Re: [Patch][aarch64] Use IFUNCs to enable LSE instructions in libatomic on aarch64

[James Greenhalgh <james.greenhalgh@arm.com>]

On Fri, Sep 29, 2017 at 09:29:37PM +0100, Steve Ellcey wrote:
> On Thu, 2017-09-28 at 12:31 +0100, Szabolcs Nagy wrote:
> > 
> > i think this should be improved, see below.
> 
> diff --git a/libatomic/Makefile.am b/libatomic/Makefile.am
> index d731406..92d19c6 100644
> --- a/libatomic/Makefile.am
> +++ b/libatomic/Makefile.am
> @@ -122,6 +122,10 @@ libatomic_la_LIBADD = $(foreach s,$(SIZES),$(addsuffix _$(s)_.lo,$(SIZEOBJS)))
>  
>  ## On a target-specific basis, include alternates to be selected by IFUNC.
>  if HAVE_IFUNC
> +if ARCH_AARCH64_LINUX
> +IFUNC_OPTIONS	     = -march=armv8.1-a
> +libatomic_la_LIBADD += $(foreach s,$(SIZES),$(addsuffix _$(s)_1_.lo,$(SIZEOBJS)))
> +endif
>  if ARCH_ARM_LINUX
>  IFUNC_OPTIONS	     = -march=armv7-a -DHAVE_KERNEL64
>  libatomic_la_LIBADD += $(foreach s,$(SIZES),$(addsuffix _$(s)_1_.lo,$(SIZEOBJS)))

One obvious thing I missed in the review is that this change will break
bootstrap on systems with older assemblers. Practically, that's those of
us who are holding out on Ubuntu 14.04. -march=armv8-a+lse would go back
a little further, so would be preferable, but even this won't get bootstrap
back on older systems.

Is there anything you can do to check for assembler support before turning
on IFUNCS for libatomic, or am I best to just start configuring with
--disable-gnu-indirect-function ?

Thanks,
James