloc();
if (c) free(o)
while(true) {
  if (c) break;
  // With the current semantics, we will hoist o.f above the loop
  v = o.f;
}

Despite this, Clang does emit the existing dereferenceable attribute in some problematic cases. We have observed miscompiles as a result, and optimizer has an assortment of hacks to try not to be too aggressive and miscompile too widely.

Haven’t we already solved this?

This has been discussed relatively extensively in the past, included an accepted review (https://reviews.llvm.org/D61652) which proposed splitting the dereferenceable attribute into two to adress this. However, this change never landed and recent findings reveal that we both need a broader solution, and have an interesting oppurtunity to take advantage of other recent work.

The need for a broader solution comes from the observation that deref(N) is not the only attribute with this problem. deref_or_null(N) is a fairly obvious case we’d known about with the previous proposal, but it was recently realized that other allocation related facts have this problem as well. We now have specific examples with allocsize(N,M) - and the baked in variants in MemoryBuiltins - and suspect there are other attributes, either current or future, with the same challenge.

The opportunity comes from the addition of “nofree” attribute. Up until recently, we really didn’t have a good notion of "free"ing an allocation in the abstract machine model. We used to comingle this with our notion of capture. (i.e. We’d assume that functions which could free must also capture.) With the explicit notion of “nofree”, we have an approach available to us we didn’t before.

The Proposal Itself

The basic idea is that we’re going to redefine the currently globally scoped attributes (deref, deref_or_null, and allocsize) such that they imply a point in time fact only and then combine that with nofree to recover the previous global semantics.

More specifically:

  • A deref attribute on a function parameter will imply that the memory is dereferenceable for a specified number of bytes at the instant the function call occurs.
  • A deref attribute on a function return will imply that the memory is dereferenceable at the moment of return.

We will then use the point in time fact combined with other information to drive inference of the global facts. While in principle we may loose optimization potential, we believe this is sufficient to infer the global facts in all practical cases we care about.

Sample inference cases:

  • A deref(N) argument to a function with the nofree and nosync function attribute is known to be globally dereferenceable within the scope of the function call. We need the nosync to ensure that no other thread is freeing the memory on behalf of the callee in a coordinated manner.
  • An argument with the attributes deref(N), noalias, and nofree is known to be globally dereferenceable within the scope of the function call. This relies on the fact that free is modeled as writing to the memory freed, and thus noalias ensures there is no other argument which can be freed. (See discussion below.)
  • A memory allocation in a function with a garbage collector which guarantees collection occurs only at explicit safepoints and uses the gc.statepoint infrastructure, is known to be globally dereferenceable if there are no calls to gc.statepoint anywhere in the module. This effectively refines the abstract machine model used for garbage collection before lowering by RS4GC to disallow explicit deallocation (for collectors which opt in).

The items above are described in terms of deref(N) for ease of description. The other attributes are handle analogously.

Explanation

The “deref(N), noalias, + nofree” argument case requires a bit of explanation as it involves a bunch of subtleties.

First, the current wording of nofree argument attribute implies that the callee can not arrange for another thread to free the object on it’s behalf. This is different than the specification of the nofree function attribute. There is no “nosync” equivalent for function attributes.

Second, the noalias argument attribute is subtle. There’s a couple of sub-cases worth discussing:

  • If the noalias argument is written to (reminder: free is modeled as a write), then it must be the only copy of the pointer passed to the function and there can be no copies passed through memory used in the scope of function.
  • If the noalias argument is only read from, then there may be other copies of the pointer. However, all of those copies must also be read only. If the object was freed through one of those other copies, then we must have at least one writeable copy and having the noalias on the read copy was undefined behavior to begin with.

Essentially, what we’re doing with noalias is using it to promote a fact about the pointer to a fact about the object being pointed to. Code structure wise, we should probably write it exactly that way.

Result

It’s important to acknowledge that with this change, we will lose the ability to specify global dereferenceability of arguments and return values in the general case. We believe the current proposal allows us to recover that fact for all interesting cases, but if we’ve missed an important use case we may need to iterate a bit.

We’ve discussed a few alternatives (below) which could be revisited if it turns out we are missing an important use case.

Use Cases

C++ References – A C++ reference implies that the value pointed to is dereferenceable at point of declaration, and that the reference itself is non-null. Of particular note, an object pointed to through a reference can be freed without introducing UB.

class A { int f; };

void ugly_delete(A &a) { delete &a; }
ugly_delete(*new A());

void ugly_delete2(A &a, A *a2) {
  if (unknown)
    // a.f can be *proven* deref here as it's deref on entry,
    // and no free on path from entry to here.
    x = a.f;
  delete a2;
}
auto *a = new A();
ugly_delete2(*a, a);

A &foo() {...}
A &a = foo();
if (unknown)
  delete b;
// If a and b point to the same object, a.f may not be deref here
if (unknown2)
  a.f;

