[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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