[PATCH 0/2] Kconfig: -O3 enablement

[PATCH 0/2] Kconfig: -O3 enablement

[Miko Larsson]

Hi,

This very small series allows -O3 to be used for all architectures. The
first patch marks -O3 as experimental, with the reasoning being that it
might expose unwanted regressions to users, and the second patch
actually allows -O3 by removing the "depend on ARC" string.

The reasoning behind this series is to open up -O3 so that bugs related
to it (both compiler-related and kernel-related) can be discovered by
eyeballs wanting to improve the "-O3 experience," as that might be
beneficial to both compilers and the kernel. This has been attempted
before [1], but unfortunately nothing ever came of it.

[1] https://lore.kernel.org/lkml/20191211104619.114557-1-oleksandr@redhat.com/

Cc: linux-kbuild@vger.kernel.org
Cc: x86@kernel.org
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Sean Christopherson <seanjc@google.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Chris Down <chris@chrisdown.name>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: John Ogness <john.ogness@linutronix.de>
Cc: Elliot Berman <quic_eberman@quicinc.com>
Cc: Oleksandr Natalenko <oleksandr@redhat.com>

Miko Larsson (2):
  Kconfig: Mark -O3 as experimental
  Kconfig: Allow -O3 for all architectures

 init/Kconfig | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

-- 
2.36.1

[PATCH 1/2] Kconfig: Mark -O3 as experimental

[Miko Larsson]

Mark -O3 as experimental, as it might cause unwanted regressions for
users.

Signed-off-by: Miko Larsson <mikoxyzzz@gmail.com>
---
 init/Kconfig | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/init/Kconfig b/init/Kconfig
index c7900e897..9c292acb2 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1402,12 +1402,15 @@ config CC_OPTIMIZE_FOR_PERFORMANCE
 	  helpful compile-time warnings.
 
 config CC_OPTIMIZE_FOR_PERFORMANCE_O3
-	bool "Optimize more for performance (-O3)"
+	bool "Optimize more for performance (-O3) (EXPERIMENTAL)"
 	depends on ARC
 	help
 	  Choosing this option will pass "-O3" to your compiler to optimize
 	  the kernel yet more for performance.
 
+	  This option is EXPERIMENTAL; you may encounter compiler bugs and/or
+	  kernel bugs with this option enabled.
+
 config CC_OPTIMIZE_FOR_SIZE
 	bool "Optimize for size (-Os)"
 	help
-- 
2.36.1

[PATCH 2/2] Kconfig: Allow -O3 for all architectures

[Miko Larsson]

This commit allows all architectures to use the experimental -O3
optimization option. Previously, only ARC was allowed to use this
option.

Signed-off-by: Miko Larsson <mikoxyzzz@gmail.com>
---
 init/Kconfig | 1 -
 1 file changed, 1 deletion(-)

diff --git a/init/Kconfig b/init/Kconfig
index 9c292acb2..b88613cb5 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1403,7 +1403,6 @@ config CC_OPTIMIZE_FOR_PERFORMANCE
 
 config CC_OPTIMIZE_FOR_PERFORMANCE_O3
 	bool "Optimize more for performance (-O3) (EXPERIMENTAL)"
-	depends on ARC
 	help
 	  Choosing this option will pass "-O3" to your compiler to optimize
 	  the kernel yet more for performance.
-- 
2.36.1

Re: [PATCH 0/2] Kconfig: -O3 enablement

[Nick Desaulniers]

On Tue, Jun 21, 2022 at 6:35 AM Miko Larsson <mikoxyzzz@gmail.com> wrote:
>
> Hi,
>
> This very small series allows -O3 to be used for all architectures. The
> first patch marks -O3 as experimental, with the reasoning being that it
> might expose unwanted regressions to users, and the second patch
> actually allows -O3 by removing the "depend on ARC" string.

I think we should just remove -O3 support from KCONFIG.

If someone wants to mess around with "experimental features," there's
nothing stopping you from doing:

$ make KCFLAGS=-O3

>
> The reasoning behind this series is to open up -O3 so that bugs related
> to it (both compiler-related and kernel-related) can be discovered by
> eyeballs wanting to improve the "-O3 experience," as that might be
> beneficial to both compilers and the kernel. This has been attempted
> before [1], but unfortunately nothing ever came of it.
>
> [1] https://lore.kernel.org/lkml/20191211104619.114557-1-oleksandr@redhat.com/
>
> Cc: linux-kbuild@vger.kernel.org
> Cc: x86@kernel.org
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: Sean Christopherson <seanjc@google.com>
> Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Chris Down <chris@chrisdown.name>
> Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> Cc: John Ogness <john.ogness@linutronix.de>
> Cc: Elliot Berman <quic_eberman@quicinc.com>
> Cc: Oleksandr Natalenko <oleksandr@redhat.com>
>
> Miko Larsson (2):
>   Kconfig: Mark -O3 as experimental
>   Kconfig: Allow -O3 for all architectures
>
>  init/Kconfig | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> --
> 2.36.1
>


