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#ifndef HeterogeneousCore_AlpakaInterface_interface_prefixScan_h
#define HeterogeneousCore_AlpakaInterface_interface_prefixScan_h
#include <alpaka/alpaka.hpp>
#include "FWCore/Utilities/interface/CMSUnrollLoop.h"
#include "HeterogeneousCore/AlpakaInterface/interface/config.h"
#include "HeterogeneousCore/AlpakaInterface/interface/workdivision.h"
namespace cms::alpakatools {
template <typename T, typename = std::enable_if_t<std::is_integral_v<T>>>
constexpr bool isPowerOf2(T v) {
// returns true iif v has only one bit set.
while (v) {
if (v & 1)
return !(v >> 1);
else
v >>= 1;
}
return false;
}
template <typename TAcc, typename T, typename = std::enable_if_t<alpaka::isAccelerator<TAcc>>>
ALPAKA_FN_ACC ALPAKA_FN_INLINE void warpPrefixScan(
const TAcc& acc, int32_t laneId, T const* ci, T* co, uint32_t i, bool active = true) {
// ci and co may be the same
T x = active ? ci[i] : 0;
CMS_UNROLL_LOOP
for (int32_t offset = 1; offset < alpaka::warp::getSize(acc); offset <<= 1) {
// Force the exact type for integer types otherwise the compiler will find the template resolution ambiguous.
using dataType = std::conditional_t<std::is_floating_point_v<T>, T, std::int32_t>;
T y = alpaka::warp::shfl(acc, static_cast<dataType>(x), laneId - offset);
if (laneId >= offset)
x += y;
}
if (active)
co[i] = x;
}
template <typename TAcc, typename T, typename = std::enable_if_t<alpaka::isAccelerator<TAcc>>>
ALPAKA_FN_ACC ALPAKA_FN_INLINE void warpPrefixScan(
const TAcc& acc, int32_t laneId, T* c, uint32_t i, bool active = true) {
warpPrefixScan(acc, laneId, c, c, i, active);
}
// limited to warpSize² elements
template <typename TAcc, typename T>
ALPAKA_FN_ACC ALPAKA_FN_INLINE void blockPrefixScan(
const TAcc& acc, T const* ci, T* co, int32_t size, T* ws = nullptr) {
if constexpr (!requires_single_thread_per_block_v<TAcc>) {
const auto warpSize = alpaka::warp::getSize(acc);
int32_t const blockDimension(alpaka::getWorkDiv<alpaka::Block, alpaka::Threads>(acc)[0u]);
int32_t const blockThreadIdx(alpaka::getIdx<alpaka::Block, alpaka::Threads>(acc)[0u]);
ALPAKA_ASSERT_ACC(ws);
ALPAKA_ASSERT_ACC(size <= warpSize * warpSize);
ALPAKA_ASSERT_ACC(0 == blockDimension % warpSize);
auto first = blockThreadIdx;
ALPAKA_ASSERT_ACC(isPowerOf2(warpSize));
auto laneId = blockThreadIdx & (warpSize - 1);
auto warpUpRoundedSize = (size + warpSize - 1) / warpSize * warpSize;
for (auto i = first; i < warpUpRoundedSize; i += blockDimension) {
// When padding the warp, warpPrefixScan is a noop
warpPrefixScan(acc, laneId, ci, co, i, i < size);
if (i < size) {
// Skipped in warp padding threads.
auto warpId = i / warpSize;
ALPAKA_ASSERT_ACC(warpId < warpSize);
if ((warpSize - 1) == laneId)
ws[warpId] = co[i];
}
}
alpaka::syncBlockThreads(acc);
if (size <= warpSize)
return;
if (blockThreadIdx < warpSize) {
warpPrefixScan(acc, laneId, ws, blockThreadIdx);
}
alpaka::syncBlockThreads(acc);
for (auto i = first + warpSize; i < size; i += blockDimension) {
int32_t warpId = i / warpSize;
co[i] += ws[warpId - 1];
}
alpaka::syncBlockThreads(acc);
} else {
co[0] = ci[0];
for (int32_t i = 1; i < size; ++i)
co[i] = ci[i] + co[i - 1];
}
}
template <typename TAcc, typename T>
ALPAKA_FN_HOST_ACC ALPAKA_FN_INLINE void blockPrefixScan(const TAcc& acc,
T* __restrict__ c,
int32_t size,
T* __restrict__ ws = nullptr) {
if constexpr (!requires_single_thread_per_block_v<TAcc>) {
const auto warpSize = alpaka::warp::getSize(acc);
int32_t const blockDimension(alpaka::getWorkDiv<alpaka::Block, alpaka::Threads>(acc)[0u]);
int32_t const blockThreadIdx(alpaka::getIdx<alpaka::Block, alpaka::Threads>(acc)[0u]);
ALPAKA_ASSERT_ACC(ws);
ALPAKA_ASSERT_ACC(size <= warpSize * warpSize);
ALPAKA_ASSERT_ACC(0 == blockDimension % warpSize);
auto first = blockThreadIdx;
auto laneId = blockThreadIdx & (warpSize - 1);
auto warpUpRoundedSize = (size + warpSize - 1) / warpSize * warpSize;
for (auto i = first; i < warpUpRoundedSize; i += blockDimension) {
// When padding the warp, warpPrefixScan is a noop
warpPrefixScan(acc, laneId, c, i, i < size);
if (i < size) {
// Skipped in warp padding threads.
