[Bug rtl-optimization/116136] New: [15 Regression] ext-dce exposes latent subreg simplification bug on m68k

[law at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116136

            Bug ID: 116136
           Summary: [15 Regression] ext-dce exposes latent subreg
                    simplification bug on m68k
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: law at gcc dot gnu.org
  Target Milestone: ---

ext-dce is exposing a latent bug in SUBREG simplification.

Recent changes in ext-dce are exhibiting regressions in the testsuite:

Tests that now fail, but worked before (3 tests):

gcc: gcc.c-torture/execute/960416-1.c   -O2  execution test
gcc: gcc.c-torture/execute/960416-1.c   -O2 -flto -fno-use-linker-plugin
-flto-partition=none  execution test
gcc: gcc.c-torture/execute/960416-1.c   -Os  execution test


Short thread on the underlying issue:

https://gcc.gnu.org/pipermail/gcc-patches/2024-July/658302.html

A patch to fix this previously latent bug is undergoing testing.