Failure to optimize malloc+memset to calloc

[GabrielRavier]

Bugzilla Link	47159
Version	trunk
OS	Linux
CC	@rotateright

Extended Description

struct S
{
    int a;
};

struct S *f()
{
    struct S *p = (struct S *)malloc(sizeof(struct S));
    memset(p, 0, sizeof(*p));
    return p;
}

This can be optimized to return calloc(sizeof(struct S), 1);. This transformation is done by GCC, but not by LLVM. See also https://godbolt.org/z/MGzfGf.

[rotateright]

We probably require 2 different patches to get this because we canonicalize small memsets like the 1 shown here into a single 'store' instruction. Ie, we should produce the same code even if the source was written as:

struct S *f()
{
struct S *p = (struct S *)malloc(sizeof(struct S));
p->a = 0;
return p;
}

Looks like gcc misses that case.

[pinskia]

#26266