From mboxrd@z Thu Jan  1 00:00:00 1970
From: Joshua Henderson <joshua.henderson@microchip.com>
Date: Thu, 5 Apr 2018 14:46:47 -0700
Subject: [Buildroot] [git commit] Config.in: add -Ofast option
In-Reply-To: <20180326201918.157AC83C5E@busybox.osuosl.org>
References: <20180326201918.157AC83C5E@busybox.osuosl.org>
Message-ID: <3fafe5de-1c7b-801c-9c14-291ebcfdc264@microchip.com>
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To: buildroot@busybox.net

Thomas, All, 

On 03/26/2018 01:19 PM, Thomas Petazzoni wrote:
> commit: https://git.buildroot.net/buildroot/commit/?id=ed6a7e18af8f4dfddee6e6b144f20dfaf6926315
> branch: https://git.buildroot.net/buildroot/commit/?id=refs/heads/master
> 
> -Ofast (introduced in GCC 4.6) It combines the existing optimization level -O3
> with options that can affect standards compliance but result in better optimized
> code. For example, -Ofast enables -ffast-math.
> 
> Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
> Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
> ---
>  Config.in           | 11 +++++++++++
>  package/Makefile.in |  3 +++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/Config.in b/Config.in
> index 0002df5176..037ea2265b 100644
> --- a/Config.in
> +++ b/Config.in
> @@ -527,6 +527,17 @@ config BR2_OPTIMIZE_S
>  	  -ftree-vect-loop-version
>  	  This is the default.
>  
> +config BR2_OPTIMIZE_FAST
> +	bool "optimize for fast"
> +	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_6
> +	help
> +	  Optimize for fast. Disregard strict standards
> +	  compliance. -Ofast enables all -O3 optimizations. It also
> +	  enables optimizations that are not valid for all
> +	  standard-compliant programs. It turns on -ffast-math and the
> +	  Fortran-specific -fstack-arrays, unless -fmax-stack-var-size
> +	  is specified, and -fno-protect-parens.
> +
>  endchoice
>  

...

For reference, I just found an unfriendly package with this option.

sqlite3.c: In function ?sqlite3IsNaN?:
sqlite3.c:28554:3: error: #error SQLite will not work correctly with the -ffast-math option of GCC.
 # error SQLite will not work correctly with the -ffast-math option of GCC.
   ^
Makefile:541: recipe for target 'sqlite3.lo' failed

sqlite3 refuses to be built with -ffast-math (a side effect of -Ofast) when it
falls back to implementing its own isnan() function.  From what I can see,
SQLITE_HAVE_ISNAN can be defined [1] to make it use the system library isnan().
But, how to tell if the system library supports a proper isnan() seems to open
up a whole new can of worms.  Thoughts?

[1]: https://www.sqlite.org/compile.html

Josh