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

[Christophe JAILLET]

Le 23/05/2022 à 20:26, Nick Desaulniers a écrit :
> On Sat, May 21, 2022 at 2:04 AM 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.
>
> Which is a tragedy.
>
> As with the Rust series, I'm not a fan of this (or
> CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3):
> https://lore.kernel.org/lkml/CAKwvOd=7QTUH69+ZbT7e8einvgcosTbDkyohmPaUBv6_y8RfrQ@xxxxxxxxxxxxxx/
>
> These feel more like attempts to wrap every conceivable command line
> flag in a kconfig option, which makes me think of that meme from
> Jurassic Park: "your scientists were so preoccupied with whether or
> not they could, they didn't stop to think if they should."
> Not a fan.  I'd ask for measurements, but that would be a request for
> a "rock fetching quest" for something I still wouldn't be a fan of.

Hi,

some preliminary tests gave roughly a 5% build time speed-up with -O1.
That is to say that the impact seems quite limited, after all.
This was done on building /drivers/net/ or /drivers/net/ethernet/ only.

I don't have numbers for the impact on memory consumption.

Given the feedbacks on -O1 which is unlikely to be able to build a full 
kernel and your position on yet another option for a very specific use 
case, I won't push the idea further.

I'll keep it in my own tree for (small) build time improvement when 
running smatch.

The idea of turning off a subset of -O2 options would just be some too 
fine tuning. I'll certainly try it for my own use, but it would end to 
an un-understandable list of flags in Makefile.
-O1 (or -O0) was simple. A more complex solution is way too much.


Thanks for your feedbacks.

CJ

> > 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
>
> s/slighly/slightly/

Thanks for taking time for a full review, even on something you are not 
fan on. This is really appreciated.

CJ

> > +         memory.
> > +
> >   config CC_OPTIMIZE_FOR_PERFORMANCE
> >          bool "Optimize for performance (-O2)"
> >          help
> > --
> > 2.34.1