[gcc r17-3498] avoid-store-forwarding: Reject BLKmode memory operands [PR126686]

https://gcc.gnu.org/g:b01cd452a6c7ba2a2f95a77f62f0727cbbe9d6a5

commit r17-3498-gb01cd452a6c7ba2a2f95a77f62f0727cbbe9d6a5
Author: Konstantinos Eleftheriou <[email protected]>
Date:   Thu Aug 20 03:57:02 2026 -0700

    avoid-store-forwarding: Reject BLKmode memory operands [PR126686]
    
    Memory operands were only rejected when their size is unknown or
    non-constant, so a BLKmode store with a known size could become a
    forwarding candidate. Building the bit insert sequence for it asserts
    in store_bit_field, as BLKmode has no corresponding integer mode.
    Reject such memory when collecting candidates.
    
            PR rtl-optimization/126686
    
    gcc/ChangeLog:
    
            * avoid-store-forwarding.cc
            (store_forwarding_analyzer::avoid_store_forwarding): Reject
            BLKmode stores and loads as forwarding candidates.
    
    gcc/testsuite/ChangeLog:
    
            * gcc.dg/torture/pr126686.c: New test.

Diff:
---
 gcc/avoid-store-forwarding.cc           | 10 +++++++---
 gcc/testsuite/gcc.dg/torture/pr126686.c | 16 ++++++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/gcc/avoid-store-forwarding.cc b/gcc/avoid-store-forwarding.cc
index 141f96c3d848..6cc3a3b4486d 100644
--- a/gcc/avoid-store-forwarding.cc
+++ b/gcc/avoid-store-forwarding.cc
@@ -565,10 +565,14 @@ store_forwarding_analyzer::avoid_store_forwarding (basic_block bb)
       /* The mem RTX if INSN is a store, NULL_RTX otherwise.  */
       rtx store_mem = MEM_P (SET_DEST (set)) ? SET_DEST (set) : NULL_RTX;
 
-      /* We cannot analyze memory RTXs that have unknown size.	*/
-      if ((store_mem && (!MEM_SIZE_KNOWN_P (store_mem)
+      /* We cannot analyze memory RTXs that have unknown size.  BLKmode
+	 memory is rejected as well, as there is no mode for the forwarded
+	 value, even when its size is known.  */
+      if ((store_mem && (GET_MODE (store_mem) == BLKmode
+			 || !MEM_SIZE_KNOWN_P (store_mem)
 			 || !MEM_SIZE (store_mem).is_constant ()))
-	  || (load_mem && (!MEM_SIZE_KNOWN_P (load_mem)
+	  || (load_mem && (GET_MODE (load_mem) == BLKmode
+			   || !MEM_SIZE_KNOWN_P (load_mem)
 			   || !MEM_SIZE (load_mem).is_constant ())))
 	{
 	  store_exprs.truncate (0);
diff --git a/gcc/testsuite/gcc.dg/torture/pr126686.c b/gcc/testsuite/gcc.dg/torture/pr126686.c
new file mode 100644
index 000000000000..b9e03ba5c473
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr126686.c
@@ -0,0 +1,16 @@
+/* PR rtl-optimization/126686 */
+/* { dg-do compile } */
+/* { dg-additional-options "-favoid-store-forwarding" } */
+
+int tmp;
+short d_e;
+
+int
+foo ()
+{
+  long f = 0;
+  __builtin_memset ((char *) &f + sizeof f - 2, d_e, 2);
+  tmp = f;
+
+  return f;
+}