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Date: Mon, 27 Jul 2015 18:13:39 +0200 From: Ingo Molnar <mingo@...nel.org> To: Uros Bizjak <ubizjak@...il.com> Cc: Uros Bizjak <uros_bizjak1@....net>, linux-kernel@...r.kernel.org, "x86@...nel.org" <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com> Subject: Re: [PATCH] x86: Introduce ASM flags to bitops * Uros Bizjak <ubizjak@...il.com> wrote: > On Mon, Jul 27, 2015 at 5:54 PM, Ingo Molnar <mingo@...nel.org> wrote: > > > > * Uros Bizjak <ubizjak@...il.com> wrote: > > > >> On Mon, Jul 27, 2015 at 5:40 PM, Ingo Molnar <mingo@...nel.org> wrote: > >> > > >> > * Uros Bizjak <uros_bizjak1@....net> wrote: > >> > > >> >> From: Uros Bizjak <ubizjak@...il.com> > >> >> > >> >> This patch introduces GCC ASM flags to bitops. Instead of e.g. > >> >> > >> >> 136d7: 48 0f a3 3d 00 00 00 bt %rdi,0x0(%rip) > >> >> 136de: 00 > >> >> 136df: 19 ff sbb %edi,%edi > >> >> 136e1: 85 ff test %edi,%edi > >> >> 136e3: 0f 95 c0 setne %al > >> >> > >> >> following code is generated: > >> >> > >> >> 13767: 48 0f a3 3d 00 00 00 bt %rdi,0x0(%rip) > >> >> 1376e: 00 > >> >> 1376f: 0f 92 c0 setb %al > >> >> > >> >> Similar improvement can be seen in following code: > >> >> > >> >> 7a6c: 48 0f a3 11 bt %rdx,(%rcx) > >> >> 7a70: 19 d2 sbb %edx,%edx > >> >> 7a72: 85 d2 test %edx,%edx > >> >> 7a74: 74 eb je 7a61 > >> >> > >> >> which becomes: > >> >> > >> >> 7a8c: 48 0f a3 11 bt %rdx,(%rcx) > >> >> 7a90: 73 ef jae 7a81 > >> >> > >> >> Signed-off-by: Uros Bizjak <ubizjak@...il.com> > >> >> --- > >> >> arch/x86/include/asm/bitops.h | 26 ++++++++++++++++++++++++-- > >> >> arch/x86/include/asm/percpu.h | 18 +++++++++++++++++- > >> >> arch/x86/include/asm/signal.h | 6 ++++++ > >> >> arch/x86/include/asm/sync_bitops.h | 18 ++++++++++++++++++ > >> >> 4 files changed, 65 insertions(+), 3 deletions(-) > >> > > >> > Nothing in your patch seems to be setting __GCC_ASM_FLAG_OUTPUTS__, and the patch > >> > does not seem to be mailed as part of a larger series ... > >> > > >> > So how is this supposed to work? > >> > >> GCC version 6+ will automatically define __GCC_ASM_FLAG_OUTPUTS__ when > >> this feature is supported. Please see [1] for RFC GCC patch series and > >> [2] for final committed patch. > >> > >> [1] https://gcc.gnu.org/ml/gcc-patches/2015-05/msg00594.html > >> [2] https://gcc.gnu.org/ml/gcc-patches/2015-06/msg02087.html > > > > Ok, great. This information should be part of the changelog and such, as it's not > > obvious. > > No problem, I'll add this information and send a v2 patch. > > > Does the GCC project treat this as an ABI kind of thing, i.e. can the kernel rely > > on it from now on, without the GCC side semantics of this feature not ever > > changing and breaking the kernel? > > Yes. It was discussed and agreed between GCC and kernel people (HPA) > on GCC mailing list. Please see [3]. > > [3] https://gcc.gnu.org/ml/gcc-patches/2015-05/msg00725.html Ok, great! One more request: please make it similar to how we handle other, similar compiler features and introduce an interim macro like: #ifdef __GCC_ASM_FLAG_OUTPUTS__ # define CC_HAVE_ASM_FLAG_OUTPUTS #endif in a suitable spot in include/linux/compiler-gcc.h, and use CC_HAVE_ASM_FLAG_OUTPUTS in the other places. this makes it easier to disable/enable this feature centrally, and it might enable other compilers to introduce a similar feature. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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