linux-kernel - Re: [PATCH] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE

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Message-ID: <20200508083502.GP5298@hirez.programming.kicks-ass.net>
Date:   Fri, 8 May 2020 10:35:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Jakub Jelinek <jakub@...hat.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, hjl.tools@...il.com
Subject: Re: [PATCH] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE_O3 for
 gcc >= 10

On Thu, May 07, 2020 at 04:45:30PM -0600, Jason A. Donenfeld wrote:
> GCC 10 appears to have changed -O2 in order to make compilation time
> faster when using -flto, seemingly at the expense of performance, in
> particular with regards to how the inliner works. Since -O3 these days
> shouldn't have the same set of bugs as 10 years ago, this commit
> defaults new kernel compiles to -O3 when using gcc >= 10.

Would be nice to get some GCC person's feedback on this. But in general,
I think you're right in that O3 isn't the code-gen disaster it used to
be.

> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
>  init/Kconfig | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index 9e22ee8fbd75..fab3f810a68d 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1245,7 +1245,8 @@ config BOOT_CONFIG
>  
>  choice
>  	prompt "Compiler optimization level"
> -	default CC_OPTIMIZE_FOR_PERFORMANCE
> +	default CC_OPTIMIZE_FOR_PERFORMANCE_O3 if GCC_VERSION >= 100000
> +	default CC_OPTIMIZE_FOR_PERFORMANCE if (GCC_VERSION < 100000 || CC_IS_CLANG)
>  
>  config CC_OPTIMIZE_FOR_PERFORMANCE
>  	bool "Optimize for performance (-O2)"
> -- 
> 2.26.2
>