-- 
Thanks,
~Nick Desaulniers

Re: [PATCH 0/2] Kconfig: -O3 enablement

[Masahiro Yamada]

On Wed, Jun 22, 2022 at 1:17 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Tue, Jun 21, 2022 at 6:35 AM Miko Larsson <mikoxyzzz@gmail.com> wrote:
> >
> > Hi,
> >
> > This very small series allows -O3 to be used for all architectures. The
> > first patch marks -O3 as experimental, with the reasoning being that it
> > might expose unwanted regressions to users, and the second patch
> > actually allows -O3 by removing the "depend on ARC" string.
>
> I think we should just remove -O3 support from KCONFIG.
>
> If someone wants to mess around with "experimental features," there's
> nothing stopping you from doing:
>
> $ make KCFLAGS=-O3
>

ARC uses -O3 since day1.

"Generic build system uses -O2, we want -O3"
in commit cfdbc2e16e65c1ec1c23057640607cee98d1a1bd

If they want -O3, it is up to the ARC maintainer.



If you want to say "use this option carefully",
EXPERT might be another option.

    depends on ARC || EXPERT







> >
> > The reasoning behind this series is to open up -O3 so that bugs related
> > to it (both compiler-related and kernel-related) can be discovered by
> > eyeballs wanting to improve the "-O3 experience," as that might be
> > beneficial to both compilers and the kernel. This has been attempted
> > before [1], but unfortunately nothing ever came of it.
> >
> > [1] https://lore.kernel.org/lkml/20191211104619.114557-1-oleksandr@redhat.com/
> >
> > Cc: linux-kbuild@vger.kernel.org
> > Cc: x86@kernel.org
> > Cc: Nathan Chancellor <nathan@kernel.org>
> > Cc: Vlastimil Babka <vbabka@suse.cz>
> > Cc: Nick Desaulniers <ndesaulniers@google.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > Cc: Sean Christopherson <seanjc@google.com>
> > Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Chris Down <chris@chrisdown.name>
> > Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> > Cc: John Ogness <john.ogness@linutronix.de>
> > Cc: Elliot Berman <quic_eberman@quicinc.com>
> > Cc: Oleksandr Natalenko <oleksandr@redhat.com>
> >
> > Miko Larsson (2):
> >   Kconfig: Mark -O3 as experimental
> >   Kconfig: Allow -O3 for all architectures
> >
> >  init/Kconfig | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > --
> > 2.36.1
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers



-- 
Best Regards
Masahiro Yamada

Re: [PATCH 0/2] Kconfig: -O3 enablement

[Miko Larsson]

On Wednesday, 22 June 2022 03:57:34 CEST Masahiro Yamada wrote:
> If you want to say "use this option carefully",
> EXPERT might be another option.
> 
>     depends on ARC || EXPERT
>

Yeah, this would be a fair compromise, though I think it would be
better to use "visible if" instead of "depends on". I can get a v2 of
the series together if this is desired.

--
~miko

Re: [PATCH 0/2] Kconfig: -O3 enablement

[Masahiro Yamada]

On Fri, Jun 24, 2022 at 12:42 AM Miko Larsson <mikoxyzzz@gmail.com> wrote:
>
> On Wednesday, 22 June 2022 03:57:34 CEST Masahiro Yamada wrote:
> > If you want to say "use this option carefully",
> > EXPERT might be another option.
> >
> >     depends on ARC || EXPERT
> >
>
> Yeah, this would be a fair compromise, though I think it would be
> better to use "visible if" instead of "depends on". I can get a v2 of
> the series together if this is desired.


Why is "visible if" better than "depends on"?



> --
> ~miko
>
>


--
Best Regards
Masahiro Yamada

Re: [PATCH 0/2] Kconfig: -O3 enablement

[Miko Larsson]

On Thursday, 23 June 2022 17:44:57 CEST Masahiro Yamada wrote:
> On Fri, Jun 24, 2022 at 12:42 AM Miko Larsson <mikoxyzzz@gmail.com> wrote:
> > On Wednesday, 22 June 2022 03:57:34 CEST Masahiro Yamada wrote:
> > > If you want to say "use this option carefully",
> > > EXPERT might be another option.
> > > 
> > >     depends on ARC || EXPERT
> > 
> > Yeah, this would be a fair compromise, though I think it would be
> > better to use "visible if" instead of "depends on". I can get a v2 of
> > the series together if this is desired.
> 
> Why is "visible if" better than "depends on"?
> 

Technically it most likely doesn't matter, but logically it makes more
sense, since we'd make CC_OPTIMIZE_FOR_PERFORMANCE_O3 be visible if
we're on ARC or if we have EXPERT enabled, instead of depending on
them. But yeah, it probably doesn't matter.

