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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/112572] [14 regression] LLVM miscompiled since
 r14-5355-g3cd3a09b3f91a1
Date: Sat, 18 Nov 2023 22:22:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112572

--- Comment #21 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So originally we had in vzeroupper:
```
(insn # # # 2 (set (reg/f:DI 43 r15 [orig:126 _84 ] [126])
        (reg/f:DI 2 cx [orig:126 _84 ] [126]))
"/home/sam/git/llvm-project/llvm/include/llvm/ADT/SmallVector.h":272:42 dis=
crim
2# {*movdi_internal}
     (expr_list:REG_UNUSED (reg/f:DI 43 r15 [orig:126 _84 ] [126])
        (nil)))
(insn # # # 2 (set (reg/f:DI 43 r15 [orig:126 _84 ] [126])
        (reg/f:DI 2 cx [orig:126 _84 ] [126]))
"/tmp/build/include/c++/14.0.0/bits/stl_algo.h":1897:7# {*movdi_internal}
     (expr_list:REG_DEAD (reg/f:DI 2 cx [orig:126 _84 ] [126])
        (nil)))
```
And then in postreload we changed it to just:
```
(insn # # # 2 (set (reg/f:DI 43 r15 [orig:126 _84 ] [126])
        (reg/f:DI 2 cx [orig:126 _84 ] [126]))
"/home/sam/git/llvm-project/llvm/include/llvm/ADT/SmallVector.h":272:42 dis=
crim
2# {*movdi_internal}
     (expr_list:REG_UNUSED (reg/f:DI 43 r15 [orig:126 _84 ] [126])
        (nil)))
```
Without updating the REG notes there. I suspect if we moved vzeroupper after
postreload it would also just had worked. reload/LRA looks like it likes to
generate extra moves that do the same thing ...
Moving vzeroupper after postreload will also reduce memory and compile time=
 due
to less REG_DEAD/REG_UNUSED being generated ...=