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SubjectRe: [RFC PATCH] kbuild: Add an option to enable -O1 and speed-up compilation time
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Le 23/05/2022 à 20:26, Nick Desaulniers a écrit :
> On Sat, May 21, 2022 at 2:04 AM Christophe JAILLET
> <christophe.jaillet@wanadoo.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@wanadoo.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
>>
>
>

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