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

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

Hi all,

There is a slight issue with the libatomic Makefile for arm linux ifunc targets. 
It adds an explicity -march=armv7-a option to the command line to enable 
building the exclusive instruction variants in libatomic. However, if the 
multilib machinery tries to add an -mcpu option that conflicts with the -march 
one (such as -mcpu=cortex-a15) gcc will give a warning about incompatible -march 
and -mcpu options, causing the -Werror build to fail.

A workaround here is to override the -mcpu option as well as the -march one. 
This patch does that by adding an EXTRA_OVERRIDE variable and setting it to 
-mcpu=cortex-a9 under the same conditions as when -march=armv7-a is selected, so 
that it's added only when -march=armv70a is added.

Can someone see a better way of achieving this?

If this is acceptable, ok to commit?

Build and test arm-none-linux-gnueabi with --enable-gnu-indirect-function
Bootstrap on x86 with --enable-gnu-indirect-function

Thanks,
Kyrill

2014-02-03  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    * Makefile.in: Override -mcpu option when building arm
    linux ifunc targets.

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=cortex-a9
 @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)