fwprop: Don't propagate asms [PR121253]

For the reasons explained in the comment, fwprop shouldn't even
try to propagate an asm definition.

gcc/
	PR rtl-optimization/121253
	* fwprop.cc (forward_propagate_into): Don't propagate asm defs.

gcc/testsuite/
	PR rtl-optimization/121253
	* gcc.target/aarch64/pr121253.c: New test.

2 files changed, 30 insertions, 0 deletions

diff --git a/gcc/fwprop.cc b/gcc/fwprop.cc
index 4ee44ad..59df7ca 100644
--- a/gcc/fwprop.cc
+++ b/gcc/fwprop.cc
@@ -836,6 +836,20 @@ forward_propagate_into (use_info *use, bool reg_prop_only = false)
   if (def_insn->is_artificial ())
     return false;
 
+  /* Do not propagate asms.  The only kind of propagation that would
+     succeed is propagation into a register move.  Such a propagation
+     is neutral if the destination of the move is a pseudo and unnecessarily
+     restricts the register allocator if the destination of the move is
+     a hard register.
+
+     Furthermore, unlike for a normal instruction, we cannot take a SET from an
+     asm and try dropping the CLOBBERs.  The recog process does not (and should
+     not try to) second-guess whether what the user wrote can be changed and
+     so it has to assume that any asm given to it is a fair reflection of
+     what the user wrote.  */
+  if (def_insn->is_asm ())
+    return false;
+
   rtx_insn *def_rtl = def_insn->rtl ();
   if (!NONJUMP_INSN_P (def_rtl))
     return false;
diff --git a/gcc/testsuite/gcc.target/aarch64/pr121253.c b/gcc/testsuite/gcc.target/aarch64/pr121253.c
new file mode 100644
index 0000000..37de605
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/pr121253.c
@@ -0,0 +1,16 @@
+/* { dg-options "-O" } */
+
+struct s128 {
+    long a, b;
+};
+
+struct s128 foo(void) {
+    struct s128 ret;
+    asm("mov %0, #0 \n\t"
+        "mov %R0, #0 \n\t"
+        "mov x0, #12345"
+        : "=r" (ret) : : "x0");
+    return ret;
+}
+
+/* { dg-final { scan-assembler-not {mov x0, #0} } } */