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localalloc::dealloc tweak size math for large objects

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Avoid computing bits::next_pow2_bits(1 << n).  Even if the compiler can see
through enough of the algebra, it's surely more direct to just use n.

While here, slightly expand documentation about what's going on with the
"sizeclass" encoded into MetaEntry-s.
This commit is contained in:
Nathaniel Wesley Filardo
2021-10-17 18:03:39 +01:00
committed by Nathaniel Wesley Filardo
parent 52a4b0c8d0
commit 554db3997d
4 changed files with 40 additions and 5 deletions
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18
src/mem/localalloc.h
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@@ -508,21 +508,31 @@ namespace snmalloc
// Large deallocation or null.
if (likely(p_tame != nullptr))
{
// Check this is managed by this pagemap.
check_client(entry.get_sizeclass() != 0, "Not allocated by snmalloc.");
size_t entry_sizeclass = entry.get_sizeclass();
size_t size = bits::one_at_bit(entry.get_sizeclass());
// Check this is managed by this pagemap.
//
// TODO: Should this be tested even in the !CHECK_CLIENT case? Things
// go fairly pear-shaped, with the ASM's ranges[] getting cross-linked
// with a ChunkAllocator's chunk_stack[0], which seems bad.
check_client(entry_sizeclass != 0, "Not allocated by snmalloc.");
size_t size = bits::one_at_bit(entry_sizeclass);
size_t slab_sizeclass =
metaentry_chunk_sizeclass_to_slab_sizeclass(entry_sizeclass);
// Check for start of allocation.
check_client(
pointer_align_down(p_tame, size) == p_tame,
"Not start of an allocation.");
size_t slab_sizeclass = large_size_to_chunk_sizeclass(size);
# ifdef SNMALLOC_TRACING
std::cout << "Large deallocation: " << size
<< " chunk sizeclass: " << slab_sizeclass << std::endl;
# else
UNUSED(size);
# endif
ChunkRecord* slab_record =
reinterpret_cast<ChunkRecord*>(entry.get_metaslab());
/*