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[libc] Search empty bits after failed allocation #149910

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Merged
merged 1 commit into from
Jul 23, 2025
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[libc] Search empty bits after failed allocation #149910

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16 changes: 13 additions & 3 deletions libc/src/__support/GPU/allocator.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -251,12 +251,18 @@ struct Slab {
// The uniform mask represents which lanes contain a uniform target pointer.
// We attempt to place these next to each other.
void *result = nullptr;
uint32_t after = ~0u;
uint32_t old_index = 0;
for (uint64_t mask = lane_mask; mask;
mask = gpu::ballot(lane_mask, !result)) {
if (result)
continue;

uint32_t start = gpu::broadcast_value(lane_mask, impl::xorshift32(state));
// We try using any known empty bits from the previous attempt first.
uint32_t start = gpu::shuffle(mask, cpp::countr_zero(uniform & mask),
~after ? (old_index & ~(BITS_IN_WORD - 1)) +
cpp::countr_zero(~after)
: impl::xorshift32(state));

uint32_t id = impl::lane_count(uniform & mask);
uint32_t index = (start + id) % usable_bits(chunk_size);
Expand All @@ -266,8 +272,9 @@ struct Slab {
// Get the mask of bits destined for the same slot and coalesce it.
uint64_t match = uniform & gpu::match_any(mask, slot);
uint32_t length = cpp::popcount(match);
uint32_t bitmask = static_cast<uint32_t>((uint64_t(1) << length) - 1)
<< bit;
uint32_t bitmask = gpu::shuffle(
mask, cpp::countr_zero(match),
static_cast<uint32_t>((uint64_t(1) << length) - 1) << bit);

uint32_t before = 0;
if (gpu::get_lane_id() == static_cast<uint32_t>(cpp::countr_zero(match)))
Expand All @@ -278,6 +285,9 @@ struct Slab {
result = ptr_from_index(index, chunk_size);
else
sleep_briefly();

after = before | bitmask;
old_index = index;
}

cpp::atomic_thread_fence(cpp::MemoryOrder::ACQUIRE);
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