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Re: [PATCH] libstdc++-v3: Have aligned_alloc() on Newlib


On 08/08/18 16:01, Jonathan Wakely wrote:
On 08/08/18 15:57 +0200, Ulrich Weigand wrote:
Jonathan Wakely wrote:
On 08/08/18 10:52 +0200, Sebastian Huber wrote:
>While building for Newlib, some configure checks must be hard coded.
>The aligned_alloc() is supported since 2015 in Newlib.
>
>libstdc++-v3
>
>    PR target/85904
>    * configure.ac): Define HAVE_ALIGNED_ALLOC if building for

There's a stray closing parenthesis here.

>    Newlib.
>    * configure: Regnerate.

Typo "Regnerate".

But the patch itself is fine - OK for trunk.

I'm ambivalent about this being backported to gcc-7 and gcc-8 branches
(gcc-6 is unaffected as it doesn't use aligned_alloc).

It's strictly speaking an ABI change, because HAVE_ALIGNED_ALLOC
affects the memory layout for allocations from operator new(size_t,
align_val_t) (in new_opa.cc) which needs to agree with the
corresponding operator delete (in del_opa.cc). Using static linking it
might be possible to create a binary that has operator new using
aligned_alloc, but operator delete expecting to do ((void**)ptr)[-1],
which would be bad.

Those operators are C++17, so "experimental", but maybe we shouldn't
make the change on release branches.

The way it is now I'm getting build failures on new SPU target
(which is newlib based):

/home/uweigand/dailybuild/spu-tc-2018-08-07/gcc-head/src/libstdc++-v3/libsupc++/new_opa.cc:58:1: error: 'void* aligned_alloc(std::size_t, std::size_t)' was declared 'extern' and later 'static' [-fpermissive]
aligned_alloc (std::size_t al, std::size_t sz)
^~~~~~~~~~~~~
In file included from /home/uweigand/dailybuild/spu-tc-2018-08-07/gcc-build/spu/libstdc++-v3/include/cstdlib:75:62,                 from /home/uweigand/dailybuild/spu-tc-2018-08-07/gcc-build/spu/libstdc++-v3/include/stdlib.h:36,                 from /home/uweigand/dailybuild/spu-tc-2018-08-07/gcc-head/src/libstdc++-v3/libsupc++/new_opa.cc:27: /home/uweigand/dailybuild/spu-tc-2018-08-07/spu-toolchain/spu/include/stdlib.h:328:8: note: previous declaration of 'void* aligned_alloc(size_t, size_t)'
void * aligned_alloc(size_t, size_t) __malloc_like __alloc_align(1)
       ^~~~~~~~~~~~~

This seems to be because configure is hard-coded to assume aligned_alloc
is not available, but then the new_opc.cc file tries to define its own
version, conflicting with the newlib prototype that is actually there.

So one way or the other this needs to be fixed ...

But that's on trunk, right? Sebastian is about to fix that.

I expect the same problem exists on the branches too though.

I'm surprised nobody has reported this until now. It's been like that
since r244933 in Jan 2017.

I notices this problem today with a build of the GCC 7 branch. I am able to build a GCC 7.3.0.

--
Sebastian Huber, embedded brains GmbH

Address : Dornierstr. 4, D-82178 Puchheim, Germany
Phone   : +49 89 189 47 41-16
Fax     : +49 89 189 47 41-09
E-Mail  : sebastian.huber@embedded-brains.de
PGP     : Public key available on request.

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