From: Linus Torvalds <torvalds@linux-foundation.org>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Juri Lelli <juri.lelli@gmail.com>, Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [RFC][PATCH 2/3] math128: Introduce {mult,add,cmp}_u128
Date: Tue, 24 Apr 2012 18:46:14 -0700 [thread overview]
Message-ID: <CA+55aFx_efN7A2H2VuRGZiyh2xcV=p6uoDLcVAW2JOqmJJNd1Q@mail.gmail.com> (raw)
In-Reply-To: <1335304497.28150.243.camel@twins>
On Tue, Apr 24, 2012 at 2:54 PM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
>
> that does generate slightly better code in that it avoids some masks on
> 64bit:
>
> @@ -7,12 +7,11 @@
> .LFB38:
> .cfi_startproc
> movq %rdi, %r8
> - movq %rdi, %rdx
> movq %rsi, %rcx
> + mov %edi, %edx
> shrq $32, %r8
> - andl $4294967295, %edx
> shrq $32, %rcx
> - andl $4294967295, %esi
> + mov %esi, %esi
Oh christ.
What insane version of gcc is that? Can you please make a gcc bug-report?
Because a compiler that generates an instruction sequence like
movq %rdi,%rsi
andl $4294967295, %esi
is just so fricking stupid that it's outright buggy. That's just
crazy. It's demented. It's an "and" with all bits set.
But yeah, I do think that in general using a cast to 32-bit instead of
a mask to 32-bit is easier for the compiler. Although that still is a
particularly stupid code sequence to use.
Linus
next prev parent reply other threads:[~2012-04-25 1:46 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-24 16:10 [RFC][PATCH 0/3] gcc work-around and math128 Peter Zijlstra
2012-04-24 16:10 ` [RFC][PATCH 1/3] kernel,sched,time: Clean up gcc work-arounds Peter Zijlstra
2012-04-24 16:10 ` Peter Zijlstra
2012-04-24 19:39 ` Linus Torvalds
2012-04-25 0:14 ` Stephen Rothwell
2012-04-25 0:14 ` Stephen Rothwell
2012-04-24 16:10 ` [RFC][PATCH 2/3] math128: Introduce {mult,add,cmp}_u128 Peter Zijlstra
2012-04-24 16:10 ` Peter Zijlstra
2012-04-24 19:37 ` Linus Torvalds
2012-04-24 19:43 ` Peter Zijlstra
2012-04-24 19:43 ` Peter Zijlstra
2012-04-24 20:12 ` Måns Rullgård
2012-04-24 20:12 ` Måns Rullgård
2012-04-24 21:54 ` Peter Zijlstra
2012-04-25 0:09 ` H. Peter Anvin
2012-04-25 1:46 ` Linus Torvalds [this message]
2012-04-25 1:46 ` Linus Torvalds
2012-04-25 8:35 ` Peter Zijlstra
2012-04-25 10:13 ` Peter Zijlstra
2012-04-25 0:23 ` Stephen Rothwell
2012-04-25 8:11 ` Peter Zijlstra
2012-04-25 11:23 ` Geert Uytterhoeven
2012-04-25 11:58 ` Peter Zijlstra
2012-04-25 14:35 ` Geert Uytterhoeven
2012-04-25 15:09 ` Peter Zijlstra
2012-04-24 16:10 ` [RFC][PATCH 3/3] math128, x86_64: Implement {mult,add}_u128 in 64bit asm Peter Zijlstra
2012-04-24 16:10 ` Peter Zijlstra
2012-04-24 16:34 ` H. Peter Anvin
2012-04-24 16:36 ` Peter Zijlstra
2012-04-24 17:17 ` H. Peter Anvin
2012-04-24 17:19 ` Peter Zijlstra
2012-04-24 17:20 ` H. Peter Anvin
2012-04-24 22:00 ` Peter Zijlstra
2012-04-24 22:03 ` H. Peter Anvin
2012-04-24 17:22 ` [RFC][PATCH 0/3] gcc work-around and math128 H. Peter Anvin
2012-04-24 17:27 ` H. Peter Anvin
2012-04-24 21:15 ` Andy Lutomirski
2012-04-24 21:18 ` Linus Torvalds
2012-04-24 21:32 ` Peter Zijlstra
2012-04-24 21:35 ` Andy Lutomirski
2012-04-24 21:35 ` Andy Lutomirski
2012-04-24 21:51 ` Peter Zijlstra
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CA+55aFx_efN7A2H2VuRGZiyh2xcV=p6uoDLcVAW2JOqmJJNd1Q@mail.gmail.com' \
--to=torvalds@linux-foundation.org \
--cc=a.p.zijlstra@chello.nl \
--cc=akpm@linux-foundation.org \
--cc=juri.lelli@gmail.com \
--cc=linux-arch@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@kernel.org \
--cc=tglx@linutronix.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).