[gentoo-user] Re: CFLAGs for kernel compilation

[Nikos Chantziaras]

On 02/05/15 14:19, Volker Armin Hemmann wrote:
> Am 02.05.2015 um 07:04 schrieb Nikos Chantziaras:
>> On 01/05/15 10:44, Andrew Savchenko wrote:
>>> On Fri, 1 May 2015 05:09:51 +0000 (UTC) Martin Vaeth wrote:
>>>> Andrew Savchenko <bircoph@g.o> wrote:
>>>>>
>>>>> That's why kernel makes sure that no floating point instructions
>>>>> sneaks in using CFLAGS, you may see a lot of -mno-${intrucion_set}
>>>>> flags when running make -V.
>>>>
>>>> So it should be sufficient that the kernel does not use "float"
>>>> or "double", shouldn't it?
>>>
>>> No. Optimizer paths may be very unobvious, i.e. I'll not be
>>> surprised if under some conditions vectorizer may use float
>>> instructions for int code.
>>
>> The kernel uses -O2 and several -march variants (e.g. -march=core2).
>> Several other options are used to prevent GCC from generating
>> unsuitable code.
>>
>> Specifying another -march variant does not affect the optimizer
>> though. It only affects the code generator. If you don't modify the
>> other CFLAGS and only change -march, you will not get FP instructions
>> unless you use FP in the code.
>>
>> Also, I'd be very interested to see *any* optimization that would
>> somehow transform integer code to FP code (note that SIMD is not FP
>> and is perfectly fine in the kernel.) In fact, optimizers tend to
>> transform FP into SIMD, at least on x86 (and other architectures that
>> have fast SIMD instructions.) If I inspect the generated assembly from
>> GCC or Clang, I cannot find FP anywhere, even for code using "float"
>> and "double" operations. They get converted to SIMD on modern CPUs
>> (unless you specify a compiler flag that tells it to use the FPU, for
>> example if you need 80-bit extended precision, which is supported by
>> the x86 FPU.)
>>
>>
>>
>
> http://www.agner.org/optimize/calling_conventions.pdf

Not sure what you're trying to say.