From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm1-f68.google.com ([209.85.128.68]:50662 "EHLO mail-wm1-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726167AbfGVV2C (ORCPT ); Mon, 22 Jul 2019 17:28:02 -0400 Date: Tue, 23 Jul 2019 00:27:57 +0300 From: Alexey Dobriyan Subject: Re: [PATCH 2/5] x86_64, -march=native: POPCNT support Message-ID: <20190722212757.GA2525@avx2> References: <20190722202723.13408-1-adobriyan@gmail.com> <20190722202723.13408-2-adobriyan@gmail.com> <20190722211210.GN6698@worktop.programming.kicks-ass.net> <20190722211539.GA29979@avx2> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190722211539.GA29979@avx2> Sender: linux-kbuild-owner@vger.kernel.org List-ID: To: Peter Zijlstra 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 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 > > > --- > > > 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.