Re: Pass --build=<triplet> to native builds by default?

[ludo@gnu.org (Ludovic Courtès)]

Mark H Weaver <mhw@netris.org> skribis:

> Mark H Weaver <mhw@netris.org> writes:
>
>> ludo@gnu.org (Ludovic Courtès) writes:
>>
>>> Mark H Weaver <mhw@netris.org> skribis:
>>>
>>>> It turns out that on ARM systems, the result of 'config.guess' depends
>>>> on the result of 'uname -m'.  In other words, details of the kernel (and
>>>> perhaps processor?) on the build machine will determine the triplet of
>>>> our builds, which in turn may affect what set of instructions is used.
>>>
>>> Do you know how the ‘uname -m’ output is used in config.guess?  What
>>> does it return on ARM?
>>
>> The output of 'uname -m' becomes the first (cpu) component of the GNU
>> triplet.  uname(1) gets its information from the kernel via the uname(2)
>> system call.  The field returned by 'uname -m' is described as "Hardware
>> identifier".  See <http://man7.org/linux/man-pages/man2/uname.2.html>.
>>
>> Here's the relevant section of config.guess from gcc-4.8.4:
>>
>>     arm*:Linux:*:*)
>> 	eval $set_cc_for_build
>> 	if echo __ARM_EABI__ | $CC_FOR_BUILD -E - 2>/dev/null \
>> 	    | grep -q __ARM_EABI__
>> 	then
>> 	    echo ${UNAME_MACHINE}-unknown-linux-${LIBC}
>> 	else
>> 	    if echo __ARM_PCS_VFP | $CC_FOR_BUILD -E - 2>/dev/null \
>> 		| grep -q __ARM_PCS_VFP
>> 	    then
>> 		echo ${UNAME_MACHINE}-unknown-linux-${LIBC}eabi
>> 	    else
>> 		echo ${UNAME_MACHINE}-unknown-linux-${LIBC}eabihf
>> 	    fi
>> 	fi
>> 	exit ;;
>
> I forgot to answer your second question.  On my Novena, 'uname -m'
> returns "armv7l".  The problem is this: I suspect that if the build
> machine has an armv8 processor, it will return something different like
> "armv8l".

But how do the armv7 and armv8 ISAs differ?  If it’s more like
additional SIMD extensions, then indeed it would make sense to use the
same name for both; but if there’s more than that, perhaps using
different triplets is the right thing?

Ludo’.