Garbage Collected Objects (Java) – LLVM supports two models of GCed objects, the abstract machine and the physical machine model. The later is essentially the same as that for c++ as deallocation points (at safepoints) are explicit. The former has objects conceptually live forever (i.e. reclaimation is handled outside the model).

class A { int f; }

void foo(A a) {
  ...
  // a.f is trivially deref anywhere in foo
  x = a.f;
}

A a = new A();
...
// a.f is trivially deref following it's definition
x = a.f;

A foo();
a = foo();
...
// a.f is (still) trivially deref
x = a.f;

Rust Borrows – A rust reference argument (e.g. “borrow”) points to an object whose lifetime is guaranteed to be longer than the reference’s defining scope. As such, the object is dereferenceable through the scope of the function. Today, rustc does emit a dereferenceable attribute using the current globally dereferenceable semantic.

pub fn square(num: &i32) -> i32 {
  num * num
}
square(&5);

// a could be noalias, but isn't today
pub fn bar(a: &mut i32, b: &i32) {
  *a = a * b
}

bar(&mut 5, &2);

// At first appearance, rust does not allow returning references.  So return
// attributes are not relevant.  This seems like a major language hole, so this
// should probably be checked with a language expert.

Migration

Existing bytecode will be upgraded to the weaker non-global semantics. This provides forward compatibility, but does lose optimization potential for previously compiled bytecode.

C++ and GC’d language frontends don’t change.

Rustc should emit noalias where possible. In particular, ‘a’ in the case ‘bar’ above is currently not marked noalias and results in lost optimization potential as a result of this change. According to the rustc code, this is legal, but currently blocked on a noalias related miscompile. See https://github.com/rust-lang/rust/issues/54462 and https://github.com/rust-lang/rust/issues/54878 for further details. (My current belief is that all llvm side blockers have been resolved.)

Frontends which want the global semantics should emit noalias, nofree, and nosync where appropriate. If this is not enough to recover optimizations in common cases, please explain why not. It’s possible we’ve failed to account for something.

Alternative Designs

All of the alternate designs listed focus on recovering the full global deref semantics. Our hope is that any common case we’ve missed can be resolved with additional inference rules instead.

Extend nofree to object semantics

The nofree argument attribute current describes whether an object can freed through some particular copy of the pointer. We could strength the semantics to imply that the object is not freed through any copy of the pointer in the specified scope.

Doing so greatly weakens our ability to infer the nofree property. The current nofree property when combined with capture tracking in the caller is enough to prove interest deref facts over calls. We don’t want to loose the ability to infer that since it enables interesting transforms (such as code reordering over calls).

Add a separate nofreeobj attribute

Rather than change nofree, we could add a parallel attribute with the stronger object property. This - combined with deref(N) as a point in time fact - would be enough to recover the current globally deferenceable semantics.

The downside of this alternative is a) possible overkill, and b) the “ugly” factor of having two similar but not quite identical attributes.

Add an orthogonal attribute to promote pointer facts to object ones

To address the weakness of the former alternative, we could specify an attribute which strengthens arbitrary pointer facts to object facts. Examples of current pointer facts are attributes such as readonly, and writeonly.

This has not been well explored; there’s a huge possible design space here.

Hi Philip,

TBH I was slightly surprised with this email, as the interpretation about deref I had in my head was exactly what you are proposing as the new semantics.

I just looked again at LangRef and I see it could be interpreted in the way you describe. Because it mentions a pointer with deref attribute can be always dereferenced, irrespective of whether the pointer is free’d or not.

FWIW, Alive2 already implements the semantics you describe. For example, introducing a load after a call with a deref attribute is UB as the pointer may be free’d by the callee: https://alive2.llvm.org/ce/z/dz2wYy (but works with nofree: https://alive2.llvm.org/ce/z/XsHEn-)

TL;DR: Please go ahead!

Nuno

Thanks Philip for driving this for a while now!

Yeah, there were always two conflicting views for this one and we now found multiple cases where that bites us.
I think the `nofree` solution is a good one, it was introduced for that reason (https://reviews.llvm.org/D49165).

(Yet another opportunity to suggest a lightweight Attributor pass run by default only for certain attributes, like
nofree.)

I'm (kinda obviously) in favor :wink:

~ Johannes

+1

I have one minor question: according to https://llvm.org/docs/Atomics.html#optimization-outside-atomic , introducing a store is problematic even if the pointer is known to be dereferenceable. Does this also apply to dereferenceable+nosync+nofree?

+1

I have one minor question: according to https://llvm.org/docs/Atomics.html#optimization-outside-atomic , introducing a store is problematic even if the pointer is known to be dereferenceable. Does this also apply to dereferenceable+nosync+nofree?

This proposal is *only* discussing dereferenceability. This question involves concurrency and the memory model which is not changing at all. So, no, there's no change to when it's safe to insert a store to a potentially shared location.

Replying to myself to clarify a bit on the scope of this proposal.

