[PATCH] 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.target/s390/pr126686.c: New test.
---
 gcc/avoid-store-forwarding.cc            | 10 +++++++---
 gcc/testsuite/gcc.target/s390/pr126686.c | 16 ++++++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/s390/pr126686.c

diff --git a/gcc/avoid-store-forwarding.cc b/gcc/avoid-store-forwarding.cc
index 141f96c3d848..99feebafe126 100644
--- a/gcc/avoid-store-forwarding.cc
+++ b/gcc/avoid-store-forwarding.cc
@@ -565,11 +565,15 @@ 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.	*/
+      /* 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 && (!MEM_SIZE_KNOWN_P (store_mem)
-			 || !MEM_SIZE (store_mem).is_constant ()))
+			 || !MEM_SIZE (store_mem).is_constant ()
+			 || GET_MODE (store_mem) == BLKmode))
 	  || (load_mem && (!MEM_SIZE_KNOWN_P (load_mem)
-			   || !MEM_SIZE (load_mem).is_constant ())))
+			   || !MEM_SIZE (load_mem).is_constant ()
+			   || GET_MODE (load_mem) == BLKmode)))
 	{
 	  store_exprs.truncate (0);
 	  continue;
diff --git a/gcc/testsuite/gcc.target/s390/pr126686.c b/gcc/testsuite/gcc.target/s390/pr126686.c
new file mode 100644
index 000000000000..ab25d109ec35
--- /dev/null
+++ b/gcc/testsuite/gcc.target/s390/pr126686.c
@@ -0,0 +1,16 @@
+/* PR rtl-optimization/126686 */
+/* { dg-do compile } */
+/* { dg-options "-O3 -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;
+}
-- 
2.55.0