https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93213
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Blocks| |83819
--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
The code gating the strlen multi-character store optimization tries to enable
it only for stores into objects of character types but in r279392, for MEM_REF
arguments, it also considers casts like in the assignment that is dropped here:
MEM <short unsigned int> [(char * {ref-all})&u16_1] = 0;
This was done to detect out-of-bounds stores to allocated objects such as in
void *p = malloc (3);
const char a[] = "123";
memcpy (p, a, sizeof a);
where the memcpy is transformed into
MEM <unsigned int> [(char * {ref-all})p_3] = 3355185;
r279392 wasn't supposed to introduce any new optimization, so I think the bug
is actually latent, in the handling of stores of single, non-zero bytes over
previously zeroed out arrays of two or more bytes. Here's a test case that
operates only on char arrays with no casts that reproduces the underlying
problem:
int main (void)
{
char a[] = { 1, 2 };
const char b[] = { 0, 0 };
const char c[] = { 2 };
__builtin_memcpy (a, b, 2);
// The above is transformed into
// MEM <short unsigned int> [(char * {ref-all})&a] = 0;
// which is then dropped because of the non-nul store below.
__builtin_memcpy (a, c, 1);
volatile char *p = a;
if (p[0] != 2 || p[1] != 0)
__builtin_abort ();
}
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83819
[Bug 83819] [meta-bug] missing strlen optimizations
