lkml.org 
[lkml]   [2019]   [Jul]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 2/5] x86_64, -march=native: POPCNT support
On Tue, Jul 23, 2019 at 12:15:39AM +0300, Alexey Dobriyan wrote:
> On Mon, Jul 22, 2019 at 11:12:10PM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 22, 2019 at 11:27:20PM +0300, Alexey Dobriyan wrote:
> > > Detect POPCNT instruction support and inline hweigth*() functions
> > > if it is supported by CPU.
> > >
> > > Detect POPCNT at boot time and conditionally refuse to boot.
> > >
> > > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> > > ---
> > > arch/x86/include/asm/arch_hweight.h | 24 +++++++++++++++++++
> > > arch/x86/include/asm/segment.h | 1 +
> > > arch/x86/kernel/verify_cpu.S | 8 +++++++
> > > arch/x86/lib/Makefile | 5 +++-
> > > .../drm/i915/display/intel_display_power.c | 2 +-
> > > drivers/misc/sgi-gru/grumain.c | 2 +-
> > > fs/btrfs/tree-checker.c | 4 ++--
> > > include/linux/bitops.h | 2 ++
> > > lib/Makefile | 2 ++
> > > scripts/kconfig/cpuid.c | 7 ++++++
> > > scripts/march-native.sh | 2 ++
> > > 11 files changed, 54 insertions(+), 5 deletions(-)
> >
> > *WHY* ?
> >
> > AFAICT this just adds lines and complexity and wins aboslutely nothing.
>
> If CPU is know to have POPCNT, it doesn't make sense to go through RDI.
> Additionally some CPUs (still?) have fake dependency on the destination,
> so "popcnt rax, rdi" is suboptimal.

More general argument is that if -march=native is accepted, compiler will
generate new instructions which will throw #UD on CPUs which aren't
capable, so it doesn't make sense to _not_ go deeper and use all the
knowledge about current CPU.

\
 
 \ /
  Last update: 2019-07-22 23:28    [W:0.056 / U:0.296 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site