Re: [Xen-devel] [PATCH 01/12] xen: clang: Support correctly cross-compile

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.03.19 at 11:14, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 28/03/2019 09:55, Jan Beulich wrote:
>>>>> On 27.03.19 at 19:45, <julien.grall@xxxxxxx> wrote:
>>> Clang uses "-target" option for cross-compilation.
>> And all possible targets are always available? I'd like to point out
>> that CROSS_COMPILE can be used for other than actual cross
>> compilation, e.g. building with just an alternative tool chain
>> built for the same target. Dropping the $(CROSS_COMPILE)
>> prefixes makes this impossible afaict. Requiring suitable wrapper
>> scripts to be put in place would seem better to me.
>>
>> I also wonder why this change is needed for Arm, but wasn't
>> needed so far for x86. But perhaps no-one ever tried using it
>> so far ...
> 
> It seems that CROSS_COMPILE is a GNU-ism, which is not shared by the
> clang world.  I can't find anything which will make you a
> $FOO-$BAR-clang binary, whereas you do typically get clang-$X aliases
> for the different versions of clang.

Oh, interesting. For my own re-built tool chains I actually can't use
CROSS_COMPILE either, because traditionally all my wrapper scripts
have a suffix like you say clang uses too. I patch in

cross-compile ?= $(CROSS_COMPILE)$(1)

locally as a fallback, to then use it as

AS = $(call cross-compile,as)

and then override things in the build root directory .config to
suite my actual needs, e.g. in its simplest possible form

cross-compile=$(1)x

But of course this doesn't fit clang either, as it's (aiui) only the
compiler which wants to be overridden this way.

> Using -target is from the Clang instructions on cross compilation, which
> say to do it this way.  https://clang.llvm.org/docs/CrossCompilation.html 
> 
> The targets supported will depend on the configuration Clang was
> compiled with, but Clang specifically opposes GCC's way of requiring the
> user to recompile for every different target.  It is expected that a
> packager of clang will enable all of the supported targets in the
> package they distribute.

Are you sure a distro caring about, say, only x86 would indeed
enable Arm and all sorts of other architectures in the compiler,
just because it can be enabled? IOW I assume the need for an
override to the system default clang binaries would still exist.

Jan



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