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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/110206] [14 Regression] wrong code with -Os
 -march=cascadelake since r14-1246
Date: Mon, 10 Jul 2023 12:37:51 +0000
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X-Bugzilla-Component: rtl-optimization
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X-Bugzilla-Keywords: patch, wrong-code
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110206

--- Comment #14 from Uro=C5=A1 Bizjak <ubizjak at gmail dot com> ---
(In reply to Uro=C5=A1 Bizjak from comment #10)
> (In reply to Uro=C5=A1 Bizjak from comment #9)
> > and simplify_replace_rtx simplifies the above to:
> >=20
> > (gdb) p debug_rtx (src)
> > (const_vector:V8HI [
> >         (const_int 204 [0xcc]) repeated x8
> >     ])
>=20
> Patched compiler simplifies to:
>=20
> (gdb) p debug_rtx (src)
> (const_vector:V8HI [
>         (const_int 204 [0xcc]) repeated x4
>         (const_int 0 [0]) repeated x4
>     ])

The patched compiler puts the above in REG_EQUAL note. While the value is "=
more
correct", I don't think the compiler has the right to set REG_EQUAL note wh=
en
the top 4 bytes are actually undefined (as a result of an operation with an
undefined input, which is the case with paradoxical subreg).=