[Bug tree-optimization/66261] operations that simplify whether the pointers are the same or not (++p;++q;--p;--q)

[pcordes at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66261

Peter Cordes <pcordes at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |pcordes at gmail dot com

--- Comment #3 from Peter Cordes <pcordes at gmail dot com> ---
Re: does this show up in real code: not exactly this, but incrementing two
array elements that might or might not be the same location defeats optimizing
an  x++  loop into x += n

From
https://stackoverflow.com/questions/77618348/why-does-gcc-fail-to-reduce-a-loop-that-increments-two-locations-of-the-same-buf

unsigned int getid();
void foo(unsigned int *counter, unsigned int n) {
        unsigned int A = getid();
        unsigned int B = getid();
        for (unsigned int i = 0; i < n; i++) {
            ++counter[A];
            ++counter[B];
        }
}



If the two locations are related so GCC knows they're not the same, 
such as arr[A%4 + 0] and arr[A%4 + 16], it can also optimize.
https://godbolt.org/z/PbeaW8nnE

But it doesn't help to do something like  if (A==B) return;
before the loop so it can't be entered with A==B

Source-level loop fission is required to get good asm (two increments to memory
locations that might be the same or not).