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

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Message-ID: <20190820031538.GC30221@archlinux-threadripper>
Date:   Mon, 19 Aug 2019 20:15:38 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        clang-built-linux@...glegroups.com, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: Don't add -mabi= flags when building with Clang

On Mon, Aug 19, 2019 at 04:19:31AM -0500, Segher Boessenkool wrote:
> On Sun, Aug 18, 2019 at 12:13:21PM -0700, Nathan Chancellor wrote:
> > When building pseries_defconfig, building vdso32 errors out:
> > 
> >   error: unknown target ABI 'elfv1'
> > 
> > Commit 4dc831aa8813 ("powerpc: Fix compiling a BE kernel with a
> > powerpc64le toolchain") added these flags to fix building GCC but
> > clang is multitargeted and does not need these flags. The ABI is
> > properly set based on the target triple, which is derived from
> > CROSS_COMPILE.
> 
> You mean that LLVM does not *allow* you to select a different ABI, or
> different ABI options, you always have to use the default.  (Everything
> else you say is true for GCC as well).

I need to improve the wording of the commit message as it is really that
clang does not allow a different ABI to be selected for 32-bit PowerPC,
as the setABI function is not overridden and it defaults to false.

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

GCC appears to just silently ignores this flag (I think it is the
SUBSUBTARGET_OVERRIDE_OPTIONS macro in gcc/config/rs6000/linux64.h).

It can be changed for 64-bit PowerPC it seems but it doesn't need to be
with clang because everything is set properly internally (I'll find a
better way to clearly word that as I am sure I'm not quite getting that
subtlety right).

> (-mabi= does not set a "target ABI", fwiw, it is more subtle; please see
> the documentation.  Unless LLVM is incompatible in that respect as well?)

Are you referring to the error message? I suppose I could file an LLVM
bug report on that but that message applies to all of the '-mabi='
options, which may refer to a target ABI.

Cheers,
Nathan