Re: [PATCH] Makerules: pass CFLAGS when building .S files, BZ #23273

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

On 11/06/2018 12:30, Sergei Trofimovich wrote:
> On Mon, 11 Jun 2018 15:47:41 +0200
> Florian Weimer <fweimer@redhat.com> wrote:
> 
>> On 06/11/2018 03:41 PM, Sergei Trofimovich wrote:
>>> The below works on unmodified glibc as well:
>>>
>>> $ ../glibc/configure \
>>>      --build=x86_64-pc-linux-gnu \
>>>      --host=armv7a-hardfloat-linux-gnueabi \
>>>      CC="armv7a-hardfloat-linux-gnueabi-gcc -march=armv7-a -mfpu=neon" \
>>>      CFLAGS="-pipe -O2" \
>>>      --prefix=/usr  
>>
>> Then please use that instead.  It's what we do on other architectures, too.
> 
> Aha. Gentoo allows user to specify CFLAGS from a limited (but not small) set of
> flags when glibc is built locally:
>     https://gitweb.gentoo.org/repo/gentoo.git/tree/eclass/flag-o-matic.eclass#n25
> 
> Does it mean that canonical way to build glibc would be to always pass all
> CFLAGS/CXXFLAGS as part of CC/CXX?
>     CC="${TARGET_CC} ${CFLAGS}"
>     CXX="${TARGET_CC} ${CXXFLAGS}"
> 
> Or should only a small subset of CFLAGS/CXXFLAGS go to CC/CXX?
>     CC="${TARGET_CC} ${filtered_CFLAGS}"
>     CXX="${TARGET_CC} ${filtered_CXXFLAGS}"
> 
> If it should be a filtered subset, how can we easily infer supported subset?
> 

It is not exactly the canonical way, but rather the usual one which should be
simpler when the compiler flags may change the target ABI.  One option is
to set the ABI flags on CC/CXX, another option is to set on both CFLAGS and
CPPFLAGS.

For armv7 neon you can either set:

./configure CC="gcc -march=armv7-a -mfpu=neon" CXX="g++ -march=armv7-a -mfpu=neon"

or

./configure CFLAGS="-O2 -march=armv7-a -mfpu=neon" CPPFLAGS="-O2 -march=armv7-a -mfpu=neon"

The only difference is you will need to explicit set the optimization level when
setting CFLAGS.

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