linux-kernel - Re: [PATCH v5 1/3] powerpc: Don't add -mabi= flags when building with Clang

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Message-Id: <47MQrd1PSrz9sPZ@ozlabs.org>
Date:   Tue, 26 Nov 2019 12:13:21 +1100 (AEDT)
From:   Michael Ellerman <patch-notifications@...erman.id.au>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        linuxppc-dev@...ts.ozlabs.org, Daniel Axtens <dja@...ens.net>
Subject: Re: [PATCH v5 1/3] powerpc: Don't add -mabi= flags when building with Clang

On Tue, 2019-11-19 at 04:57:10 UTC, Nathan Chancellor wrote:
> 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/clang/include/clang/Basic/TargetInfo.h#L1073-L1078
> 
> https://github.com/llvm/llvm-project/blob/llvmorg-9.0.0/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/clang/lib/Driver/Driver.cpp#L450-L463
> 
> https://github.com/llvm/llvm-project/blob/llvmorg-9.0.0/llvm/lib/Support/Triple.cpp#L1432-L1516
> 
> https://github.com/llvm/llvm-project/blob/llvmorg-9.0.0/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>

Series applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/465bfd9c44dea6b55962b5788a23ac87a467c923

cheers