(+CC: Arnd, Changbin Du)
If -O1 support does not require additional effort,
I have no objection to this patch.
(but I do not have enough insight about
the compiler's inlining heuristic)
BTW, when we attempted to add the -Og support,
we fixed various parts, and Linus rejected it.
https://lore.kernel.org/linux-kbuild/CAK7LNARQggM3aKEPRKJqa4tunFAfmfErMZuS-rrnRv6UB1VpPQ@mail.gmail.com/
On Sat, May 21, 2022 at 6:04 PM Christophe JAILLET
<[email protected]> wrote:
>
> Add a new compilation option which speeds-up compilation time.
> This can be useful when using static checker such as smatch or build-bots.
> In such cases, the speed and quality of the generated code is not
> important.
>
> Using -O0 would be even better, but unfortunately, building fails with
> this option.
>
> Signed-off-by: Christophe JAILLET <[email protected]>
> ---
> Makefile | 5 ++++-
> init/Kconfig | 8 ++++++++
> 2 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/Makefile b/Makefile
> index 1f8bef92868f..14467386f947 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -817,7 +817,10 @@ KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation)
> KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow)
> KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
>
> -ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
> +ifdef CONFIG_CC_OPTIMIZE_FOR_COMPILATION_SPEED
> +KBUILD_CFLAGS += -O1
> +KBUILD_RUSTFLAGS_OPT_LEVEL_MAP := 1
> +else ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
> KBUILD_CFLAGS += -O2
> KBUILD_RUSTFLAGS_OPT_LEVEL_MAP := 2
> else ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3
> diff --git a/init/Kconfig b/init/Kconfig
> index a96776a9b080..3177a1830c9a 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1384,6 +1384,14 @@ choice
> prompt "Compiler optimization level"
> default CC_OPTIMIZE_FOR_PERFORMANCE
>
> +config CC_OPTIMIZE_FOR_COMPILATION_SPEED
> + bool "Optimize for compilation speed (-O1)"
> + help
> + This option can be useful when running a static checker such as smatch
> + or a build-bot.
> + Compilation time is slighly faster than -O2 and it requires less
> + memory.
> +
> config CC_OPTIMIZE_FOR_PERFORMANCE
> bool "Optimize for performance (-O2)"
> help
> --
> 2.34.1
>
--
Best Regards
Masahiro Yamada
On Sun, May 22, 2022 at 12:38:17AM +0900, Masahiro Yamada wrote:
> (+CC: Arnd, Changbin Du)
>
>
> If -O1 support does not require additional effort,
> I have no objection to this patch.
>
> (but I do not have enough insight about
> the compiler's inlining heuristic)
>
>
>
> BTW, when we attempted to add the -Og support,
> we fixed various parts, and Linus rejected it.
>
> https://lore.kernel.org/linux-kbuild/CAK7LNARQggM3aKEPRKJqa4tunFAfmfErMZuS-rrnRv6UB1VpPQ@mail.gmail.com/
>
>
>
>
>
>
>
I am afraid that '-O1' has the same situation with '-Og'. As described in GCC
mannual:
Like -O0, -Og completely disables a number of optimization passes so that
individual options controlling them have no effect. Otherwise -Og enables all
-O1 optimization flags except for those that may interfere with debugging:
-fbranch-count-reg -fdelayed-branch
-fdse -fif-conversion -fif-conversion2
-finline-functions-called-once
-fmove-loop-invariants -fmove-loop-stores -fssa-phiopt
-ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra
> On Sat, May 21, 2022 at 6:04 PM Christophe JAILLET
> <[email protected]> wrote:
> >
> > Add a new compilation option which speeds-up compilation time.
> > This can be useful when using static checker such as smatch or build-bots.
> > In such cases, the speed and quality of the generated code is not
> > important.
> >
> > Using -O0 would be even better, but unfortunately, building fails with
> > this option.
