Re: [PATCH v3 0/7] bitops: let optimize out non-atomic bitops on compile-time constants

Alexander Lobakin
2022-06-21T18:51:41Z

From: Yury Norov <yury.norov@gmail.com>
Date: Tue, 21 Jun 2022 10:39:44 -0700

> On Mon, Jun 20, 2022 at 05:08:55PM +0200, Alexander Lobakin wrote:
> > From: Mark Rutland <mark.rutland@arm.com>
> > Date: Mon, 20 Jun 2022 15:19:42 +0100
> >
> > > On Fri, Jun 17, 2022 at 04:40:24PM +0200, Alexander Lobakin wrote:
> > > > So, in order to let the compiler optimize out such cases, expand the
> > > > test_bit() and __*_bit() definitions with a compile-time condition
> > > > check, so that they will pick the generic C non-atomic bitop
> > > > implementations when all of the arguments passed are compile-time
> > > > constants, which means that the result will be a compile-time
> > > > constant as well and the compiler will produce more efficient and
> > > > simple code in 100% cases (no changes when there's at least one
> > > > non-compile-time-constant argument).
> > >
> > > > The savings are architecture, compiler and compiler flags dependent,
> > > > for example, on x86_64 -O2:
> > > >
> > > > GCC 12: add/remove: 78/29 grow/shrink: 332/525 up/down: 31325/-61560 (-30235)
> > > > LLVM 13: add/remove: 79/76 grow/shrink: 184/537 up/down: 55076/-141892 (-86816)
> > > > LLVM 14: add/remove: 10/3 grow/shrink: 93/138 up/down: 3705/-6992 (-3287)
> > > >
> > > > and ARM64 (courtesy of Mark[0]):
> > > >
> > > > GCC 11: add/remove: 92/29 grow/shrink: 933/2766 up/down: 39340/-82580 (-43240)
> > > > LLVM 14: add/remove: 21/11 grow/shrink: 620/651 up/down: 12060/-15824 (-3764)
> > >
> > > Hmm... with *this version* of the series, I'm not getting results nearly as
> > > good as that when building defconfig atop v5.19-rc3:
> > >
> > > GCC 8.5.0: add/remove: 83/49 grow/shrink: 973/1147 up/down: 32020/-47824 (-15804)
> > > GCC 9.3.0: add/remove: 68/51 grow/shrink: 1167/592 up/down: 30720/-31352 (-632)
> > > GCC 10.3.0: add/remove: 84/37 grow/shrink: 1711/1003 up/down: 45392/-41844 (3548)
> > > GCC 11.1.0: add/remove: 88/31 grow/shrink: 1635/963 up/down: 51540/-46096 (5444)
> > > GCC 11.3.0: add/remove: 89/32 grow/shrink: 1629/966 up/down: 51456/-46056 (5400)
> > > GCC 12.1.0: add/remove: 84/31 grow/shrink: 1540/829 up/down: 48772/-43164 (5608)
> > >
> > > LLVM 12.0.1: add/remove: 118/58 grow/shrink: 437/381 up/down: 45312/-65668 (-20356)
> > > LLVM 13.0.1: add/remove: 35/19 grow/shrink: 416/243 up/down: 14408/-22200 (-7792)
> > > LLVM 14.0.0: add/remove: 42/16 grow/shrink: 415/234 up/down: 15296/-21008 (-5712)
> > >
> > > ... and that now seems to be regressing codegen with recent versions of GCC as
> > > much as it improves it LLVM.
> > >
> > > I'm not sure if we've improved some other code and removed the benefit between
> > > v5.19-rc1 and v5.19-rc3, or whether something else it at play, but this doesn't
> > > look as compelling as it did.
> >
> > Mostly likely it's due to that in v1 I mistakingly removed
> > `volatile` from gen[eric]_test_bit(), so there was an impact for
> > non-constant cases as well.
> > +5 Kb sounds bad tho. Do you have CONFIG_TEST_BITMAP enabled, does
> > it work? Probably the same reason as for m68k, more constant
> > optimization -> more aggressive inlining or inlining rebalance ->
> > larger code. OTOH I've no idea why sometimes compiler decides to
> > uninline really tiny functions where due to this patch series some
> > bitops have been converted to constants, like it goes on m68k.
> >
> > >
> > > Overall that's mostly hidden in the Image size, due to 64K alignment and
> > > padding requirements:
> > >
> > > Toolchain Before After Difference
> > >
> > > GCC 8.5.0 36178432 36178432 0
> > > GCC 9.3.0 36112896 36112896 0
> > > GCC 10.3.0 36442624 36377088 -65536
> > > GCC 11.1.0 36311552 36377088 +65536
> > > GCC 11.3.0 36311552 36311552 0
> > > GCC 12.1.0 36377088 36377088 0
> > >
> > > LLVM 12.0.1 31418880 31418880 0
> > > LLVM 13.0.1 31418880 31418880 0
> > > LLVM 14.0.0 31218176 31218176 0
> > >
> > > ... so aside from the blip around GCC 10.3.0 and 11.1.0, there's not a massive
> > > change overall (due to 64KiB alignment restrictions for portions of the kernel
> > > Image).
>
> I gave it a try on v5.19-rc3 for arm64 with my default GCC 11.2, and it's:
> add/remove: 89/33 grow/shrink: 1629/966 up/down: 51456/-46064 (5392)
>
> Which is not great in terms of layout size. But I don't think we should
> focus too much on those numbers. The goal of the series is not to shrink
> the image; the true goal is to provide more information to the compiler
> in a hope that it will make a better decision regarding optimizations.
>
> Looking at results provided by Mark, both GCC and LLVM have a tendency
> to inline and use other techniques that increase the image more
> aggressively in newer releases, comparing to old ones. From this
> perspective, unless we find some terribly wrong behavior, I'm OK with
> +5K for the Image, because I trust my compiler and believe it spent
> those 5K wisely.
>
> For the reasons said above, I think we shouldn't disable const
> bitops for -Os build.
>
> I think this series has total positive impact because it adds a lot
> of information for compiler with just a few lines of code.

Right, that was the primary intention. But then I got some text size
decreases and thought this applies to any setup :)

>
> If no objections, I think it's good to try it in -next. Alexander,
> would you like me to fix gen/generic typo in comment and take it in
> bitmap-for-next, or you'd prefer to send v4?

I'm sending v4 in a couple minutes, lkp reported that on ARC GCC
never expands mem*() builtins to plain assignments, which sucks,
but failed my compile-time tests, so I adjusted code a bit. Hope
that change will be okay for everyone, so that you could pick it.

>
> Thanks,
> Yury

Thanks,
Olek