Re: [PATCH v2] powerpc/32: Avoid unsupported flags with clang

[Nick Desaulniers <ndesaulniers@google.com>]

On Sun, Nov 11, 2018 at 9:28 PM Joel Stanley <joel@jms.id.au> wrote:
>
> When building for ppc32 with clang these flags are unsupported:
>
>   -ffixed-r2 and -mmultiple
>
> llvm's lib/Target/PowerPC/PPCRegisterInfo.cpp marks r2 as reserved on
> when building for SVR4ABI and !ppc64:
>
>   // The SVR4 ABI reserves r2 and r13
>   if (Subtarget.isSVR4ABI()) {
>     // We only reserve r2 if we need to use the TOC pointer. If we have no
>     // explicit uses of the TOC pointer (meaning we're a leaf function with
>     // no constant-pool loads, etc.) and we have no potential uses inside an
>     // inline asm block, then we can treat r2 has an ordinary callee-saved
>     // register.
>     const PPCFunctionInfo *FuncInfo = MF.getInfo<PPCFunctionInfo>();
>     if (!TM.isPPC64() || FuncInfo->usesTOCBasePtr() || MF.hasInlineAsm())
>       markSuperRegs(Reserved, PPC::R2);  // System-reserved register
>     markSuperRegs(Reserved, PPC::R13); // Small Data Area pointer register
>   }
>
> This means we can safely omit -ffixed-r2 when building for 32-bit
> targets.
>
> The -mmultiple/-mno-multiple flags are not supported by clang, so
> platforms that might support multiple miss out on using multiple word
> instructions.
>
> We wrap these flags in cc-option so that when Clang gains support the
> kernel will be able use these flags.
>
> Clang 8 can then build a ppc44x_defconfig which boots in Qemu:
>
>   make CC=clang-8 ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu-  ppc44x_defconfig
>   ./scripts/config -e CONFIG_DEVTMPFS -d DEVTMPFS_MOUNT
>   make CC=clang-8 ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu-
>
>   qemu-system-ppc -M bamboo \
>    -kernel arch/powerpc/boot/zImage \
>    -dtb arch/powerpc/boot/dts/bamboo.dtb \
>    -initrd ~/ppc32-440-rootfs.cpio \
>    -nographic -serial stdio -monitor pty -append "console=ttyS0"
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/261
> Link: https://bugs.llvm.org/show_bug.cgi?id=39556
> Link: https://bugs.llvm.org/show_bug.cgi?id=39555
> Signed-off-by: Joel Stanley <joel@jms.id.au>

Hopefully we can get these fixed up soon for you, thanks Joel!
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>  arch/powerpc/Makefile | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
> index 8a2ce14d68d0..4685671dfb4f 100644
> --- a/arch/powerpc/Makefile
> +++ b/arch/powerpc/Makefile
> @@ -152,7 +152,14 @@ endif
>  CFLAGS-$(CONFIG_PPC64) += $(call cc-option,-mcmodel=medium,$(call cc-option,-mminimal-toc))
>  CFLAGS-$(CONFIG_PPC64) += $(call cc-option,-mno-pointers-to-nested-functions)
>
> -CFLAGS-$(CONFIG_PPC32) := -ffixed-r2 $(MULTIPLEWORD)
> +# Clang unconditionally reserves r2 on ppc32 and does not support the flag
> +# https://bugs.llvm.org/show_bug.cgi?id=39555
> +CFLAGS-$(CONFIG_PPC32) := $(call cc-option, -ffixed-r2)
> +
> +# Clang doesn't support -mmultiple / -mno-multiple
> +# https://bugs.llvm.org/show_bug.cgi?id=39556
> +CFLAGS-$(CONFIG_PPC32) += $(call cc-option, $(MULTIPLEWORD))
> +
>  CFLAGS-$(CONFIG_PPC32) += $(call cc-option,-mno-readonly-in-sdata)
>
>  ifdef CONFIG_PPC_BOOK3S_64
> --
> 2.19.1
>


-- 
Thanks,
~Nick Desaulniers