[gcc r14-1405] PR target/107172: Avoid "unusual" MODE_CC comparisons in simplify-rtx.cc

[gcc r14-1405] PR target/107172: Avoid "unusual" MODE_CC comparisons in simplify-rtx.cc

[Roger Sayle]

https://gcc.gnu.org/g:69185294f322dd53d4e1592115014c5488302e2e

commit r14-1405-g69185294f322dd53d4e1592115014c5488302e2e
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Tue May 30 14:40:50 2023 +0100

    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:
---
 gcc/simplify-rtx.cc | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
index d4aeebc7a5f..d6444b40802 100644
--- 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)
     {