inttoptr and noalias returns

Hi,

I’m a bit confused about the interaction between inttoptr and noalias, and would like to better understand our model.

I realize there’s a bunch of in-flight work around restrict modeling and that ptrtoint was on the agenda for last week’s AA call. I’m interested in understanding both the current state and the thinking/plans for the future. And I’m happy for pointers to anywhere this is already written down, I didn’t find it from skimming the AA call minutes or the mailing list archive, but I could easily have overlooked it, and haven’t really dug into the set of restrict patches (nor do I know where to get a list of those).

I also realize that with aliasing questions there can always be a gap between what the model says we can infer and how aggressive analyses and optimizations are about actually making use of those inferences. Again I’m interested in both answers (and happy for either).

In the LangRef section on pointer aliasing rules [1], I see

An integer constant other than zero or a pointer value returned from a function not defined within LLVM may be associated with address ranges allocated through mechanisms other than those provided by LLVM. Such ranges shall not overlap with any ranges of addresses allocated by mechanisms provided by LLVM.

And I’m curious what “mechanisms provided by LLVM” for allocation means. Alloca, presumably. Global variables? Certain intrinsics? Any function with a noalias return value?

In the LangRef description of the noalias attribute [2], I see

This indicates that memory locations accessed via pointer values based on the argument or return value are not also accessed, during the execution of the function, via pointer values not based on the argument or return value … On function return values, the noalias attribute indicates that the function acts like a system memory allocation function, returning a pointer to allocated storage disjoint from the storage for any other object accessible to the caller.

The phrase “the storage for any other object accessible to the caller” in the noalias description sounds like a broader category than the phrase “mechanisms provided by LLVM” from the pointer aliasing section, so I would expect that if the pointer returned from a call to a function with return attribute noalias does not escape, then loads/stores through it would not alias loads/stores through a pointer produced by inttoptr. Am I interpreting that correctly?

I wrote some snippets [3] to see what the optimizer would do. Each case has a store of value 86 via pointer %p that I’d expect dead store elimination to remove if we think it does not alias the subsequent load via pointer %q (because immediately after that is another store to %p).

In each case, %q is the result of a call to a function whose return value is annotated noalias.

When %p is a pointer parameter, I indeed see the optimizer removing the dead store:

define i8 @test1(i8* %p) {

%q = call i8* @allocate()

store i8 86, i8* %p ; ← this gets removed

%result = load i8, i8* %q

store i8 0, i8* %p

ret i8 %result

}

When %p is the result of inttoptr, I do not see the store being removed, and I’m wondering if this is because of a subtle aliasing rule or an intentional conservativism in the optimizer or just a blind spot in the analysis:

define i8 @test2(i64 %p_as_int) {

%p = inttoptr i64 %p_as_int to i8*

%q = call i8* @allocate()

store i8 86, i8* %p ; ← this does not get removed

%result = load i8, i8* %q

store i8 0, i8* %p

ret i8 %result

}

When I outline the inttoptr into a separate function, I again see the optimizer remove the dead store, which again I’m wondering if the difference between this and the previous case is an intentional subtle point or what.

define i8* @launder(i64 %int) noinline {

%ptr = inttoptr i64 %int to i8*

ret i8* %ptr

}

define i8 @test3(i64 %p_as_int) {

%p = call i8* @launder(i64 %p_as_int)

%q = call i8* @allocate()

store i8 86, i8* %p ; ← this gets removed

%result = load i8, i8* %q

store i8 0, i8* %p

ret i8 %result

}

Happy for any insights you can share.

Thanks,

-Joseph

1 - https://llvm.org/docs/LangRef.html#pointeraliasing

2 - https://llvm.org/docs/LangRef.html#parameter-attributes

3 - https://godbolt.org/z/x8e41G33Y

Stepping through this in the debugger, I see this code in BasicAliasAnalysis doing a check similar to the sort that I would have expected to see proving NoAlias for this case, but it’s not because (ISTM) it’s being pretty conservative:

// If one pointer is the result of a call/invoke or load and the other is a

// non-escaping local object within the same function, then we know the

// object couldn’t escape to a point where the call could return it.