> >
> > Signed-off-by: Christophe JAILLET <[email protected]>
> > ---
> > Makefile | 5 ++++-
> > init/Kconfig | 8 ++++++++
> > 2 files changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/Makefile b/Makefile
> > index 1f8bef92868f..14467386f947 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -817,7 +817,10 @@ KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation)
> > KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow)
> > KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
> >
> > -ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
> > +ifdef CONFIG_CC_OPTIMIZE_FOR_COMPILATION_SPEED
> > +KBUILD_CFLAGS += -O1
> > +KBUILD_RUSTFLAGS_OPT_LEVEL_MAP := 1
> > +else ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
> > KBUILD_CFLAGS += -O2
> > KBUILD_RUSTFLAGS_OPT_LEVEL_MAP := 2
> > else ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3
> > diff --git a/init/Kconfig b/init/Kconfig
> > index a96776a9b080..3177a1830c9a 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -1384,6 +1384,14 @@ choice
> > prompt "Compiler optimization level"
> > default CC_OPTIMIZE_FOR_PERFORMANCE
> >
> > +config CC_OPTIMIZE_FOR_COMPILATION_SPEED
> > + bool "Optimize for compilation speed (-O1)"
> > + help
> > + This option can be useful when running a static checker such as smatch
> > + or a build-bot.
> > + Compilation time is slighly faster than -O2 and it requires less
> > + memory.
> > +
> > config CC_OPTIMIZE_FOR_PERFORMANCE
> > bool "Optimize for performance (-O2)"
> > help
> > --
> > 2.34.1
> >
>
>
> --
> Best Regards
> Masahiro Yamada
--
Cheers,
Changbin Du
On Mon, May 23, 2022 at 2:34 PM Changbin Du <[email protected]> wrote:
> On Sun, May 22, 2022 at 12:38:17AM +0900, Masahiro Yamada wrote:
> > (+CC: Arnd, Changbin Du)
> >
> >
> > If -O1 support does not require additional effort,
> > I have no objection to this patch.
> >
> > (but I do not have enough insight about
> > the compiler's inlining heuristic)
> >
> >
> >
> > BTW, when we attempted to add the -Og support,
> > we fixed various parts, and Linus rejected it.
> >
> > https://lore.kernel.org/linux-kbuild/CAK7LNARQggM3aKEPRKJqa4tunFAfmfErMZuS-rrnRv6UB1VpPQ@mail.gmail.com/
> >
> >
> >
> >
> >
> >
> >
> I am afraid that '-O1' has the same situation with '-Og'. As described in GCC
> mannual:
>
> Like -O0, -Og completely disables a number of optimization passes so that
> individual options controlling them have no effect. Otherwise -Og enables all
> -O1 optimization flags except for those that may interfere with debugging:
> -fbranch-count-reg -fdelayed-branch
> -fdse -fif-conversion -fif-conversion2
> -finline-functions-called-once
> -fmove-loop-invariants -fmove-loop-stores -fssa-phiopt
> -ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra
Correct, I'm fairly sure this never worked. It may be possible to narrow down
the individual optimization flags to have something that works inbetween
-O1 and -O2, but that requires a lot of testing work, and it is questionable
whether there are any practical upsides.
Note that we already disable some optimizations for certain configurations,
e.g. CONFIG_READABLE_ASM, CONFIG_FRAME_POINTER or
CONFIG_DEBUG_SECTION_MISMATCH. The gcc manual suggests
listing all optimization flags with
$ gcc -c -Q -O1 --help=optimizers > O1-opts
$ gcc -c -Q -O2 --help=optimizers > O2-opts
$ diff -u O1-opts O2-opts
If the goal is to speed up compilation, there may be value in trying
which of the various options make the most difference here and
still produce a working kernel when disabled. I know we need the
inlining to happen or things go badly wrong, but I don't know if e.g.
-fno-expensive-optimizations would work.
Arnd