[gcc r17-2669] ifcvt: do not emit a store flag for an irreversible condition [PR126347]

[Philipp Tomsich]

https://gcc.gnu.org/g:dcb8c8d3e7333b09081f81c29358e03e73d59a12

commit r17-2669-gdcb8c8d3e7333b09081f81c29358e03e73d59a12
Author: Philipp Tomsich <philipp.tomsich@vrull.eu>
Date:   Tue Jul 21 21:19:53 2026 +0200

    ifcvt: do not emit a store flag for an irreversible condition [PR126347]
    
    noce_emit_store_flag feeds the possibly-reversed comparison code to a
    store-flag insn, and to emit_store_flag, without checking that the
    reversal succeeded.  reversed_comparison_code returns UNKNOWN for
    UNLT/UNLE/UNGT/UNGE, and for a MODE_CC comparison it cannot trace back
    to its COMPARE -- common for a floating-point condition on targets that
    compare into a condition-code register.  emit_store_flag then reaches
    its floating-point path and calls swap_condition (UNKNOWN), which aborts.
    
    This was latent until r17-2519-ga33f26607eb4f3 made
    noce_try_shifted_store_flag the first caller to reach
    noce_emit_store_flag with REVERSEP set for such a condition.
    
    Bail out early when the reversal failed.
    
    Bootstrapped and regression tested on aarch64-unknown-linux-gnu with no
    regressions.
    
            PR rtl-optimization/126347
    gcc/
            * ifcvt.cc (noce_emit_store_flag): Return NULL_RTX when the
            comparison code is UNKNOWN.

Diff:
---
 gcc/ifcvt.cc | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/gcc/ifcvt.cc b/gcc/ifcvt.cc
index 7c50ab06e2f7..6b169e69741d 100644
--- a/gcc/ifcvt.cc
+++ b/gcc/ifcvt.cc
@@ -865,6 +865,11 @@ noce_emit_store_flag (struct noce_if_info *if_info, rtx x, bool reversep,
   else
     code = GET_CODE (cond);
 
+  /* reversed_comparison_code returns UNKNOWN for an unordered code, or a
+     CC-mode compare it cannot trace; neither path below can use that.  */
+  if (code == UNKNOWN)
+    return NULL_RTX;
+
   if ((if_info->cond_earliest == if_info->jump || cond_complex)
       && (normalize == 0 || STORE_FLAG_VALUE == normalize))
     {