linux-kernel - [PATCH v2] powerpc: Don't add -mabi= flags when building with Clang

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Message-Id: <20190820232921.102673-1-natechancellor@gmail.com>
Date:   Tue, 20 Aug 2019 16:29:22 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        Daniel Axtens <dja@...ens.net>
Subject: [PATCH v2] powerpc: Don't add -mabi= flags when building with Clang

When building pseries_defconfig, building vdso32 errors out:

  error: unknown target ABI 'elfv1'

This happens because -m32 in clang changes the target to 32-bit,
which does not allow the ABI to be changed, as the setABI virtual
function is not overridden:

https://github.com/llvm/llvm-project/blob/llvmorg-9.0.0-rc2/clang/include/clang/Basic/TargetInfo.h#L1073-L1078

https://github.com/llvm/llvm-project/blob/llvmorg-9.0.0-rc2/clang/lib/Basic/Targets/PPC.h#L327-L365

Commit 4dc831aa8813 ("powerpc: Fix compiling a BE kernel with a
powerpc64le toolchain") added these flags to fix building big endian
kernels with a little endian GCC.

Clang doesn't need -mabi because the target triple controls the default
value. -mlittle-endian and -mbig-endian manipulate the triple into
either powerpc64-* or powerpc64le-*, which properly sets the default
ABI:

https://github.com/llvm/llvm-project/blob/llvmorg-9.0.0-rc2/clang/lib/Driver/Driver.cpp#L450-L463

https://github.com/llvm/llvm-project/blob/llvmorg-9.0.0-rc2/llvm/lib/Support/Triple.cpp#L1432-L1516

https://github.com/llvm/llvm-project/blob/llvmorg-9.0.0-rc2/clang/lib/Basic/Targets/PPC.h#L377-L383

Adding a debug print out in the PPC64TargetInfo constructor after line
383 above shows this:

$ echo | ./clang -E --target=powerpc64-linux -mbig-endian -o /dev/null -
Default ABI: elfv1

$ echo | ./clang -E --target=powerpc64-linux -mlittle-endian -o /dev/null -
Default ABI: elfv2

$ echo | ./clang -E --target=powerpc64le-linux -mbig-endian -o /dev/null -
Default ABI: elfv1

$ echo | ./clang -E --target=powerpc64le-linux -mlittle-endian -o /dev/null -
Default ABI: elfv2

Don't specify -mabi when building with clang to avoid the build error
with -m32 and not change any code generation.

-mcall-aixdesc is not an implemented flag in clang so it can be
safely excluded as well, see commit 238abecde8ad ("powerpc: Don't
use gcc specific options on clang").

pseries_defconfig successfully builds after this patch and
powernv_defconfig and ppc44x_defconfig don't regress.

Link: https://github.com/ClangBuiltLinux/linux/issues/240
Reviewed-by: Daniel Axtens <dja@...ens.net>
Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
---

v1 -> v2:

* Improve commit message wording and explanation.
* Add Daniel's reviewed-by.

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

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index c345b79414a9..971b04bc753d 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -93,11 +93,13 @@ MULTIPLEWORD	:= -mmultiple
 endif
 
 ifdef CONFIG_PPC64
+ifndef CONFIG_CC_IS_CLANG
 cflags-$(CONFIG_CPU_BIG_ENDIAN)		+= $(call cc-option,-mabi=elfv1)
 cflags-$(CONFIG_CPU_BIG_ENDIAN)		+= $(call cc-option,-mcall-aixdesc)
 aflags-$(CONFIG_CPU_BIG_ENDIAN)		+= $(call cc-option,-mabi=elfv1)
 aflags-$(CONFIG_CPU_LITTLE_ENDIAN)	+= -mabi=elfv2
 endif
+endif
 
 ifndef CONFIG_CC_IS_CLANG
   cflags-$(CONFIG_CPU_LITTLE_ENDIAN)	+= -mno-strict-align
@@ -144,6 +146,7 @@ endif
 endif
 
 CFLAGS-$(CONFIG_PPC64)	:= $(call cc-option,-mtraceback=no)
+ifndef CONFIG_CC_IS_CLANG
 ifdef CONFIG_CPU_LITTLE_ENDIAN
 CFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mabi=elfv2,$(call cc-option,-mcall-aixdesc))
 AFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mabi=elfv2)
@@ -152,6 +155,7 @@ CFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mabi=elfv1)
 CFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mcall-aixdesc)
 AFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mabi=elfv1)
 endif
+endif
 CFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mcmodel=medium,$(call cc-option,-mminimal-toc))
 CFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mno-pointers-to-nested-functions)
 
-- 
2.23.0