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

[Hans Wennborg via llvm-dev]

On Thu, Sep 24, 2015 at 12:06 PM, Aaron Ballman <aaron at aaronballman.com> wrote:
> On Thu, Sep 24, 2015 at 2:42 PM, Hans Wennborg <hans at chromium.org> wrote:
>> 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.

Right, I'm not suggesting that :-)

The code I looked at went something like this:

  struct S {
    S& operator=(const S& other) {
      if (&other != this)
        val = other.val;
      return *this;
    }
    void foo();
    int val;
  };
  void S::foo() {
    S tmp;
    tmp.val = 42;
    *this = tmp; // operator= gets inlined; we should know(?) that &tmp != this
  }

This is of course a silly example, but with GCC we get:

  movl $42, (%rdi)
  ret

whereas Clang generates:

  movl $42, -8(%rsp)
  leaq -8(%rsp), %rax
  cmpq %rdi, %rax
  je .LBB0_2
  movl $42, (%rdi)
  .LBB0_2:
  retq

which made me sad.


>> 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