//

// Note that if the pointers are in different functions, there are a

// variety of complications. A call with a nocapture argument may still

// temporary store the nocapture argument’s value in a temporary memory

// location if that memory location doesn’t escape. Or it may pass a

// nocapture value to other functions as long as they don’t capture it.

if (isEscapeSource(O1) &&

isNonEscapingLocalObject(O2, &AAQI.IsCapturedCache))

return NoAlias;

if (isEscapeSource(O2) &&

isNonEscapingLocalObject(O1, &AAQI.IsCapturedCache))

return NoAlias;

}

and

/// Returns true if the pointer is one which would have been considered an

/// escape by isNonEscapingLocalObject.

static bool isEscapeSource(const Value *V) {

if (isa(V))

return true;

if (isa(V))

return true;

// The load case works because isNonEscapingLocalObject considers all

// stores to be escapes (it passes true for the StoreCaptures argument

// to PointerMayBeCaptured).

if (isa(V))

return true;

return false;

}

Since we have to look through all the uses of O1/O2 (including certain transitive ones) to prove isNonEscapingLocalObject, an expensive-but-more-precise analysis could just check if O2/O1 is in that set, IIUC. I get why BasicAliasAnalysis isn’t the right place to do that. Is there some more expensive alias analysis that I could opt into and get that sort of check?

Alternatively, following the logic that we can assume isEscapeSource for loads because we treat stores as escapes, is there room to assume isEscapeSource for inttoptrs because we treat ptrtoints, and things that let you subtly intify pointers such as certain compares, as escapes?

Thanks,

-Joseph

You’re right that LLVM is very conservative in handling inttoptr. And otherwise relies on the incorrect transformation “inttoptr(ptrtoint(x)) → x” to get rid of inttoptr.

I agree the store should have been removed in your second example. I guess inttoptr is not frequently used, and even less after a bunch of fixes to prevent optimizers from creating new ones.

BasicAA is quite basic, but that’s all LLVM has. The other alias analyses in git are either not useful in practice, unfinished or buggy. (I haven’t looked into that dir in a couple of years, so things may have changed in the meantime).

A big issue with LLVM’s static analysis is caching, since everything is done lazily. If you want to add something more expensive to BasicAA, you need to make sure that information is cached somehow to avoid recomputing it a thousand times. Compilation time is quite sensitive to the performance of BasicAA.

Although there’s no definitive semantics for pointer comparisons yet (soonish I hope), LLVM’s behavior implies that pointer comparisons indeed escape pointers just like ptrtoint does (except if the two pointers being compared are inbounds and point to the same object, and therefore the comparison is only around offsets and thus their address doesn’t leak).

Nuno

otherwise relies on the incorrect transformation “inttoptr(ptrtoint(x)) → x”

Could you point me to an example/explanation of why that transformation is incorrect? It’s not clear to me from the LangRef.

A big issue with LLVM’s static analysis is caching, since everything is done lazily. If you want to add something more expensive to BasicAA, you need to make sure that information is cached somehow to avoid recomputing it a thousand times. Compilation time is quite sensitive to the performance of BasicAA.

The IsCapturedCache in AAQueryInfo is pretty close to what I’m after, but I don’t really understand why the code in aliasCheck is using the weaker isEscapeSource as opposed to !isNonEscapingLocalObject.

escape pointers just like ptrtoint does

Yeah, so if the rule for ptrtoint is simply that the source pointer escapes, then I’d think we could take advantage of the flip side of that and isEscapeSource could return true for inttoptr, without needing expensive analysis/caching. But I know this can be a subtle area, so I’m not sure that’s the rule. I see [1] that Ryan Taylor added discussing it to the agenda for the February AA conference call, I’m curious what the outcome of that was.

Thanks,

-Joseph

1 - https://lists.llvm.org/pipermail/llvm-dev/2021-February/148671.html

otherwise relies on the incorrect transformation “inttoptr(ptrtoint(x)) → x”

Could you point me to an example/explanation of why that transformation is incorrect? It’s not clear to me from the LangRef.

A big issue with LLVM’s static analysis is caching, since everything is done lazily. If you want to add something more expensive to BasicAA, you need to make sure that information is cached somehow to avoid recomputing it a thousand times. Compilation time is quite sensitive to the performance of BasicAA.

The IsCapturedCache in AAQueryInfo is pretty close to what I’m after, but I don’t really understand why the code in aliasCheck is using the weaker isEscapeSource as opposed to !isNonEscapingLocalObject.

escape pointers just like ptrtoint does

Yeah, so if the rule for ptrtoint is simply that the source pointer escapes, then I’d think we could take advantage of the flip side of that and isEscapeSource could return true for inttoptr, without needing expensive analysis/caching. But I know this can be a subtle area, so I’m not sure that’s the rule. I see [1] that Ryan Taylor added discussing it to the agenda for the February AA conference call, I’m curious what the outcome of that was.

Yes, I believe adding inttoptr to isEscapeSource() should be safe, as any way to produce an integer from a pointer (via ptrtoint, store+load, icmp) will have counted as an escape.

Nikita