On Thu, Nov 22, 2012 at 12:10:41PM +0400, Kirill Smelkov wrote:
> Linus, maybe you missed this. What is the reason telling people
> CC_OPTIMIZE_FOR_SIZE should be Y if we actually do otherwise?
>
>
> commit 281dc5c5ec0fb299514567cbc358562649c1af95
> Author: Linus Torvalds <torva...@linux-foundation.org>
> Date: Sun May 22 14:30:36 2011 -0700
>
> Give up on pushing CC_OPTIMIZE_FOR_SIZE
>
> I still happen to believe that I$ miss costs are a major thing, but
> sadly, -Os doesn't seem to be the solution. With or without it, gcc
> will miss some obvious code size improvements, and with it enabled gcc
> will sometimes make choices that aren't good even with high I$ miss
> ratios.
>
> For example, with -Os, gcc on x86 will turn a 20-byte constant memcpy
> into a "rep movsl". While I sincerely hope that x86 CPU's will some day
> do a good job at that, they certainly don't do it yet, and the cost is
> higher than a L1 I$ miss would be.
>
> Some day I hope we can re-enable this.
>
> Signed-off-by: Linus Torvalds <torva...@linux-foundation.org>
>
> diff --git a/init/Kconfig b/init/Kconfig
> index 4986ecc..ffcdad7 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -908,7 +908,6 @@ endif
>
> config CC_OPTIMIZE_FOR_SIZE
> bool "Optimize for size"
> - default y
> help
> Enabling this option will pass "-Os" instead of "-O2" to gcc
> resulting in a smaller kernel.
FWIW, we gave up using this in Fedora quite a few releases back because we
kept finding workloads that it truly sucked on.
For code that is quite bloaty and not particularly performance critical,
maybe we could move -Os to individual directories.
Dave
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