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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/99434] std::bit_cast generates more instructions than __builtin_bit_cast and memcpy with -march=native
Date: Sat, 06 Mar 2021 22:50:19 +0000	[thread overview]
Message-ID: <bug-99434-4-wZqlP7x4KR@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-99434-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99434

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to cqwrteur from comment #2)
> (In reply to Andrew Pinski from comment #1)
> > This is just a register allocation issue dealing with mulx and TImode.
> > 
> > If mulq was used instead (that is without -march=native), all of the
> > functions are done correctly.
> 
> I do not think so. I think GCC generally did things like this wrong. I have
> even found out how to produce different wrong results deterministically.
> 
> For example like this
> https://godbolt.org/z/PbobYG
> 
> Any time it deals with things like >>32 or >>64, it produces a slower result.
> This even compiles without -march=native.

This is still a register allocation issue, this time dealing with DImode on
32bit. GCC has a known issue with register allocation when dealing with values
stored into two registers.  See PR 21150, PR 43644, PR 50339, etc.

  parent reply	other threads:[~2021-03-06 22:50 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-06 20:12 [Bug tree-optimization/99434] New: " unlvsur at live dot com
2021-03-06 21:25 ` [Bug target/99434] " pinskia at gcc dot gnu.org
2021-03-06 21:36 ` unlvsur at live dot com
2021-03-06 22:50 ` pinskia at gcc dot gnu.org [this message]
2021-03-08 16:49 ` jakub at gcc dot gnu.org
2021-03-08 16:50 ` jakub at gcc dot gnu.org
2021-03-08 20:40 ` unlvsur at live dot com

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