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Re: mips RDHWR instruction in glibc

To: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Subject: Re: mips RDHWR instruction in glibc
From: Daniel Jacobowitz <dan@debian.org>
Date: Wed, 14 Jun 2006 12:50:40 -0400
Cc: libc-ports@sourceware.org, linux-mips@linux-mips.org
In-reply-to: <20060615.001238.65193088.anemo@mba.ocn.ne.jp>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <20060615.001238.65193088.anemo@mba.ocn.ne.jp>
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User-agent: Mutt/1.5.11+cvs20060403
On Thu, Jun 15, 2006 at 12:12:38AM +0900, Atsushi Nemoto wrote:
> If a system call returned an error, glibc must save the result to
> errno, which is thread-local, so RDHWR used.  I can understand this
> scenario.  But it seems the RDHWR is often called on non-error cases.

Libc uses TLS for many things other than just errno.  The GCC port
knows how to generate the agreed-upon rdhwr instruction directly.

> For example, in the code below, RDHWR is placed _before_ checking the
> error.  I suppose these instructions were reordered by gcc's
> optimization, but the optimization would have large negative effect in
> this case.

You'd have to figure out how to get GCC not to eagerly schedule the
rdhwr.  This might be quite hard.  I don't know much about this part of
the scheduler.

-- 
Daniel Jacobowitz
CodeSourcery

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