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

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Message-ID: <20190820124033.GQ31406@gate.crashing.org>
Date:   Tue, 20 Aug 2019 07:40:33 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Nathan Chancellor <natechancellor@...il.com>
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 08:15:38PM -0700, Nathan Chancellor wrote:
> 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.

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

What flag?  -mabi=elfv[12]?

(Only irrelevant things are ever ignored; otherwise, please do a bug
report).

> 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).

You can have elfv2 on BE, and e.g. the sysv ABI on LE.  Neither of those
is tested a lot.

> > (-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?

Yup.

> 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.

That depends on what you call "an ABI", I guess.  You can call any ABI
variant a separate ABI: you'll have to rebuild all of userland.  You can
also says ELFv1 and ELFv2 are pretty much the same thing, which is true
as well.  The way -mabi= is defined is the latter:

'-mabi=ABI-TYPE'
     Extend the current ABI with a particular extension, or remove such
     extension.  Valid values are 'altivec', 'no-altivec',
     'ibmlongdouble', 'ieeelongdouble', 'elfv1', 'elfv2'.


Segher