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

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Date:   Mon, 23 May 2022 22:16:05 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     dan.carpenter@...cle.com, Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-kbuild@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>
Subject: Re: [RFC PATCH] kbuild: Add an option to enable -O1 and speed-up
 compilation time

Le 23/05/2022 à 20:26, Nick Desaulniers a écrit :
> On Sat, May 21, 2022 at 2:04 AM Christophe JAILLET
> <christophe.jaillet@...adoo.fr> 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@mail.gmail.com/
> 
> 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@...adoo.fr>
>> ---
>>   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
>>
> 
>