| Message ID | 20220215112858.304779-1-mpe@ellerman.id.au (mailing list archive) |
|---|---|
| State | Accepted |
| Headers | show |
| Series | powerpc/Makefile: Don't pass -mcpu=powerpc64 when building 32-bit | expand |
| Context | Check | Description |
|---|---|---|
| snowpatch_ozlabs/github-powerpc_selftests | success | Successfully ran 8 jobs. |
| snowpatch_ozlabs/github-powerpc_ppctests | success | Successfully ran 8 jobs. |
| snowpatch_ozlabs/github-powerpc_kernel_qemu | success | Successfully ran 24 jobs. |
| snowpatch_ozlabs/github-powerpc_sparse | success | Successfully ran 4 jobs. |
| snowpatch_ozlabs/github-powerpc_clang | success | Successfully ran 7 jobs. |
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
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
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(-)