linux-kernel - Re: [PATCH] Kbuild: Add option to set -femit-struct-debug-baseonly

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Date:	Wed, 14 Jul 2010 17:22:51 +0200
From:	Michal Marek <mmarek@...e.cz>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
	ak@...ux.intel.com
Subject: Re: [PATCH] Kbuild: Add option to set -femit-struct-debug-baseonly

On 07/14/2010 03:43 PM, Andi Kleen wrote:
> Kbuild: Add option to set -femit-struct-debug-baseonly 
> 
> Newer gcc has a -femit-struct-debug-baseonly option that dramatically
> reduces the size of object files with debug info. What it does
> is to only emit type information for structures when the structures
> are defined in the same file or in a header file.
> 
> This means the type information for most headers are not included.
> This is not good when the type information is actually
> needed (e.g. with kgdb or systemtap) 
> 
> But often kernel hackers only care about line numbers and don't
> need all the type information anyways. In this case setting
> the option can be a big win:
> 
> A build dir for a specific x86-64 configuration with gcc 4.5
> shrunk from 2.3G to 1.2G. The compilation was also nearly a minute
> faster.
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> 
> ---
>  Makefile          |    4 ++++
>  lib/Kconfig.debug |   12 ++++++++++++
>  2 files changed, 16 insertions(+)
> 
> Index: linux-2.6.34-ak/Makefile
> ===================================================================
> --- linux-2.6.34-ak.orig/Makefile
> +++ linux-2.6.34-ak/Makefile
> @@ -554,6 +554,10 @@ KBUILD_CFLAGS	+= -g
>  KBUILD_AFLAGS	+= -gdwarf-2
>  endif
>  
> +ifdef CONFIG_DEBUG_INFO_REDUCED
> +KBUILD_CFLAGS 	+= $(call cc-option, -femit-struct-debug-baseonly)
> +endif
> +
>  ifdef CONFIG_FUNCTION_TRACER
>  KBUILD_CFLAGS	+= -pg
>  endif
> Index: linux-2.6.34-ak/lib/Kconfig.debug
> ===================================================================
> --- linux-2.6.34-ak.orig/lib/Kconfig.debug
> +++ linux-2.6.34-ak/lib/Kconfig.debug
> @@ -616,6 +616,18 @@ config DEBUG_INFO
>  
>  	  If unsure, say N.
>  
> +config DEBUG_INFO_REDUCED
> +	bool "Reduce debugging information"
> +	depends on DEBUG_INFO
> +	help
> +	  If you say Y here gcc is instructed to generate less debugging information
> +          for structure types. This means that tools that need full debugging information
> +          (like kgdb or systemtap) won't be happy. But if you merely need debugging
> +          information to resolve line numbers there is no loss.
> +          Advantage is that build directory object sizes shrink dramatically over
> +          a full DEBUG_INFO build and compile times are reduced too.
> +          Only works with newer gcc versions.
> +

I reformatted this help text to not wrap in a 80-column terminal and
applied the patch to the kbuild tree.

thanks,
Michal
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