[Bug tree-optimization/127521] [15/16/17 Regression] `ifcombine` incorrectly removes a masked sign-bit condition

[mikpelinux at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127521

--- Comment #5 from Mikael Pettersson <mikpelinux at gmail dot com> ---
Started with r15-6774-g740c84975ceb74:

commit 740c84975ceb7426da656dc7115445872a9e5b6f
Author: Alexandre Oliva <ol...@adacore.com>
Date:   Fri Jan 10 09:32:38 2025 -0300

    [ifcombine] reuse left-hand mask to decode right-hand xor operand

With the test case from https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127521#c1,
the impact of that commit on x86_64 is:

--- pr127521.s.a        2026-09-26 18:24:44.302571185 +0200
+++ pr127521.s.b        2026-09-26 17:12:33.134915069 +0200
@@ -6,15 +6,9 @@
 cmp3:
 .LFB0:
        .cfi_startproc
-       movl    %esi, %edx
-       xorl    %eax, %eax
-       xorw    %di, %dx
-       js      .L1
-       andw    $32767, %si
        xorl    %eax, %eax
        cmpw    %di, %si
        sete    %al
-.L1:
        ret
        .cfi_endproc
 .LFE0: