amd64 arch and sse optimization

[Reinhard Tartler]

On Mon, Feb 3, 2014 at 10:56 AM, Reinhard Tartler <siretart at gmail.com> wrote:
> On Mon, Feb 3, 2014 at 10:43 AM, Felipe Sateler <fsateler at debian.org> wrote:

>>> Please be careful with adding those flags. They instruct gcc to emit
>>> code that does not work on some machines. The admitedlyonly way to use
>>> them correctly is to ensure that the emitted code is only used on
>>> machines that actually supports that.
>>
>> The mtune flag instructs the compiler to optimize for a certain
>> instruction set, but still provide a fallback for when the instrucions
>> are not available. I don't know if this includes the use of SSE or
>> other coprocessor.
>>
>> http://stackoverflow.com/questions/10559275/gcc-how-is-march-different-from-mtune
>
> OK, then we will only see performance impact on machines not matching
> the optimization target. Which seems OK to me, if the benefit for the
> targeted architectures justify this. Sorry for the noise.

Please note that Jaromir proposed to use the following flags:

ifeq ($(DEB_HOST_ARCH_CPU),amd64)
CFLAGS += -msse -msse2 -mfpmath=sse
endif


None of them fall in the "mtune" category, that is, none of them are
"safe" to use without further precaution measures!

-- 
regards,
    Reinhard