auto warpId = i / warpSize;
ALPAKA_ASSERT_ACC(warpId < warpSize);
if ((warpSize - 1) == laneId)
ws[warpId] = c[i];
}
}
alpaka::syncBlockThreads(acc);
if (size <= warpSize)
return;
if (blockThreadIdx < warpSize) {
warpPrefixScan(acc, laneId, ws, blockThreadIdx);
}
alpaka::syncBlockThreads(acc);
for (auto i = first + warpSize; i < size; i += blockDimension) {
auto warpId = i / warpSize;
c[i] += ws[warpId - 1];
}
alpaka::syncBlockThreads(acc);
} else {
for (int32_t i = 1; i < size; ++i)
c[i] += c[i - 1];
}
}
// in principle not limited....
template <typename T>
struct multiBlockPrefixScan {
template <typename TAcc>
ALPAKA_FN_ACC void operator()(
const TAcc& acc, T const* ci, T* co, uint32_t size, int32_t numBlocks, int32_t* pc, std::size_t warpSize) const {
// Get shared variable. The workspace is needed only for multi-threaded accelerators.
T* ws = nullptr;
if constexpr (!requires_single_thread_per_block_v<TAcc>) {
ws = alpaka::getDynSharedMem<T>(acc);
}
ALPAKA_ASSERT_ACC(warpSize == static_cast<std::size_t>(alpaka::warp::getSize(acc)));
[[maybe_unused]] const auto elementsPerGrid = alpaka::getWorkDiv<alpaka::Grid, alpaka::Elems>(acc)[0u];
const auto elementsPerBlock = alpaka::getWorkDiv<alpaka::Block, alpaka::Elems>(acc)[0u];
const auto threadsPerBlock = alpaka::getWorkDiv<alpaka::Block, alpaka::Threads>(acc)[0u];
const auto blocksPerGrid = alpaka::getWorkDiv<alpaka::Grid, alpaka::Blocks>(acc)[0u];
const auto blockIdx = alpaka::getIdx<alpaka::Grid, alpaka::Blocks>(acc)[0u];
const auto threadIdx = alpaka::getIdx<alpaka::Block, alpaka::Threads>(acc)[0u];
ALPAKA_ASSERT_ACC(elementsPerGrid >= size);
// first each block does a scan
[[maybe_unused]] int off = elementsPerBlock * blockIdx;
if (size - off > 0) {
blockPrefixScan(acc, ci + off, co + off, std::min(elementsPerBlock, size - off), ws);
}
// count blocks that finished
auto& isLastBlockDone = alpaka::declareSharedVar<bool, __COUNTER__>(acc);
//__shared__ bool isLastBlockDone;
if (0 == threadIdx) {
alpaka::mem_fence(acc, alpaka::memory_scope::Device{});
auto value = alpaka::atomicAdd(acc, pc, 1, alpaka::hierarchy::Blocks{}); // block counter
isLastBlockDone = (value == (int(blocksPerGrid) - 1));
}
alpaka::syncBlockThreads(acc);
if (!isLastBlockDone)
return;
ALPAKA_ASSERT_ACC(int(blocksPerGrid) == *pc);
// good each block has done its work and now we are left in last block
// let's get the partial sums from each block except the last, which receives 0.
T* psum = nullptr;
if constexpr (!requires_single_thread_per_block_v<TAcc>) {
psum = ws + warpSize;
} else {
psum = alpaka::getDynSharedMem<T>(acc);
}
for (int32_t i = threadIdx, ni = blocksPerGrid; i < ni; i += threadsPerBlock) {
auto j = elementsPerBlock * i + elementsPerBlock - 1;
psum[i] = (j < size) ? co[j] : T(0);
}
alpaka::syncBlockThreads(acc);
blockPrefixScan(acc, psum, psum, blocksPerGrid, ws);
// now it would have been handy to have the other blocks around...
// Simplify the computation by having one version where threads per block = block size
// and a second for the one thread per block accelerator.
if constexpr (!requires_single_thread_per_block_v<TAcc>) {
// Here threadsPerBlock == elementsPerBlock
for (uint32_t i = threadIdx + threadsPerBlock, k = 0; i < size; i += threadsPerBlock, ++k) {
co[i] += psum[k];
}
} else {
// We are single threaded here, adding partial sums starting with the 2nd block.
for (uint32_t i = elementsPerBlock; i < size; i++) {
co[i] += psum[i / elementsPerBlock - 1];
}
}
}
};
} // namespace cms::alpakatools
// declare the amount of block shared memory used by the multiBlockPrefixScan kernel
namespace alpaka::trait {
// Variable size shared mem
template <typename TAcc, typename T>
struct BlockSharedMemDynSizeBytes<cms::alpakatools::multiBlockPrefixScan<T>, TAcc> {
template <typename TVec>
ALPAKA_FN_HOST_ACC static std::size_t getBlockSharedMemDynSizeBytes(
cms::alpakatools::multiBlockPrefixScan<T> const& /* kernel */,
TVec const& /* blockThreadExtent */,
TVec const& /* threadElemExtent */,
T const* /* ci */,
T const* /* co */,
int32_t /* size */,
int32_t numBlocks,
int32_t const* /* pc */,
// This trait function does not receive the accelerator object to look up the warp size
std::size_t warpSize) {
// We need workspace (T[warpsize]) + partial sums (T[numblocks]).
if constexpr (cms::alpakatools::requires_single_thread_per_block_v<TAcc>) {
return sizeof(T) * numBlocks;
} else {
return sizeof(T) * (warpSize + numBlocks);
}
}
};
} // namespace alpaka::trait
#endif // HeterogeneousCore_AlpakaInterface_interface_prefixScan_h
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