[PATCH AUTOSEL 4.19 119/219] powerpc/32: Avoid unsupported flags with clang

Sasha Levin
2019-11-22T06:24:34Z

From: Joel Stanley <joel@jms.id.au>

[ Upstream commit 72e7bcc2cdf82bf03caaa5e6c9b0134c2fc2ee7d ]

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>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/powerpc/Makefile | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index c4c03992ee828..dfcb698ec8f3b 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -145,7 +145,14 @@ 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
--
2.20.1