linux-kernel - Re: [RFC: 2.6 patch] add -fno-tree-scev-cprop to KBUILD

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Date:	Mon, 12 Nov 2007 17:24:18 +0100
From:	Bernd Schmidt <bernds_cb1@...nline.de>
To:	Adrian Bunk <bunk@...nel.org>
CC:	Sam Ravnborg <sam@...nborg.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFC: 2.6 patch] add -fno-tree-scev-cprop to KBUILD_CFLAGS

Adrian Bunk wrote:
> The gcc from svn that will become gcc 4.3 generates libgcc calls in 
> cases like the following (on 32bit architectures):
> 
> <--  snip  -->
> 
> static inline void timespec_add_ns(struct timespec *a, u64 ns)
> {
> ...
>         while(ns >= NSEC_PER_SEC) {
>                 ns -= NSEC_PER_SEC;
>                 a->tv_sec++;
>         }
> ...
> 
> <--  snip  -->
> 
> It can make sense to emit assembler code doing division for such C code -
> that doesn't seem to be something that would generally be wrong.

It can be a pretty huge performance regression, so gcc ought to be fixed.


Bernd
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