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From: Alexey Dobriyan <adobriyan@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
	hpa@zytor.com, linux-kernel@vger.kernel.org, x86@kernel.org,
	linux-kbuild@vger.kernel.org, yamada.masahiro@socionext.com,
	michal.lkml@markovi.net
Subject: Re: [PATCH 2/5] x86_64, -march=native: POPCNT support
Date: Tue, 23 Jul 2019 00:15:39 +0300	[thread overview]
Message-ID: <20190722211539.GA29979@avx2> (raw)
In-Reply-To: <20190722211210.GN6698@worktop.programming.kicks-ass.net>

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.

  reply	other threads:[~2019-07-22 21:15 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-22 20:27 [PATCH 1/5] x86_64: -march=native support Alexey Dobriyan
2019-07-22 20:27 ` [PATCH 2/5] x86_64, -march=native: POPCNT support Alexey Dobriyan
2019-07-22 21:12   ` Peter Zijlstra
2019-07-22 21:15     ` Alexey Dobriyan [this message]
2019-07-22 21:27       ` Alexey Dobriyan
2019-07-23  7:20       ` Peter Zijlstra
2019-07-23 20:04         ` Alexey Dobriyan
2019-07-22 20:27 ` [PATCH 3/5] x86_64, -march=native: REP MOVSB support Alexey Dobriyan
2019-07-22 20:27 ` [PATCH 4/5] x86_64, -march=native: REP STOSB support Alexey Dobriyan
2019-07-22 20:27 ` [PATCH 5/5] x86_64, -march=native: MOVBE support Alexey Dobriyan
  -- strict thread matches above, loose matches on Subject: below --
2019-07-04 20:47 [PATCH 1/5] x86_64: -march=native support Alexey Dobriyan
2019-07-04 20:47 ` [PATCH 2/5] x86_64, -march=native: POPCNT support Alexey Dobriyan

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