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Small sizeclass lookup improvement, (#777)

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* Fix off by one in deciding where to switch from small to large.
* Improve codegen on Windows by making it simpler for compiler to remove a redundant branch
This commit is contained in:
Matthew Parkinson
2025-07-01 12:02:10 +01:00
committed by GitHub
parent 2d33e4f33b
commit 012138e29f
1 changed files with 6 additions and 5 deletions
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11
src/snmalloc/mem/sizeclasstable.h
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@@ -32,7 +32,7 @@ namespace snmalloc
}
constexpr size_t NUM_SMALL_SIZECLASSES =
size_to_sizeclass_const(MAX_SMALL_SIZECLASS_SIZE);
size_to_sizeclass_const(MAX_SMALL_SIZECLASS_SIZE) + 1;
// Large classes range from [MAX_SMALL_SIZECLASS_SIZE, ADDRESS_SPACE).
constexpr size_t NUM_LARGE_CLASSES =
@@ -41,7 +41,7 @@ namespace snmalloc
// How many bits are required to represent either a large or a small
// sizeclass.
constexpr size_t TAG_SIZECLASS_BITS = bits::max<size_t>(
bits::next_pow2_bits_const(NUM_SMALL_SIZECLASSES + 1),
bits::next_pow2_bits_const(NUM_SMALL_SIZECLASSES),
bits::next_pow2_bits_const(NUM_LARGE_CLASSES + 1));
// Number of bits required to represent a tagged sizeclass that can be
@@ -410,7 +410,7 @@ namespace snmalloc
}
constexpr size_t sizeclass_lookup_size =
sizeclass_lookup_index(MAX_SMALL_SIZECLASS_SIZE);
sizeclass_lookup_index(MAX_SMALL_SIZECLASS_SIZE) + 1;
/**
* This struct is used to statically initialise a table for looking up
@@ -470,9 +470,10 @@ namespace snmalloc
constexpr smallsizeclass_t size_to_sizeclass(size_t size)
{
auto index = sizeclass_lookup_index(size);
if (index < sizeclass_lookup_size)
if (SNMALLOC_LIKELY(is_small_sizeclass(size)))
{
auto index = sizeclass_lookup_index(size);
SNMALLOC_ASSERT(index < sizeclass_lookup_size);
return sizeclass_lookup.table[index];
}