I am proposing changes to semantics of the dereferenceable, dereferenceable_or_null, and allocsize attributes. I am also proposing changes to semantics of the analogous dereferenceable, and dereferenceable_or_null metadata since they are directly analogous. I am also proposing changes to how we infer dereferenceability for known allocation sites (as this essentially infers the attributes even though we don’t express it that way in code).

I am not proposing changes to the dereferenceability handling of any other IR construct. Specifically, this proposal does not involve any changes to handling of allocas or lifetime intrinsics. The semantics of those, and all other unmentioned IR features, should be unchanged after this work.

Philip

At this point, I find myself needing to declare that the proposal below is a failure, and ask the community what next steps we’d prefer.

This effort stumbled into the fact that we don’t seem to have any actual agreement on what the semantics of various attributes are. In particular, the semantics of nofree don’t appear to be in a usable state, and my attempts at driving consensus have failed. I am not willing to continue investing effort in that direction.

Given that, I see three options, and need input from the community as to which we should chose.

Option 1 - Back out the couple of changes which have landed, update LangRef to be explicit about the scoped dereferenceability we had historically, and consider this effort a failure.

Option 2 - Change the semantic of the attributes to the point in time semantic without attempting any further inference of the scoped semantics. At the current moment, the Java use case is covered (via the GC rule), no one seems to care about the lost optimization power for C/C++, and I am unclear on the practical impact (if any) on rust.

Option 3 - Introduce a new ‘nofreeobj’ attribute whose semantics would be specifically that an object is not freed in the dynamic scope of the function through any mechanism (including concurrency). This attribute would be basically uninferrable, and would exist only to support language guarantees being encoded by frontends.

My recommendation would be for option 2, than 3, than 1. It’s worth noting that we could also chose option 2, then implement option 3 lazily if anyone reports a practical performance regression.

Philip

// At first appearance, rust does not allow returning references.  So return
// attributes are not relevant.  This seems like a major language hole, so this
// should probably be checked with a language expert.

I’ll note that Rust totally allows returning references:
https://rust.godbolt.org/z/rcvdEPGW4

pub fn f(a: &mut [i32; 3]) → &mut i32 {
&mut a[2]
}

Jacob Lifshay

Hi Philip,

Regarding nofree, I think one of the confusions was around implementation. There was a mix between semantics and static analyses. These are different things and static analyses implement a sound approximation of the semantics. Of course it’s useful to define the semantics such that we can develop a useful approximation, but that’s it.

For nofree, I believe we concluded that it’s strictly more expressive to not have to rely on nosync. Nosync is all or nothing; we couldn’t mark a function that uses a mutex as nofree if relying on nosync. Keeping things separated strictly allows more functions to be marked nofree. Which doesn’t mean the first implementation will. But the semantics isn’t written to track the first implementation, but to be future proof.

That’s not to say nosync isn’t useful. I can totally see clang exposing a -fsingle-threaded flag or whatever so we get it for free.

Until we can disentangle implementation (of static analyses) and semantics I don’t think we will be able to reach any consensus around attributes.

I don’t think there’s anything to rollback. The changes that you did were a step in the right direction. It’s half backed, yes, but hopefully that work will be finished at some point. So option 2. seems the way to go. Thank you!

Nuno

Regarding nofree, I think one of the confusions was around implementation. There was a mix between semantics and static analyses. These are different things and static analyses implement a sound approximation of the semantics. Of course it’s useful to define the semantics such that we can develop a useful approximation, but that’s it.

For nofree, I believe we concluded that it’s strictly more expressive to not have to rely on nosync. Nosync is all or nothing; we couldn’t mark a function that uses a mutex as nofree if relying on nosync. Keeping things separated strictly allows more functions to be marked nofree. Which doesn’t mean the first implementation will. But the semantics isn’t written to track the first implementation, but to be future proof.

That’s not to say nosync isn’t useful. I can totally see clang exposing a -fsingle-threaded flag or whatever so we get it for free.

That is mostly my recollection as well, except that I ended up feeling less certain about how nofree+nosync should work together. My intuition agrees with Nuno, but I felt like the part of the discussion with Johannes hadn’t reached a conclusion. We definitely need a decision on this semantics question either way, and I would want it to be consistent across all affected attributes, to reduce the complexity of LLVM IR overall. argmemonly and perhaps others have the same problem, and last time I checked, miscompilations can be triggered on main because of this confusion.

I don’t have much time to spend on this, but I’m willing to put in at least some work to try to drive a consensus on this if needed. I’m curious to hear from Johannes.

Until we can disentangle implementation (of static analyses) and semantics I don’t think we will be able to reach any consensus around attributes.

I don’t think there’s anything to rollback. The changes that you did were a step in the right direction. It’s half backed, yes, but hopefully that work will be finished at some point. So option 2. seems the way to go. Thank you!

Agreed.

Cheers,
Nicolai

The consensus in the responses to my previous email was that we should go ahead and redefine dereferenceable to mean dereferenceable at the point-in-time the instruction executes (i.e. my option 2 below). I finally got the patch which does this posted for review. Interested readers should see .

Philip