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.
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