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Re: [PATCH]: R10000 Needs LL/SC Workaround in Glibc

To: Kumba <kumba@gentoo.org>
Subject: Re: [PATCH]: R10000 Needs LL/SC Workaround in Glibc
From: Daniel Jacobowitz <dan@debian.org>
Date: Tue, 27 Jan 2009 10:29:24 -0500
Cc: libc-ports@sources.redhat.com, Linux MIPS List <linux-mips@linux-mips.org>
In-reply-to: <4928D912.4050103@gentoo.org>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <490A912A.8030901@gentoo.org> <490C907A.40005@loowit.net> <4928D912.4050103@gentoo.org>
Sender: linux-mips-bounce@linux-mips.org
User-agent: Mutt/1.5.17 (2008-05-11)
On Sat, Nov 22, 2008 at 11:16:18PM -0500, Kumba wrote:
> Here's try #2.  The gcc-side is already sent in and accepted.  If I'm 
> still missing anything, please let me know!
>
> Joshua Kinard
> Gentoo/MIPS
> kumba@gentoo.org
>
>
> 2008-11-22  Joshua Kinard  <kumba@gentoo.org>
>
>         * ports/sysdeps/mips/bits/atomic.h
>       (R10K_BEQZ_INSN, R10K_NOPS_INSN): Define depending on ISA.
>         (__arch_compare_and_exchange_xxx_32_int): Replace 'beqz' insn with
>       R10K_BEQZ_INSN and add R10K_NOPS_INSN.
>         (__arch_compare_and_exchange_xxx_64_int): Likewise
>         (__arch_exchange_xxx_32_int): Likewise
>       (__arch_exchange_xxx_64_int): Likewise
>         (__arch_exchange_and_add_32_int): Likewise
>       (__arch_exchange_and_add_64_int): Likewise

Thinking about this...

MIPS I: 28 NOPs is really horrid.  Not so much on this processor if
the code is all in cache, but I guess that older/simpler processors
are going to sit for a number of cycles chewing through those NOPs.
Are distributions still building MIPS I code?  Can we assume that
people who want to run glibc on an R10K can at least get something
for MIPS II?

MIPS II, MIPS III, MIPS IV: Using beqzl does not seem particularly
horrid - although it's still a shame since this branch is in fact
anti-likely.  It will almost never be taken.

Other platforms: !(MIPS II or MIPS III or MIPS IV) is not the same as
(MIPS I)!  Please don't activate this workaround on builds that won't
run on an R10K, like MIPS32.

-- 
Daniel Jacobowitz
CodeSourcery

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