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Fix redundant usage of -mcpu and -march/-mtune

Thomas Petazzoni buildroot 1 version New

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Cover letter [Buildroot] [PATCH 0/1] Fix redundant usage of -mcpu and -march/-mtune

Dear K?roly Kasza,

Quoted 9 lines · K?roly Kasza
Quoted 6 lines

So normally, we should set BR2_GCC_TARGET_ARCH to armv7ve for those Cortex-A variants. I'm currently experimenting with this, but I'm having a few issues. Also, it's likely that armv7ve is quite recent, and may not be present in older gcc versions.

That's the case with the Linaro 2014.02 toolchain - in GCC 4.8.2 the armv7ve does not exist
(http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/ARM-Options.html#ARM-Options).

Yeah, in gcc 4.8.2 vanilla. But Linaro toolchains don't use a vanilla gcc, they have lots of ARM related patches. And if the Linaro toolchains complains that -march=armv7-a is incompatible with -mcpu=cortex-a7, then quite certainly it means that cortex-a7 is part of a different -march family, according to this toolchain.

Quoted 8 lines
Quoted 4 lines

This is already the case. They are only passed if BR2_GCC_TARGET_{ARCH,CPU,TUNE} are defined. Some architectures (such as ARM) defines all three, but other architectures (such as x86) do not.

Now I see. In this case, maybe the simplest would be to just leave BR2_GCC_TARGET_ARCH
undefined in arch/Config.in.arm if the user selected a proper CPU (and not just some generic one).

Not sure what you mean by "not just some generic one". In the menuconfig, what we ask the user is to define the specific ARM core being used. From that we can derive the -march and -mcpu values. But indeed, the -march value is quite useless.

However, I believe there are place in Buildroot where we do use the BR2_GCC_TARGET_ARCH value for other things than just the compiler flags. We would have to audit those before removing BR2_GCC_TARGET_ARCH from Config.in.arm.

Quoted 2 lines

Also, based on the documentation it should be completely unnecessary to define -march if we have the accurate CPU with -mcpu (on ARM at least).

Correct.

Best regards,

Thomas

Signature · 3 lines
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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