PR target/107172: Avoid "unusual" MODE_CC comparisons in simplify-rtx.cc

I believe that a better (or supplementary) fix to PR target/107172 is to
avoid producing incorrect (but valid) RTL in
simplify_const_relational_operation when presented with questionable
(obviously invalid) expressions, such as those produced during combine.
Just as with the "first do no harm" clause with the Hippocratic Oath,
simplify-rtx (probably) shouldn't unintentionally transform invalid RTL
expressions, into incorrect (non-equivalent) but valid RTL that may be
inappropriately recognized by recog.

In this specific case, many GCC backends represent their flags register via
MODE_CC, whose representation is intentionally "opaque" to the middle-end.
The only use of MODE_CC comprehensible to the middle-end's RTL optimizers
is relational comparisons between the result of a COMPARE rtx (op0) and zero
(op1).  Any other uses of MODE_CC should be left alone, and some might argue
indicate representational issues in the backend.

In practice, CPUs occasionally have numerous instructions that affect the
flags register(s) other than comparisons [AVR's setc, powerpc's mtcrf,
x86's clc, stc and cmc and x86_64's ptest that sets C and Z flags in
non-obvious ways, c.f. PR target/109973].  Currently care has to be taken,
wrapping these in UNSPEC, to avoid combine inappropriately merging flags
setters with flags consumers (such as conditional jumps).  It's safer to
teach simplify_const_relational_operation not to modify expressions that
it doesn't understand/recognize.

2023-05-30  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/107172
	* simplify-rtx.cc (simplify_const_relational_operation): Return
	early if we have a MODE_CC comparison that isn't a COMPARE against
	const0_rtx.

diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
index d4aeebc7a5f2dcbb19290078d56ce6ab55e83862..d6444b40802628aa4971985dbcd32dc510d606e4 100644 (file)
--- a/gcc/simplify-rtx.cc
+++ b/gcc/simplify-rtx.cc
@@ -6120,6 +6120,12 @@ simplify_const_relational_operation (enum rtx_code code,
              || (GET_MODE (op0) == VOIDmode
                  && GET_MODE (op1) == VOIDmode));
 
+  /* We only handle MODE_CC comparisons that are COMPARE against zero.  */
+  if (GET_MODE_CLASS (mode) == MODE_CC
+      && (op1 != const0_rtx
+         || GET_CODE (op0) != COMPARE))
+    return NULL_RTX;
+
   /* If op0 is a compare, extract the comparison arguments from it.  */
   if (GET_CODE (op0) == COMPARE && op1 == const0_rtx)
     {