[PATCH] builtins: Fix up ICE in inline_string_cmp [PR109258]

[Jakub Jelinek <jakub@redhat.com>]

Hi!

The PR109086 r13-6690 inline_string_cmp change to
      if (diff != result)
	emit_move_insn (result, diff);
regressed
FAIL: go.test/test/fixedbugs/bug207.go,  -O2 -g  (internal compiler error: in emit_move_insn, at expr.cc:4224)
The problem is the Go FE doesn't mark __builtin_memcmp as pure (I'll also
send patch for that) and so result is const0_rtx when the call lost its lhs
and the above move ICEs because moving something into const0_rtx is obviously
invalid.
I think it is better not to rely on all FEs having these *cmp functions
pure anD DCE being performed.  The following patch just punts from the
inline expansion in that case, so we just emit normal library call.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2023-03-24  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/109258
	* builtins.cc (inline_expand_builtin_bytecmp): Return NULL_RTX early
	if target == const0_rtx.

--- gcc/builtins.cc.jj	2023-03-23 10:00:58.308100548 +0100
+++ gcc/builtins.cc	2023-03-23 11:05:38.308135309 +0100
@@ -7178,8 +7178,8 @@ inline_expand_builtin_bytecmp (tree exp,
   bool is_ncmp = (fcode == BUILT_IN_STRNCMP || fcode == BUILT_IN_MEMCMP);
 
   /* Do NOT apply this inlining expansion when optimizing for size or
-     optimization level below 2.  */
-  if (optimize < 2 || optimize_insn_for_size_p ())
+     optimization level below 2 or if unused *cmp hasn't been DCEd.  */
+  if (optimize < 2 || optimize_insn_for_size_p () || target == const0_rtx)
     return NULL_RTX;
 
   gcc_checking_assert (fcode == BUILT_IN_STRCMP

	Jakub