Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932801Ab2KVVyh (ORCPT ); Thu, 22 Nov 2012 16:54:37 -0500 Received: from mail.mnsspb.ru ([84.204.75.2]:53856 "EHLO mail.mnsspb.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932728Ab2KVVya (ORCPT ); Thu, 22 Nov 2012 16:54:30 -0500 Date: Thu, 22 Nov 2012 12:10:41 +0400 From: Kirill Smelkov To: Linus Torvalds Cc: linux-kernel@vger.kernel.org Subject: Re: [PATCH] Tell the world we gave up on pushing CC_OPTIMIZE_FOR_SIZE Message-ID: <20121122081041.GA380@tugrik.mns.mnsspb.ru> References: <1351856461-3662-1-git-send-email-kirr@mns.spb.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1351856461-3662-1-git-send-email-kirr@mns.spb.ru> Organization: Marine Bridge & Navigation Systems User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4590 Lines: 124 On Fri, Nov 02, 2012 at 03:41:01PM +0400, Kirill Smelkov wrote: > [continuing 281dc5c5 "Give up on pushing CC_OPTIMIZE_FOR_SIZE"] > > Recently I've been beaten hard by CC_OPTIMIZE_FOR_SIZE=y on X86 > performance-wise. The problem turned out to be for -Os gcc wants to > inline __builtin_memcpy, to which x86 memcpy directly refers, > > ---- 8< ---- arch/x86/include/asm/string_32.h > #if (__GNUC__ >= 4) > #define memcpy(t, f, n) __builtin_memcpy(t, f, n) > > to "rep; movsb" which is several times slower compared to "rep; movsl". > > For me this turned out in vivi driver, where memcpy is used to copy > lines with colorbars, and this is one of the most significant parts of > the workload: > > ---- 8< ---- drivers/media/platform/vivi.c > static void vivi_fillbuff(struct vivi_dev *dev, struct vivi_buffer *buf) > { > ... > > for (h = 0; h < hmax; h++) > memcpy(vbuf + h * wmax * dev->pixelsize, > dev->line + (dev->mv_count % wmax) * dev->pixelsize, > wmax * dev->pixelsize); > > Gcc insists on using movb, even if it knows dest and src alignment. For > example with gcc-4.4, -4.7 and yesterday's gcc trunk, for following function > > ---- 8< ---- > void doit(unsigned long *dst, unsigned long *src, unsigned n) > { > void *__d = __builtin_assume_aligned(dst, 4); > void *__s = __builtin_assume_aligned(src, 4); > > __builtin_memcpy(__d, __s, n); > } > > it still wants to use movsb with -Os: > > 00000000 : > 0: 55 push %ebp > 1: 89 e5 mov %esp,%ebp > 3: 57 push %edi > 4: 8b 4d 10 mov 0x10(%ebp),%ecx > 7: 56 push %esi > 8: 8b 7d 08 mov 0x8(%ebp),%edi > b: 8b 75 0c mov 0xc(%ebp),%esi > e: f3 a4 rep movsb %ds:(%esi),%es:(%edi) > 10: 5e pop %esi > 11: 5f pop %edi > 12: 5d pop %ebp > 13: c3 ret > > and even if I change "n" to "4*n"... > > On the other hand, with -O2, it generates call to memcpy, which at least > has rep; movsl inside it, and things works several times faster. > > So tell people to not enable CC_OPTIMIZE_FOR_SIZE by default. > > Signed-off-by: Kirill Smelkov > --- > init/Kconfig | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/init/Kconfig b/init/Kconfig > index 6fdd6e3..6a448d5 100644 > --- a/init/Kconfig > +++ b/init/Kconfig > @@ -1119,7 +1119,7 @@ config CC_OPTIMIZE_FOR_SIZE > Enabling this option will pass "-Os" instead of "-O2" to gcc > resulting in a smaller kernel. > > - If unsure, say Y. > + If unsure, say N. > > config SYSCTL > bool 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 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 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/