[PATCH] powerpc/32: Avoid unsupported flags with clang

[Segher Boessenkool]

On Mon, Nov 05, 2018 at 02:48:49PM +1100, Michael Ellerman wrote:
> Joel Stanley <joel at jms.id.au> writes:
> > When building for ppc32 with clang these flags are unsupported:
> >
> >   -ffixed-r2 and -mmultiple

> > -CFLAGS-$(CONFIG_PPC32)	:= -ffixed-r2 $(MULTIPLEWORD)
> > +ifndef CONFIG_CC_IS_CLANG
> > +#CFLAGS-$(CONFIG_PPC32)	:= -ffixed-r2 $(MULTIPLEWORD)
> > +endif
> 
> I think using cc-option with a comment would be nicer, you could avoid
> the ifdef and it would also future-proof it against the option either
> appearing in a future clang or disappearing in a future gcc.

These options will never be removed, lol.  -ffixed-<reg> is ancient and
generic, and -mmultiple is useful so not in any danger, and besides, we
would leave the command line option even if the feature is gone.

> eg ~=:
> 
> # Clang doesn't need or support -ffixed-r2
> CFLAGS-$(CONFIG_PPC32)	+= $(cc-option,-ffixed-r2)

This option changes the ABI.  The compiler had better handle it!  If it
doesn't, the only safe cause of action is to fail loudly.

If LLVM always makes r2 fixed, it is better to special-case it here.
(Even better would be to fix the compiler, of course).


Segher