linux-kernel - Re: [PATCH] Add option to build with -O3

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Date:	Thu, 06 Mar 2014 07:28:50 -0500
From:	Austin S Hemmelgarn <ahferroin7@...il.com>
To:	jon@...gle.org, linux-kernel@...r.kernel.org
CC:	Jon Ringle <jringle@...dpoint.com>
Subject: Re: [PATCH] Add option to build with -O3

On 2014-03-05 02:36, jon@...gle.org wrote:
> From: Jon Ringle <jringle@...dpoint.com>
> 
> Signed-off-by: Jon Ringle <jringle@...dpoint.com>
> ---
>  Makefile     |  2 ++
>  init/Kconfig | 19 ++++++++++++++++---
>  2 files changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index 78209ee..e7f0b3c 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -581,6 +581,8 @@ all: vmlinux
>  
>  ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
>  KBUILD_CFLAGS	+= -Os $(call cc-disable-warning,maybe-uninitialized,)
> +else ifdef CONFIG_CC_OPTIMIZE_FOR_SPEED
> +KBUILD_CFLAGS   += -O3
>  else
>  KBUILD_CFLAGS	+= -O2
>  endif
> diff --git a/init/Kconfig b/init/Kconfig
> index 009a797..17d4c62 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1233,13 +1233,26 @@ source "usr/Kconfig"
>  
>  endif
>  
> +choice
> +    prompt "Optimize"
> +
> +config CC_OPTIMIZE_NORMAL
> +    bool "Optimize Normal (-O2)"
> +    help
> +      Enabling this option will pass "-O2" to gcc
>  config CC_OPTIMIZE_FOR_SIZE
> -	bool "Optimize for size"
> +	bool "Optimize for size (-Os)"
>  	help
> -	  Enabling this option will pass "-Os" instead of "-O2" to gcc
> +	  Enabling this option will pass "-Os" to gcc
>  	  resulting in a smaller kernel.
>  
> -	  If unsure, say N.
> +config CC_OPTIMIZE_FOR_SPEED
> +    bool "Optimze for speed (-O3)"
> +    help
> +      Enabling this option will pass "-O3" to gcc
> +      resulting in a larger kernel (but possibly faster)
> +
> +endchoice
>  
>  config SYSCTL
>  	bool
> 
I would suggest putting a warning in the Kconfig text that this might
make things break in new and unusual ways (and possibly making it depend
on EXPERT).  All of the cases of -O3 breaking userspace software that I
have seen involve really edgy and complex (and often potentially
ambiguous) code, and while the core kernel doesn't have much of this,
there are a number drivers that do (The Synopsis DWC USB OTG driver
immediately comes to mind, as well as a number of drivers from staging).
--
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