Am 02.05.2015 um 13:25 schrieb 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 <birc...@gentoo.org> 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.
>
>
>that simd is not save in kernel if not carefully guarded. Really people, just don't fuck around with the cflags.
