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support aligned allocation calls with alignments < pointer size #246

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support aligned allocation calls with alignments < pointer size #246

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7 changes: 5 additions & 2 deletions src/alloc-aligned.c
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Original file line number Diff line number Diff line change
Expand Up @@ -14,13 +14,16 @@ terms of the MIT license. A copy of the license can be found in the file
// Aligned Allocation
// ------------------------------------------------------

static void* mi_heap_malloc_zero_aligned_at(mi_heap_t* const heap, const size_t size, const size_t alignment, const size_t offset, const bool zero) mi_attr_noexcept {
static void* mi_heap_malloc_zero_aligned_at(mi_heap_t* const heap, const size_t size, size_t alignment, const size_t offset, const bool zero) mi_attr_noexcept {
// note: we don't require `size > offset`, we just guarantee that
// the address at offset is aligned regardless of the allocated size.
mi_assert(alignment > 0 && alignment % sizeof(void*) == 0);

if (mi_unlikely(size > PTRDIFF_MAX)) return NULL; // we don't allocate more than PTRDIFF_MAX (see <https://sourceware.org/ml/libc-announce/2019/msg00001.html>)
if (mi_unlikely(alignment==0 || !_mi_is_power_of_two(alignment))) return NULL; // require power-of-two (see <https://en.cppreference.com/w/c/memory/aligned_alloc>)

// if the passed alignment is too small, correct it upwards to the minimal internal required alignment
// fixes issues in user code calling aligned variants for types with small alignment requirements below pointer alignment
alignment = alignment < sizeof(void*) ? sizeof(void*) : alignment;
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I think this should be below the if()s below which check that we were fed with a non-0 power-of-two first, before alignment is changed. Those requirements are sane.

const uintptr_t align_mask = alignment-1; // for any x, `(x & align_mask) == (x % alignment)`

// try if there is a small block available with just the right alignment
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