* 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.
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?
Thanks,
Ingo
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