--
~miko

Re: [PATCH 0/2] Kconfig: -O3 enablement

[Masahiro Yamada]

On Fri, Jun 24, 2022 at 2:00 AM Miko Larsson <mikoxyzzz@gmail.com> wrote:
>
> On Thursday, 23 June 2022 17:44:57 CEST Masahiro Yamada wrote:
> > On Fri, Jun 24, 2022 at 12:42 AM Miko Larsson <mikoxyzzz@gmail.com> wrote:
> > > On Wednesday, 22 June 2022 03:57:34 CEST Masahiro Yamada wrote:
> > > > If you want to say "use this option carefully",
> > > > EXPERT might be another option.
> > > >
> > > >     depends on ARC || EXPERT
> > >
> > > Yeah, this would be a fair compromise, though I think it would be
> > > better to use "visible if" instead of "depends on". I can get a v2 of
> > > the series together if this is desired.
> >
> > Why is "visible if" better than "depends on"?
> >
>
> Technically it most likely doesn't matter, but logically it makes more
> sense, since we'd make CC_OPTIMIZE_FOR_PERFORMANCE_O3 be visible if
> we're on ARC or if we have EXPERT enabled, instead of depending on
> them. But yeah, it probably doesn't matter.


Did you write and test the code?


"visible if" is only supported for "menu".
This is clearly documented at line 207
of Documentation/kbuild/kconfig-language.rst


Using "visible if" for config entry will just
result in the syntax error.





> --
> ~miko
>
>
>


--
Best Regards
Masahiro Yamada

Re: [PATCH 0/2] Kconfig: -O3 enablement

[Miko Larsson]

On Thursday, 23 June 2022 19:15:14 CEST Masahiro Yamada wrote:
> On Fri, Jun 24, 2022 at 2:00 AM Miko Larsson <mikoxyzzz@gmail.com> wrote:
> > On Thursday, 23 June 2022 17:44:57 CEST Masahiro Yamada wrote:
> > > On Fri, Jun 24, 2022 at 12:42 AM Miko Larsson <mikoxyzzz@gmail.com> 
wrote:
> > > > On Wednesday, 22 June 2022 03:57:34 CEST Masahiro Yamada wrote:
> > > > > If you want to say "use this option carefully",
> > > > > EXPERT might be another option.
> > > > > 
> > > > >     depends on ARC || EXPERT
> > > > 
> > > > Yeah, this would be a fair compromise, though I think it would be
> > > > better to use "visible if" instead of "depends on". I can get a v2 of
> > > > the series together if this is desired.
> > > 
> > > Why is "visible if" better than "depends on"?
> > 
> > Technically it most likely doesn't matter, but logically it makes more
> > sense, since we'd make CC_OPTIMIZE_FOR_PERFORMANCE_O3 be visible if
> > we're on ARC or if we have EXPERT enabled, instead of depending on
> > them. But yeah, it probably doesn't matter.
> 
> Did you write and test the code?
>

Admittedly, I didn't, since I had falsely assumed that "visible if" was
just an "alternative" to "depends on".

> "visible if" is only supported for "menu".
> This is clearly documented at line 207
> of Documentation/kbuild/kconfig-language.rst
> 
> 
> Using "visible if" for config entry will just
> result in the syntax error.
>

Oops, yeah, I wasn't aware of this. Sorry.

--
~miko

Re: [PATCH 0/2] Kconfig: -O3 enablement

[Arnd Bergmann]

On Wed, Jun 22, 2022 at 3:57 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
> On Wed, Jun 22, 2022 at 1:17 AM Nick Desaulniers <ndesaulniers@google.com> wrote:
> >
> > On Tue, Jun 21, 2022 at 6:35 AM Miko Larsson <mikoxyzzz@gmail.com> wrote:
> > >
> > > Hi,
> > >
> > > This very small series allows -O3 to be used for all architectures. The
> > > first patch marks -O3 as experimental, with the reasoning being that it
> > > might expose unwanted regressions to users, and the second patch
> > > actually allows -O3 by removing the "depend on ARC" string.
> >
> > I think we should just remove -O3 support from KCONFIG.

I agree that would be best

> > If someone wants to mess around with "experimental features," there's
> > nothing stopping you from doing:
> >
> > $ make KCFLAGS=-O3
> >
>
> ARC uses -O3 since day1.
>
> "Generic build system uses -O2, we want -O3"
> in commit cfdbc2e16e65c1ec1c23057640607cee98d1a1bd
>
> If they want -O3, it is up to the ARC maintainer.

I suppose whatever the reason for using -O3 at the time has
likely changed by now.

> If you want to say "use this option carefully",
> EXPERT might be another option.
>
>     depends on ARC || EXPERT

This probably also needs a dependency on !COMPILE_TEST so we don't
report compile-time problems that are specific to -O3. Maybe a good first
step would be to turn this into

      depends on ARCH && EXPERT && !COMPILE_TEST

which should help both with compile-testing on ARC, and it would
prevent it from being visible on other architectures.

        Arnd