[Buildroot] [git commit] Config.in: add -Ofast option

Joshua Henderson joshua.henderson at microchip.com
Fri Apr 13 20:22:07 UTC 2018


Yann, All,

On 04/07/2018 07:37 AM, Yann E. MORIN wrote:
> Joshua, All,
> 
> On 2018-04-05 14:46 -0700, Joshua Henderson spake thusly:
>> 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 at microchip.com>
>>> Signed-off-by: Thomas Petazzoni <thomas.petazzoni at 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?
> 
> Assuming those packages are not too many, I'd say we go to overriding
> the CFLAGS for those, with code something like:
> 
>     SQLITE_CFLAGS = $(subst -Ofast,-O3,$(TARGET_CFLAGS))

Something like this gets the job done.  The -Ofast still hangs around in other
*_ENV flags because it's still in TARGET_CFLAGS, but not where I can find it
matters.  Patch to follow.

> 
> If it turns out that the number of broken packages is too high, then we
> can come up with an alternate solution...

This is the only one I have found so far.  sqlite3 was diligent enough to make
this a build time issue.  Other packages may not.

Josh



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