Re: [PATCH][RFC][libatomic] Override -mcpu option for arm linux ifunc targets

[Kyrill Tkachov <kyrylo dot tkachov at arm dot com>]

On 13/02/14 18:23, Richard Henderson wrote:
> On 02/03/2014 03:50 AM, Kyrill Tkachov wrote:
> > +# For ARM, the -march option by itself conflicts with any -mcpu option that
> > +# we might end up passing to the build, causing an error.
> > +# Therefore we override the -mcpu option as well.
> > +# This shouldn't affect tuning much because the affected code is mostly
> > +# in inline assembly anyway.
> >   @ARCH_ARM_LINUX_TRUE@@HAVE_IFUNC_TRUE@IFUNC_OPTIONS = -march=armv7-a -DHAVE_KERNEL64
> > +@ARCH_ARM_LINUX_TRUE@@HAVE_IFUNC_TRUE@EXTRA_OVERRIDE = -mcpu=cortex-a9
> Why would you want to split these across two different variables?  It's easier
> to just add the -march and -mcpu to the same IFUNC_OPTIONS variable.

Because IFUNC_OPTIONS is not used as is. It goes through some kind of machinery 
with the PAT_N, PAT_S, PAT_BASe, PAT_SPLIT above that is used to extract just 
one of the options from IFUNC_OPTIONS and adding a new entry there ends up 
messing up the expected order.
If there's an easier way to specify the extra option I'll be happy to use it, 
but I couldn't find a way of doing it without rewriting a whole lot of Makefile 
magic.

> Why the choice of cortext-a9, as opposed to any of the other v7-a
> possibilities?  If we're going to force anything, perhaps generic-armv7-a is
> more appropriate?

Hmmm, you're right. Might as well make it explicit that we want armv7-a.

How about this then?

Thanks for the review,
Kyrill

diff --git a/libatomic/Makefile.in b/libatomic/Makefile.in
index 46e60c9..b51910a 100644
--- a/libatomic/Makefile.in
+++ b/libatomic/Makefile.in
@@ -297,7 +297,7 @@ PAT_BASE = $(word 1,$(PAT_SPLIT))
 PAT_N = $(word 2,$(PAT_SPLIT))
 PAT_S = $(word 3,$(PAT_SPLIT))
 IFUNC_DEF = -DIFUNC_ALT=$(PAT_S)
-IFUNC_OPT = $(word $(PAT_S),$(IFUNC_OPTIONS))
+IFUNC_OPT = $(word $(PAT_S),$(IFUNC_OPTIONS)) $(word $(PAT_S),$(EXTRA_OVERRIDE))
 M_DEPS = -MT $@ -MD -MP -MF $(DEPDIR)/$(@F).Ppo
 M_SIZE = -DN=$(PAT_N)
 M_IFUNC = $(if $(PAT_S),$(IFUNC_DEF) $(IFUNC_OPT))
@@ -313,7 +313,13 @@ M_SRC = $(firstword $(filter %/$(M_FILE), $(all_c_files)))
 libatomic_la_LIBADD = $(foreach s,$(SIZES),$(addsuffix \
 	_$(s)_.lo,$(SIZEOBJS))) $(am__append_1) $(am__append_2) \
 	$(am__append_3)
+# For ARM, the -march option by itself conflicts with any -mcpu option that
+# we might end up passing to the build, causing an error.
+# Therefore we override the -mcpu option as well.
+# This shouldn't affect tuning much because the affected code is mostly
+# in inline assembly anyway.
 @ARCH_ARM_LINUX_TRUE@@HAVE_IFUNC_TRUE@IFUNC_OPTIONS = -march=armv7-a -DHAVE_KERNEL64
+@ARCH_ARM_LINUX_TRUE@@HAVE_IFUNC_TRUE@EXTRA_OVERRIDE = -mcpu=generic-armv7-a
 @ARCH_I386_TRUE@@HAVE_IFUNC_TRUE@IFUNC_OPTIONS = -march=i586
 @ARCH_X86_64_TRUE@@HAVE_IFUNC_TRUE@IFUNC_OPTIONS = -mcx16
 libatomic_convenience_la_SOURCES = $(libatomic_la_SOURCES)