[PATCH] D55057: [Headers] Make max_align_t match GCC's implementation.

[Ulrich Weigand via Phabricator via cfe-commits]

uweigand added inline comments.


================
Comment at: lib/Headers/__stddef_max_align_t.h:40
       __attribute__((__aligned__(__alignof__(long double))));
+#ifdef __i386__
+  __float128 __clang_max_align_nonce3
----------------
jyknight wrote:
> EricWF wrote:
> > uweigand wrote:
> > > jyknight wrote:
> > > > uweigand wrote:
> > > > > jyknight wrote:
> > > > > > Can you fix clang to consistently define `__SIZEOF_FLOAT128__` in InitPreprocessor alongside the rest of the SIZEOF macros?
> > > > > > 
> > > > > > And then use that to determine whether to add float128 to the union? This change, as is, will break on any target which is i386 but doesn't define __float128, e.g. i386-unknown-unknown.
> > > > > > 
> > > > > > The only additional target which will be modified with that (that is: the only other target which has a float128 type, but for which max_align isn't already 16) is systemz-*-linux.
> > > > > > 
> > > > > > But I think that's actually indicating a pre-existing bug in the SystemZ config -- it's configured for a 16-byte long double, with 8-byte alignment, but the ABI doc seems to call for 16-byte alignment. +Ulrich for comment on that.
> > > > > That's a bug in the ABI doc which we'll fix once we get around to releasing an updated version.
> > > > > 
> > > > > long double on SystemZ must be 8-byte aligned, which is the maximum alignment of all standard types on Z, and this was chosen because historically the ABI only guarantees an 8-byte stack alignment, so 16-byte aligned standard types would be awkward.
> > > > Then perhaps it's a bug that `__alignof__(__float128)` returns 16 with `-target systemz-linux`?
> > > Huh, really __float128 should not be supported at all on SystemZ.  It's not supported with GCC either (GCC never provides __float128 on targets where long double is already IEEE-128).  (GCC does support the new _Float128 on SystemZ, but that's not yet supported by clang anywhere.)
> > > 
> > > If it were supported, I agree it should be an alias for long double, and also have an alignof of 8.
> > @jyknight Ack. I'll add `__SIZEOF_FLOAT128__`.
> @uweigand : One of those fixes needs to land before this, so that systemz's max_align_t doesn't change to 16 in the meantime. I think your preference would be for it to be simply removed, right? Looks like the type was originally added in https://reviews.llvm.org/D19125 -- possibly in error, since the focus was x86_64.
@jyknight : Yes, this seems to have been simply an error.  I'll check in a patch to remove `__float128` on SystemZ.



Repository:
  rC Clang

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D55057/new/

https://reviews.llvm.org/D55057