linux-kernel - Re: [PATCH 0/6] Clean up ticketlock implementation

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Date:	Tue, 25 Jan 2011 12:08:50 +1100
From:	Nick Piggin <npiggin@...il.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 0/6] Clean up ticketlock implementation

On Tue, Jan 25, 2011 at 10:41 AM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
>
> Hi all,
>
> This series cleans up the x86 ticketlock implementation by converting
> a large proportion of it to C.  This eliminates the need for having
> separate implementations for "large" (NR_CPUS >= 256) and "small"
> (NR_CPUS < 256) ticket locks.
>
> This also lays the groundwork for future changes to the ticketlock
> implementation.
>
> Of course, the big question when converting from assembler to C is
> what the compiler will do to the code.  In general, the results are
> very similar.
>
> For example, the original hand-coded small-ticket ticket_lock is:
>      movl   $256, %eax
>      lock xadd %ax,(%rdi)
>   1: cmp    %ah,%al
>      je     2f
>      pause
>      mov    (%rdi),%al
>      jmp    1b
>   2:
>
> The C version, compiled by gcc 4.5.1 is:
>        movl   $256, %eax
>        lock; xaddw %ax, (%rdi)
>        movzbl  %ah, %edx
> .L3:    cmpb    %dl, %al
>        je      .L2
>        rep; nop
>        movb    (%rdi), %al     # lock_1(D)->D.5949.tickets.head, inc$head
>        jmp     .L3     #
> .L2:
>
> So very similar, except the compiler misses directly comparing
> %ah to %al.

Oh :(

Have you filed a bug with gcc?
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