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

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.

Ping?
http://gcc.gnu.org/ml/gcc-patches/2014-02/msg00069.html

On 03/02/14 11:50, Kyrill Tkachov wrote:
> 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.

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.

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?


r~