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From: Richard Sandiford <richard.sandiford@arm.com>
To: Jakub Jelinek <jakub@redhat.com>
Mail-Followup-To: Jakub Jelinek <jakub@redhat.com>,
 Uros Bizjak <ubizjak@gmail.com>, Hongtao Liu <crazylht@gmail.com>,
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Cc: Uros Bizjak <ubizjak@gmail.com>, Hongtao Liu <crazylht@gmail.com>,
 GCC Patches <gcc-patches@gcc.gnu.org>, "H. J. Lu" <hjl.tools@gmail.com>
Subject: Re: [PATCH] [i386] Fix _mm256_zeroupper to notify LRA that vzeroupper
 will kill sse registers. [PR target/82735]
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Date: Thu, 13 May 2021 12:32:26 +0100
In-Reply-To: <20210513095433.GH1179226@tucnak> (Jakub Jelinek's message of
 "Thu, 13 May 2021 11:54:33 +0200")
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Jakub Jelinek <jakub@redhat.com> writes:
> On Thu, May 13, 2021 at 11:43:19AM +0200, Uros Bizjak wrote:
>> > >   Bootstrapped and regtested on X86_64-linux-gnu{-m32,}
>> > >   Ok for trunk?
>> >
>> > Some time ago a support for CLOBBER_HIGH RTX was added (and later
>> > removed for some reason). Perhaps we could resurrect the patch for the
>> > purpose of ferrying 128bit modes via vzeroupper RTX?
>> 
>> https://gcc.gnu.org/legacy-ml/gcc-patches/2017-11/msg01325.html
>
> https://gcc.gnu.org/legacy-ml/gcc-patches/2019-09/msg01468.html
> is where it got removed, CCing Richard.

Yeah.  Initially clobber_high seemed like the best appraoch for
handling the tlsdesc thing, but in practice it was too difficult
to shoe-horn the concept in after the fact, when so much rtl
infrastructure wasn't prepared to deal with it.  The old support
didn't handle all cases and passes correctly, and handled others
suboptimally.

I think it would be worth using the same approach as
https://gcc.gnu.org/legacy-ml/gcc-patches/2019-09/msg01466.html for
vzeroupper: represent the instructions as call_insns in which the
call has a special vzeroupper ABI.  I think that's likely to lead
to better code than clobber_high would (or at least, it did for tlsdesc).

Thanks,
Richard