[PATCH] powerpc/Makefile: Don't pass -mcpu=powerpc64 when building 32-bit

[PATCH] powerpc/Makefile: Don't pass -mcpu=powerpc64 when building 32-bit

[Michael Ellerman]

When CONFIG_GENERIC_CPU=y (true for all our defconfigs) we pass
-mcpu=powerpc64 to the compiler, even when we're building a 32-bit
kernel.

This happens because we have an ifdef CONFIG_PPC_BOOK3S_64/else block in
the Makefile that was written before 32-bit supported GENERIC_CPU. Prior
to that the else block only applied to 64-bit Book3E.

The GCC man page says -mcpu=powerpc64 "[specifies] a pure ... 64-bit big
endian PowerPC ... architecture machine [type], with an appropriate,
generic processor model assumed for scheduling purposes."

It's unclear how that interacts with -m32, which we are also passing,
although obviously -m32 is taking precedence in some sense, as the
32-bit kernel only contains 32-bit instructions.

This was noticed by inspection, not via any bug reports, but it does
affect code generation. Comparing before/after code generation, there
are some changes to instruction scheduling, and the after case (with
-mcpu=powerpc64 removed) the compiler seems more keen to use r8.

Fix it by making the else case only apply to Book3E 64, which excludes
32-bit.

Fixes: 0e00a8c9fd92 ("powerpc: Allow CPU selection also on PPC32")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index ddc5a706760a..1e1ef4352f62 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -171,7 +171,7 @@ else
 CFLAGS-$(CONFIG_GENERIC_CPU) += $(call cc-option,-mtune=power7,$(call cc-option,-mtune=power5))
 CFLAGS-$(CONFIG_GENERIC_CPU) += $(call cc-option,-mcpu=power5,-mcpu=power4)
 endif
-else
+else ifdef CONFIG_PPC_BOOK3E_64
 CFLAGS-$(CONFIG_GENERIC_CPU) += -mcpu=powerpc64
 endif
 
-- 
2.34.1

Re: [PATCH] powerpc/Makefile: Don't pass -mcpu=powerpc64 when building 32-bit

[Michael Ellerman]

On Tue, 15 Feb 2022 22:28:58 +1100, Michael Ellerman wrote:
> When CONFIG_GENERIC_CPU=y (true for all our defconfigs) we pass
> -mcpu=powerpc64 to the compiler, even when we're building a 32-bit
> kernel.
> 
> This happens because we have an ifdef CONFIG_PPC_BOOK3S_64/else block in
> the Makefile that was written before 32-bit supported GENERIC_CPU. Prior
> to that the else block only applied to 64-bit Book3E.
> 
> [...]

Applied to powerpc/next.

[1/1] powerpc/Makefile: Don't pass -mcpu=powerpc64 when building 32-bit
      https://git.kernel.org/powerpc/c/2863dd2db23e0407f6c50b8ba5c0e55abef894f1

cheers