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Improve codegen for start of object.

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By doubling the mod_mult array, we cause the division part of the
calculation to completely overflow, and thus remove one instruction from
fast path.
This commit is contained in:
Matthew Parkinson
2021-04-06 12:17:36 +01:00
committed by Matthew Parkinson
parent 9ca73b8153
commit d1acb7979a
1 changed files with 6 additions and 2 deletions
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8
src/mem/sizeclasstable.h
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@@ -65,6 +65,10 @@ namespace snmalloc
(bits::one_at_bit(bits::BITS - 1) / size[sizeclass]);
if (!bits::is_pow2(size[sizeclass]))
mod_mult[sizeclass]++;
// Shift multiplier, so that the result of division completely
// overflows, and thus the top SUPERSLAB_BITS will be zero if the mod is
// zero.
mod_mult[sizeclass] *= 2;
if (sizeclass < NUM_SMALL_CLASSES)
{
@@ -214,8 +218,8 @@ namespace snmalloc
// square of the offset to be representable.
static_assert(
SUPERSLAB_BITS <= 24, "The following code assumes max of 24 bits");
static constexpr size_t MASK = (bits::one_at_bit(bits::BITS - 1) - 1) ^
(bits::one_at_bit(bits::BITS - 1 - SUPERSLAB_BITS) - 1);
static constexpr size_t MASK =
~(bits::one_at_bit(bits::BITS - 1 - SUPERSLAB_BITS) - 1);
return ((offset * sizeclass_metadata.mod_mult[sc]) & MASK) == 0;
}