LKML Archive on lore.kernel.org
 help / color / Atom feed
From: Peter Zijlstra <peterz@infradead.org>
To: David Laight <David.Laight@aculab.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	"mingo@kernel.org" <mingo@kernel.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kan.liang@linux.intel.com" <kan.liang@linux.intel.com>,
	"acme@kernel.org" <acme@kernel.org>,
	"mark.rutland@arm.com" <mark.rutland@arm.com>,
	"alexander.shishkin@linux.intel.com" 
	<alexander.shishkin@linux.intel.com>,
	"jolsa@redhat.com" <jolsa@redhat.com>,
	"namhyung@kernel.org" <namhyung@kernel.org>,
	"ak@linux.intel.com" <ak@linux.intel.com>,
	"eranian@google.com" <eranian@google.com>
Subject: Re: [PATCH 4/6] perf: Optimize get_recursion_context()
Date: Mon, 9 Nov 2020 13:12:37 +0100
Message-ID: <20201109121237.GJ2594@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <6371740df7704217926315e97294a894@AcuMS.aculab.com>

On Sat, Oct 31, 2020 at 12:11:42PM +0000, David Laight wrote:
> The gcc 7.5.0 I have handy probably generates the best code for:
> 
> unsigned char q_2(unsigned int pc)
> {
>         unsigned char rctx = 0;
> 
>         rctx += !!(pc & (NMI_MASK));
>         rctx += !!(pc & (NMI_MASK | HARDIRQ_MASK));
>         rctx += !!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET));
> 
>         return rctx;
> }
> 
> 0000000000000000 <q_2>:
>    0:   f7 c7 00 00 f0 00       test   $0xf00000,%edi     # clock 0
>    6:   0f 95 c0                setne  %al                # clock 1
>    9:   f7 c7 00 00 ff 00       test   $0xff0000,%edi     # clock 0
>    f:   0f 95 c2                setne  %dl                # clock 1
>   12:   01 c2                   add    %eax,%edx          # clock 2
>   14:   81 e7 00 01 ff 00       and    $0xff0100,%edi
>   1a:   0f 95 c0                setne  %al
>   1d:   01 d0                   add    %edx,%eax          # clock 3
>   1f:   c3                      retq
> 
> I doubt that is beatable.
> 
> I've annotated the register dependency chain.
> Likely to be 3 (or maybe 4) clocks.
> The other versions are a lot worse (7 or 8) without allowing
> for 'sbb' taking 2 clocks on a lot of Intel cpus.

https://godbolt.org/z/EfnG8E

Recent GCC just doesn't want to do that. Still, using u8 makes sense, so
I've kept that.

  parent reply index

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-30 15:13 [PATCH 0/6] perf: Reduce stack usage (and misc bits) Peter Zijlstra
2020-10-30 15:13 ` [PATCH 1/6] perf: Reduce stack usage of perf_output_begin() Peter Zijlstra
2020-11-10 12:45   ` [tip: perf/urgent] " tip-bot2 for Peter Zijlstra
2020-10-30 15:13 ` [PATCH 2/6] perf/x86: Reduce stack usage for x86_pmu::drain_pebs() Peter Zijlstra
2020-11-10 12:45   ` [tip: perf/urgent] " tip-bot2 for Peter Zijlstra
2020-10-30 15:13 ` [PATCH 3/6] perf: Fix get_recursion_context() Peter Zijlstra
2020-11-10 12:45   ` [tip: perf/urgent] " tip-bot2 for Peter Zijlstra
2020-10-30 15:13 ` [PATCH 4/6] perf: Optimize get_recursion_context() Peter Zijlstra
2020-10-30 17:11   ` Jesper Dangaard Brouer
2020-10-30 20:22     ` Steven Rostedt
2020-10-30 22:14       ` Thomas Gleixner
2020-10-30 23:31         ` Steven Rostedt
2020-10-31 11:23           ` Peter Zijlstra
2020-10-30 23:01       ` Peter Zijlstra
2020-10-31 12:11         ` David Laight
2020-10-31 13:18           ` David Laight
2020-11-09 12:12           ` Peter Zijlstra [this message]
2020-11-09 14:14             ` David Laight
2020-11-10 12:45   ` [tip: perf/urgent] " tip-bot2 for Peter Zijlstra
2020-10-30 15:13 ` [PATCH 5/6] perf/arch: Remove perf_sample_data::regs_user_copy Peter Zijlstra
2020-11-10 12:45   ` [tip: perf/urgent] " tip-bot2 for Peter Zijlstra
2020-10-30 15:13 ` [PATCH 6/6] perf/x86: Make dummy_iregs static Peter Zijlstra
2020-11-10 12:45   ` [tip: perf/urgent] " tip-bot2 for Peter Zijlstra

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20201109121237.GJ2594@hirez.programming.kicks-ass.net \
    --to=peterz@infradead.org \
    --cc=David.Laight@aculab.com \
    --cc=acme@kernel.org \
    --cc=ak@linux.intel.com \
    --cc=alexander.shishkin@linux.intel.com \
    --cc=brouer@redhat.com \
    --cc=eranian@google.com \
    --cc=jolsa@redhat.com \
    --cc=kan.liang@linux.intel.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mark.rutland@arm.com \
    --cc=mingo@kernel.org \
    --cc=namhyung@kernel.org \
    --cc=rostedt@goodmis.org \
    --cc=tglx@linutronix.de \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git