linux-kernel - RE: [PATCH 4/6] perf: Optimize get_recursion

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Date:   Mon, 9 Nov 2020 14:14:43 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>
CC:     Steven Rostedt <rostedt@...dmis.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kan.liang@...ux.intel.com" <kan.liang@...ux.intel.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "eranian@...gle.com" <eranian@...gle.com>
Subject: RE: [PATCH 4/6] perf: Optimize get_recursion_context()



> -----Original Message-----
> From: Peter Zijlstra <peterz@...radead.org>
> Sent: 09 November 2020 12:13
> To: David Laight <David.Laight@...LAB.COM>
> Cc: Steven Rostedt <rostedt@...dmis.org>; Jesper Dangaard Brouer <brouer@...hat.com>;
> mingo@...nel.org; tglx@...utronix.de; linux-kernel@...r.kernel.org; kan.liang@...ux.intel.com;
> acme@...nel.org; mark.rutland@....com; alexander.shishkin@...ux.intel.com; jolsa@...hat.com;
> namhyung@...nel.org; ak@...ux.intel.com; eranian@...gle.com
> Subject: Re: [PATCH 4/6] perf: Optimize get_recursion_context()
> 
> 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.

u8 helps x86 because its 'setne' only affects the low 8 bits.
I guess that seemed a good idea when it was added (386).
It doesn't seem to make the other architectures much worse.

gcc 10.x can be persuaded to generate the above code.

https://godbolt.org/z/6GoT94

It sometimes seems to me that every new version of gcc is
larger, slower and generates worse code than the previous one.

	David

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