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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/104869] [12 Regression] Miscompilation of
 qt5-qtdeclarative since r12-6342-ge7a7dbb5ca5dd696
Date: Thu, 10 Mar 2022 19:32:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104869

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |liuhongt at gcc dot gnu.or=
g,
                   |                            |rsandifo at gcc dot gnu.org
            Summary|[12 Regression]             |[12 Regression]
                   |Miscompilation of           |Miscompilation of
                   |qt5-qtdeclarative           |qt5-qtdeclarative since
                   |                            |r12-6342-ge7a7dbb5ca5dd696

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Bisected to r12-6342-ge7a7dbb5ca5dd69689f1a462ba7620180acfe8b0
Assembly difference is:
--- qv4codegen.s.r12-6341       2022-03-10 20:27:09.392500655 +0100
+++ qv4codegen.s.r12-6342       2022-03-10 20:23:00.256971680 +0100
@@ -52895,9 +52895,11 @@ _ZN3QV48Compiler7Codegen5visitEPN6QQmlJS
        ld 2,24(1)
        mr 9,3
        mr 3,31
-       cmpdi 4,9,0
-       beq 4,.L7388
+       cmpdi 0,9,0
+       beq 0,.L7388
+       li 9,0
        mr 31,4
+       cmpdi 4,9,0
 .L7389:
        ld 3,32(1)
        lwz 9,0(3)
@@ -53428,9 +53430,11 @@ _ZN3QV48Compiler7Codegen5visitEPN6QQmlJS
        ld 2,24(1)
        mr 9,3
        mr 3,31
-       cmpdi 4,9,0
-       beq 4,.L7477
+       cmpdi 0,9,0
+       beq 0,.L7477
+       li 9,0
        mr 31,4
+       cmpdi 4,9,0
 .L7478:
        ld 3,32(1)
        lwz 9,0(3)

That said, I suspect that the r12-6342 change just uncovered a latent fwpro=
p or
rtl-ssa bug, because the commit didn't do anything with propagation across
clobbers.=