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

Yury Norov
2022-06-21T17:39:44Z

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.

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?

Thanks,
Yury