Hi!
On gcc.c-torture/execute/pr103376.c in the following code we trigger UB
in the compiler. n->range is 8 because it is 64-bit load and rsize is 0
because it is a bswap sequence with load and known to be 0:
/* Find real size of result (highest non-zero byte). */
if (n->base_addr)
for (tmpn = n->n, rsize = 0; tmpn; tmpn >>= BITS_PER_MARKER, rsize++);
else
rsize = n->range;
The shifts then shift uint64_t by 64 bits. For this case mask is 0
and we want both *cmpxchg and *cmpnop as 0, the operation can be done as
both nop and bswap and callers will prefer nop.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2021-11-27 Jakub Jelinek <ja...@redhat.com>
PR tree-optimization/103435
* gimple-ssa-store-merging.c (find_bswap_or_nop_finalize): Avoid UB if
n->range - rsize == 8, just clear both *cmpnop and *cmpxchg in that
case.
--- gcc/gimple-ssa-store-merging.c.jj 2021-11-25 10:47:07.000000000 +0100
+++ gcc/gimple-ssa-store-merging.c 2021-11-26 10:54:11.959800560 +0100
@@ -871,12 +871,18 @@ find_bswap_or_nop_finalize (struct symbo
{
mask = ((uint64_t) 1 << (rsize * BITS_PER_MARKER)) - 1;
*cmpxchg &= mask;
- *cmpnop >>= (n->range - rsize) * BITS_PER_MARKER;
+ if (n->range - rsize == sizeof (int64_t))
+ *cmpnop = 0;
+ else
+ *cmpnop >>= (n->range - rsize) * BITS_PER_MARKER;
}
else
{
mask = ((uint64_t) 1 << (rsize * BITS_PER_MARKER)) - 1;
- *cmpxchg >>= (n->range - rsize) * BITS_PER_MARKER;
+ if (n->range - rsize == sizeof (int64_t))
+ *cmpxchg = 0;
+ else
+ *cmpxchg >>= (n->range - rsize) * BITS_PER_MARKER;
*cmpnop &= mask;
}
n->range = rsize;
Jakub
