[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 5:15 PM, Hans Wennborg <hans at chromium.org> wrote:
> 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.

Ah, yes, this makes perfect sense to me. Thank you for the explanation!

~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