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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/103393] [12 Regression] Generating 256bit register
 usage with -mprefer-avx128 -mprefer-vector-width=128
Date: Fri, 26 Nov 2021 11:37:13 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103393

--- Comment #16 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Richard Earnshaw from comment #15)
> It seems perverse to me that you have a standard named pattern in the x86
> backend that is enabled, but then you somehow expect the generic parts of
> the compiler to know that it shouldn't be used.

They should be used, but only if the user code asks for it explicitly.
So, say a 32-byte generic vector in user code, or the <x86intrin.h> intrins=
ics
that need 32-byte vectors are just fine.
The option just asks that the compiler tries hard not to introduce those on=
 its
own (e.g. vectorization but this string ops expansion is similar to that).

With those selected ISAs, such instructions are available, but on some CPUs=
 use
of those is not really performance beneficial and using smaller vectors mig=
ht
get better results.=