On Mon, Jul 27, 2015 at 5:54 PM, Ingo Molnar <[email protected]> wrote: > > * Uros Bizjak <[email protected]> wrote: > >> On Mon, Jul 27, 2015 at 5:40 PM, Ingo Molnar <[email protected]> wrote: >> > >> > * Uros Bizjak <[email protected]> wrote: >> > >> >> From: Uros Bizjak <[email protected]> >> >> >> >> 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 <[email protected]> >> >> --- >> >> 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 Uros. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
