[committed][PR rtl-optimization/120550] Drop REG_EQUAL note after ext-dce transformation

This bug was found by Edwin's fuzzing efforts on RISC-V, though it likely
affects other targets.

In simplest terms when ext-dce converts an extension into a (possibly
simplified) subreg copy it may make an attached REG_EQUAL note invalid.

In the case Edwin found the note was an extension, but I don't think that would
necessarily always be the case.  The note could have other forms which
potentially need invalidation.  So the safest thing to do is just remove any
attached REG_EQUAL or REG_EQUIV note.

Note adjusting Edwin's testcase in the obvious way to avoid having to interpret
printf output for pass/fail status makes the bug go latent.  That's why no
testcase is included with this patch.

Bootstrapped and regression tested on x86_64.  Obviously also verified it fixes
the testcase Edwin filed.

This is a good candidate for cherry-picking to the gcc-15 release branch after
simmering on the trunk a bit.

	PR rtl-optimization/120550
gcc/
	* ext-dce.cc (ext_dce_try_optimize_insn): Drop REG_EQUAL/REG_EQUIV
	notes on modified insns.

diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc
index aa80c0483d610cf76969cc5131a4e2051b4ab80f..b1d5ee4b36c44564067bd545c046bb4a6f8a04bd 100644 (file)
--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -442,6 +442,11 @@ ext_dce_try_optimize_insn (rtx_insn *insn, rtx set)
          print_rtl_single (dump_file, new_pattern);
          fprintf (dump_file, "\n");
        }
+
+      /* INSN may have a REG_EQUAL note indicating that the value was
+        sign or zero extended.  That note is no longer valid since we've
+        just removed the extension.  Just wipe the notes.  */
+      remove_reg_equal_equiv_notes (insn, true);
     }
   else
     {