[llvm-dev] Comparing stack addresses and function args (Was: [llvm] r174131 - Add a comment explaining an unavailable optimization)

[Aaron Ballman via llvm-dev]

On Thu, Sep 24, 2015 at 2:42 PM, Hans Wennborg <hans at chromium.org> wrote:
> Apologies if this has come up before. I couldn't find any previous
> discussion, but I did find this commit.
>
> I was wondering why LLVM cannot optimize this code (which GCC does optimize):
>
>   int f(int *p) { int x; return p == &x; }
>
> it would seem that this must always return 0. (This occurs as a
> self-assignment check in the code I was looking at; I was hoping we
> could fold that check away.)

This is different than a self-assignment check, is it not?

blah& operator=(const blah &b) {
  if (&b == this) {}
  // ...
}

(Because it gets the pointer from the parameter and compares against a
"local" pointer?)

I just want to make sure that you're not suggesting we should optimize
away self-assignment checks in the general case. I agree that the code
you provided should optimize away, based on my understanding of the C
and C++ standards (which may be faulty, and that's why I've CCed in
Richard).

~Aaron

>
> I'd be interested to hear what those with a stronger understanding of
> the standard than myself think about this, and also if there is any
> example of something that could break because of this optimization. If
> not, I'd like us to optimize it :-)
>
>
> On Thu, Jan 31, 2013 at 4:49 PM, Dan Gohman <dan433584 at gmail.com> wrote:
>> Author: djg
>> Date: Thu Jan 31 18:49:06 2013
>> New Revision: 174131
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=174131&view=rev
>> Log:
>> Add a comment explaining an unavailable optimization.
>>
>> Modified:
>>     llvm/trunk/lib/Analysis/InstructionSimplify.cpp
>>
>> Modified: llvm/trunk/lib/Analysis/InstructionSimplify.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?rev=174131&r1=174130&r2=174131&view=diff
>> ==============================================================================
>> --- llvm/trunk/lib/Analysis/InstructionSimplify.cpp (original)
>> +++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp Thu Jan 31 18:49:06 2013
>> @@ -1688,6 +1688,34 @@ static Value *ExtractEquivalentCondition
>>    return 0;
>>  }
>>
>> +// A significant optimization not implemented here is assuming that alloca
>> +// addresses are not equal to incoming argument values. They don't *alias*,
>> +// as we say, but that doesn't mean they aren't equal, so we take a
>> +// conservative approach.
>> +//
>> +// This is inspired in part by C++11 5.10p1:
>> +//   "Two pointers of the same type compare equal if and only if they are both
>> +//    null, both point to the same function, or both represent the same
>> +//    address."
>> +//
>> +// This is pretty permissive.
>
> Indeed :-/
>
>> +// It's also partly due to C11 6.5.9p6:
>> +//   "Two pointers compare equal if and only if both are null pointers, both are
>> +//    pointers to the same object (including a pointer to an object and a
>> +//    subobject at its beginning) or function, both are pointers to one past the
>> +//    last element of the same array object, or one is a pointer to one past the
>> +//    end of one array object and the other is a pointer to the start of a
>> +//    different array object that happens to immediately follow the ï¬ rst array
>> +//    object in the address space.)
>> +//
>> +// C11's version is more restrictive, however there's no reason why an argument
>> +// couldn't be a one-past-the-end value for a stack object in the caller and be
>> +// equal to the beginning of a stack object in the callee.
>
> This is interesting.
>
> For the one-past-the-end pointer to point into the callee, the stack
> would have to be growing upwards. So this won't happen on X86. Can we
> turn this optimization on for downward-growing-stack targets?
>
> Second, if the stack grows upward, and the function argument does
> point into the callee stack frame, "p" and "&x" could have the same
> contents. So per the "represent the same address" part above, they
> should compare equal? But they're noalias? Are we allowed to write
> through p? It wasn't a pointer to a valid object when we made the
> call, but it became valid in the callee? This is all terrifying.
>
> I suppose one could store the value of &x though, and then use it
> again later, i.e.:
>
>   int *global;
>   int f(int *p) {
>     int x;
>     global = &x;
>     return p == &x;
>   }
>   int g() {
>     f(0);
>     return f(global);
>   }
>
> Is g() guaranteed to return 1 here? Maybe we could claim it's
> implementation dependent? GCC does not seem fold p==&x to 0 here. I
> suppose we could make sure to check whether &x escapes the function?
>
>  - Hans