[GVN] Should be able to PRE a load from memset

[llvmbot]

Bugzilla Link	25559
Version	trunk
OS	Windows NT
Reporter	LLVM Bugzilla Contributor
CC	@hfinkel

Extended Description

Currently, GVN can pre loads from calloc like functions. For example, in the below test case the store to 'res' will be converted from a load into a constant zero.

#include <strings.h>
#include <stdlib.h>

int *test_calloc(int n, int *res)
{
if (n == 0)
return 0;

int *ptr = (int *)calloc(n, sizeof(int));
*res = ptr[n-1];
return ptr;
}

However, PRE falls apart with this example:

int *test_malloc_memset_zero(int n, int *res)
{
if (n == 0)
return 0;

int *ptr = (int *)malloc(n * sizeof(int));
bzero(ptr, n * sizeof(int));
*res = ptr[n-1];
return ptr;
}

[llvmbot]

FWIW, I saw this in passing, but I bet this happens fairly often in the wild.

[llvmbot]

I suggest limiting this transformation to memsets of constant values as a first pass. I would expect that to almost always be an improvement (as you're replacing a load with a constant). In the non-constant case, you may insert a phi node that prevents other optimizations (e.g., loop vectorization).

[llvmbot]

It appears GVN does have support for this optimization, but not when the loop trip count is unknown at compile-time.