linux-kernel - Re: [RFC][PATCH 2/3] math128: Introduce {mult,add,cmp}

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Date:	Tue, 24 Apr 2012 18:46:14 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Juri Lelli <juri.lelli@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC][PATCH 2/3] math128: Introduce {mult,add,cmp}_u128

On Tue, Apr 24, 2012 at 2:54 PM, Peter Zijlstra <a.p.zijlstra@...llo.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
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