Re: [RFC PATCH] kbuild: Add an option to enable -O1 and speed-up compilation time

[Changbin Du]

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@xxxxxxxxxxxxxx/
> 
> 
> 
> 
> 
> 
> 
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
> <christophe.jaillet@xxxxxxxxxx> 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 <christophe.jaillet@xxxxxxxxxx>
> > ---
> >  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