On Mon, Apr 23, 2018 at 04:49:24PM -0400, Jason Merrill wrote:
> On Tue, Apr 17, 2018 at 8:50 AM, Joseph Myers <jos...@codesourcery.com> wrote:
> > On Tue, 10 Apr 2018, Jason Merrill wrote:
> >
> >> But really this is beside the point: the x86 ABI says that the
> >> alignment of double is 4, so alignof(double) should be 4 regardless of
> >> what GCC wants to do internally. And I think the same is true of
> >> __alignof__.
> >
> > __alignof__ needs to stay reflecting the preferred, standalone alignment
> > of 8 bytes; changing that is ABI-incompatible; code such as that in
> > stddef.h uses __attribute__((__aligned__(__alignof__(long long)))) to give
> > structure members the same alignment those types would have for standalone
> > objects. (This doesn't affect the alignment of max_align_t *now* on i386,
> > but only because that now includes __float128 as well.)
>
> In that case, here's a patch following the 52023 change to the C front
> end, to change only "alignof".
>
> Tested x86_64-pc-linux-gnu, applying to trunk.
This regressed:
+FAIL: c-c++-common/attr-aligned-1.c -std=c++11 (test for excess errors)
+UNRESOLVED: c-c++-common/attr-aligned-1.c -std=c++11 compilation failed to
produce executable
+FAIL: c-c++-common/attr-aligned-1.c -std=c++14 (test for excess errors)
+UNRESOLVED: c-c++-common/attr-aligned-1.c -std=c++14 compilation failed to
produce executable
+FAIL: g++.dg/cpp0x/alignas4.C -std=c++11 scan-assembler align 8
+FAIL: g++.dg/cpp0x/alignas4.C -std=c++14 scan-assembler align 8
on i686-linux, can be reproduced even with just
make check-gcc check-g++ RUNTESTFLAGS="--target_board=unix\{-m64,-m32\}
dg.exp='attr-aligned-1.c alignas4.C'"
on x86_64-linux.
On alignas4.C, this was changed with r249409, perhaps we can limit
the target to && { ! ia32 } or similar, or do we want to expect align 4
for ia32?
attr-aligned-1.c testcase doesn't work if
__alignof__ (double) != alignof (double). Either we should use __alignof__
in the static_asserts, or guard the static_asserts for targets known not to
have different results between the two.
> commit c42f365685b86fa7cecc2666adf93037f7bd0289
> Author: Jason Merrill <ja...@redhat.com>
> Date: Tue Apr 10 11:48:51 2018 -0400
>
> PR c++/69560 - wrong alignof(double) on x86.
>
> CWG 1879 - Inadequate definition of alignment requirement.
> * cp-tree.h (ALIGNOF_EXPR_STD_P): New.
> * typeck.c (cxx_sizeof_or_alignof_type): Add std_alignof parm.
> (cxx_sizeof_expr, cxx_sizeof_nowarn, cxx_alignas_expr)
> (cxx_alignof_expr): Pass it.
> * parser.c (cp_parser_unary_expression): Pass it.
> * pt.c (tsubst_copy): Copy it.
> (tsubst_copy_and_build): Pass it.
> * decl.c (fold_sizeof_expr): Pass it.
Jakub
