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PR libstdc++/90046 fix build failure on epiphany-elf

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The epiphany-elf target aligns structs to 8 bytes, which causes the
static_assert(alignof(_Chunk) == 1) to fail.

Instead of requiring _Chunks to be positionable at any alignment, ensure
new buffers are aligned to alignof(_Chunk). Because the buffer size is a
power of two, we know that both the buffer size and sizeof(_Chunk) are
multiples of alignof(_Chunk). So is p is aligned to alignof(_Chunk) then
so is (p + size - sizeof(_Chunk)). So just ensure the new buffer is
aligned to at least alignof(_Chunk), which should already be true
because the caller requests at least alignof(max_align_t).

	PR libstdc++/90046
	* src/c++17/memory_resource.cc
	(monotonic_buffer_resource::_Chunk::allocate): Increase alignment if
	needed to allow placing a _Chunk at the end of the buffer.
	(monotonic_buffer_resource::_M_new_buffer): Remove static_assert.

From-SVN: r270291
This commit is contained in:
Jonathan Wakely 2019-04-11 20:58:14 +01:00 committed by Jonathan Wakely
parent 76d7e3f024
commit 2c390a483b
2 changed files with 22 additions and 3 deletions
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8
libstdc++-v3/ChangeLog
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@ -1,3 +1,11 @@
2019-04-11 Jonathan Wakely <jwakely@redhat.com>
PR libstdc++/90046
* src/c++17/memory_resource.cc
(monotonic_buffer_resource::_Chunk::allocate): Increase alignment if
needed to allow placing a _Chunk at the end of the buffer.
(monotonic_buffer_resource::_M_new_buffer): Remove static_assert.
2019-04-10 Jonathan Wakely <jwakely@redhat.com>
* doc/xml/faq.xml: Add information about emergency EH pool.

17
libstdc++-v3/src/c++17/memory_resource.cc
View File

@ -182,7 +182,21 @@ namespace pmr
_Chunk*& __head)
{
__size = std::__ceil2(__size + sizeof(_Chunk));
if constexpr (alignof(_Chunk) > 1)
{
// PR libstdc++/90046
// For targets like epiphany-elf where alignof(_Chunk) != 1
// ensure that the last sizeof(_Chunk) bytes in the buffer
// are suitably-aligned for a _Chunk.
// This should be unnecessary, because the caller already
// passes in max(__align, alignof(max_align_t)).
if (__align < alignof(_Chunk))
__align = alignof(_Chunk);
}
void* __p = __r->allocate(__size, __align);
// Add a chunk defined by (__p, __size, __align) to linked list __head.
void* const __back = (char*)__p + __size - sizeof(_Chunk);
__head = ::new(__back) _Chunk(__size, __align, __head);
@ -231,9 +245,6 @@ namespace pmr
void
monotonic_buffer_resource::_M_new_buffer(size_t bytes, size_t alignment)
{
// Need to check this somewhere, so put it here:
static_assert(alignof(monotonic_buffer_resource::_Chunk) == 1);
const size_t n = std::max(bytes, _M_next_bufsiz);
const size_t m = std::max(alignment, alignof(std::max_align_t));
auto [p, size] = _Chunk::allocate(_M_upstream, n, m, _M_head);