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

[Joel Stanley <joel@jms.id.au>]

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.

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
  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
Signed-off-by: Joel Stanley <joel@jms.id.au>
---
I considered putting these behind a cc-option test. If someone has an
opinion on that I'm happy to change.

 arch/powerpc/Makefile | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index 17be664dafa2..5fa678eee747 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -152,7 +152,9 @@ 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 $(MULTIPLEWORD)
+ifndef CONFIG_CC_IS_CLANG
+#CFLAGS-$(CONFIG_PPC32)	:= -ffixed-r2 $(MULTIPLEWORD)
+endif
 CFLAGS-$(CONFIG_PPC32)	+= $(call cc-option,-mno-readonly-in-sdata)
 
 ifdef CONFIG_PPC_BOOK3S_64
-- 
2.19.1