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fix: heap-allocate thread-local random buffer to reduce thread creation overhead #20488

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Merged
johnathan79717 merged 1 commit into from
Feb 24, 2026
+9 −4
Merged

fix: heap-allocate thread-local random buffer to reduce thread creation overhead #20488

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13 changes: 9 additions & 4 deletions barretenberg/cpp/src/barretenberg/numeric/random/engine.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@
#include <array>
#include <cstring>
#include <functional>
#include <memory>
#include <random>
#if defined(__APPLE__)
#include <TargetConditionals.h>
Expand Down Expand Up @@ -44,8 +45,9 @@ constexpr size_t RANDOM_BUFFER_SIZE = 1UL << 20;

#endif
struct RandomBufferWrapper {
// Buffer with randomness sampled from a CSPRNG
uint8_t buffer[RANDOM_BUFFER_SIZE];
// Buffer with randomness sampled from a CSPRNG (heap-allocated on first use to avoid
// bloating TLS — a 1 MiB inline array adds ~0.6 ms per thread creation)
std::unique_ptr<uint8_t[]> buffer;
// Offset into the unused part of the buffer
ssize_t offset = -1;
};
Expand All @@ -67,8 +69,11 @@ template <size_t size_in_unsigned_ints> std::array<unsigned int, size_in_unsigne
// We could preserve the leftover bytes, but it's a bit messy
if (random_buffer_wrapper.offset == -1 ||
(static_cast<size_t>(random_buffer_wrapper.offset) + random_data_buffer_size) > RANDOM_BUFFER_SIZE) {
if (!random_buffer_wrapper.buffer) {
random_buffer_wrapper.buffer = std::make_unique<uint8_t[]>(RANDOM_BUFFER_SIZE);
}
size_t bytes_left = RANDOM_BUFFER_SIZE;
uint8_t* current_offset = random_buffer_wrapper.buffer;
uint8_t* current_offset = random_buffer_wrapper.buffer.get();
// Sample until we fill the buffer
while (bytes_left != 0) {
#if defined(__wasm__) || defined(__APPLE__)
Expand Down Expand Up @@ -97,7 +102,7 @@ template <size_t size_in_unsigned_ints> std::array<unsigned int, size_in_unsigne
random_buffer_wrapper.offset = 0;
}

memcpy(&random_data, random_buffer_wrapper.buffer + random_buffer_wrapper.offset, random_data_buffer_size);
memcpy(&random_data, random_buffer_wrapper.buffer.get() + random_buffer_wrapper.offset, random_data_buffer_size);
random_buffer_wrapper.offset += static_cast<ssize_t>(random_data_buffer_size);
return random_data;
}
Expand Down
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