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:

quoted

On Sun, Aug 18, 2019 at 12:13:21PM -0700, Nathan Chancellor wrote:

quoted

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.

quoted

(-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