[gcc r11-9339] bswap: Fix UB in find_bswap_or_nop_finalize [PR103435]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:3479e49be805b077b215a1547aad20409e69af35

commit r11-9339-g3479e49be805b077b215a1547aad20409e69af35
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Sat Nov 27 13:00:55 2021 +0100

    bswap: Fix UB in find_bswap_or_nop_finalize [PR103435]
    
    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.
    
    2021-11-27  Jakub Jelinek  <jakub@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.
    
    (cherry picked from commit 567d5f3d62fba2a23a9e975f7e7c7b61bb67cf24)

Diff:
---
 gcc/gimple-ssa-store-merging.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/gcc/gimple-ssa-store-merging.c b/gcc/gimple-ssa-store-merging.c
index 7eb50d65a20..0bd5cab3c54 100644
--- a/gcc/gimple-ssa-store-merging.c
+++ b/gcc/gimple-ssa-store-merging.c
@@ -826,12 +826,18 @@ find_bswap_or_nop_finalize (struct symbolic_number *n, uint64_t *cmpxchg,
 	{
 	  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;