OpenFPM_pdata  3.0.0 Project that contain the implementation of distributed structures
cub::WarpReduce< T, LOGICAL_WARP_THREADS, PTX_ARCH > Class Template Reference

The WarpReduce class provides collective methods for computing a parallel reduction of items partitioned across a CUDA thread warp. More...

## Detailed Description

### template<typename T, int LOGICAL_WARP_THREADS = CUB_PTX_WARP_THREADS, int PTX_ARCH = CUB_PTX_ARCH> class cub::WarpReduce< T, LOGICAL_WARP_THREADS, PTX_ARCH >

The WarpReduce class provides collective methods for computing a parallel reduction of items partitioned across a CUDA thread warp.

Template Parameters
 T The reduction input/output element type LOGICAL_WARP_THREADS [optional] The number of threads per "logical" warp (may be less than the number of hardware warp threads). Default is the warp size of the targeted CUDA compute-capability (e.g., 32 threads for SM20). PTX_ARCH [optional] \ptxversion
Overview
• A reduction (or fold) uses a binary combining operator to compute a single aggregate from a list of input elements.
• Supports "logical" warps smaller than the physical warp size (e.g., logical warps of 8 threads)
• The number of entrant threads must be an multiple of LOGICAL_WARP_THREADS
Performance Considerations
• Uses special instructions when applicable (e.g., warp SHFL instructions)
• Uses synchronization-free communication between warp lanes when applicable
• Incurs zero bank conflicts for most types
• Computation is slightly more efficient (i.e., having lower instruction overhead) for:
• Summation (vs. generic reduction)
• The architecture's warp size is a whole multiple of LOGICAL_WARP_THREADS
Simple Examples
\warpcollective{WarpReduce}
The code snippet below illustrates four concurrent warp sum reductions within a block of 128 threads (one per each of the 32-thread warps).
#include <cub/cub.cuh>
__global__ void ExampleKernel(...)
{
// Specialize WarpReduce for type int
// Allocate WarpReduce shared memory for 4 warps
__shared__ typename WarpReduce::TempStorage temp_storage[4];
// Obtain one input item per thread
// Return the warp-wide sums to each lane0 (threads 0, 32, 64, and 96)
int warp_id = threadIdx.x / 32;
Suppose the set of input thread_data across the block of threads is {0, 1, 2, 3, ..., 127}. The corresponding output aggregate in threads 0, 32, 64, and 96 will 496, 1520, 2544, and 3568, respectively (and is undefined in other threads).
The code snippet below illustrates a single warp sum reduction within a block of 128 threads.
#include <cub/cub.cuh>
__global__ void ExampleKernel(...)
{
// Specialize WarpReduce for type int
// Allocate WarpReduce shared memory for one warp
__shared__ typename WarpReduce::TempStorage temp_storage;
...
// Only the first warp performs a reduction
{
// Obtain one input item per thread
// Return the warp-wide sum to lane0
Suppose the set of input thread_data across the warp of threads is {0, 1, 2, 3, ..., 31}. The corresponding output aggregate in thread0 will be 496 (and is undefined in other threads).

Definition at line 141 of file warp_reduce.cuh.

## Data Structures

struct  TempStorage
\smemstorage{WarpReduce} More...

## Public Types

typedef If<(PTX_ARCH >=300) &&(IS_POW_OF_TWO), WarpReduceShfl< T, LOGICAL_WARP_THREADS, PTX_ARCH >, WarpReduceSmem< T, LOGICAL_WARP_THREADS, PTX_ARCH > >::Type InternalWarpReduce
Internal specialization. Use SHFL-based reduction if (architecture is >= SM30) and (LOGICAL_WARP_THREADS is a power-of-two)

## Public Member Functions

Collective constructors
__device__ __forceinline__ WarpReduce (TempStorage &temp_storage)
Collective constructor using the specified memory allocation as temporary storage. Logical warp and lane identifiers are constructed from threadIdx.x. More...

Summation reductions
__device__ __forceinline__ T Sum (T input)
Computes a warp-wide sum in the calling warp. The output is valid in warp lane0. More...

__device__ __forceinline__ T Sum (T input, int valid_items)
Computes a partially-full warp-wide sum in the calling warp. The output is valid in warp lane0. More...

template<typename FlagT >
Computes a segmented sum in the calling warp where segments are defined by head-flags. The sum of each segment is returned to the first lane in that segment (which always includes lane0). More...

template<typename FlagT >
__device__ __forceinline__ T TailSegmentedSum (T input, FlagT tail_flag)
Computes a segmented sum in the calling warp where segments are defined by tail-flags. The sum of each segment is returned to the first lane in that segment (which always includes lane0). More...

Generic reductions
template<typename ReductionOp >
__device__ __forceinline__ T Reduce (T input, ReductionOp reduction_op)
Computes a warp-wide reduction in the calling warp using the specified binary reduction functor. The output is valid in warp lane0. More...

template<typename ReductionOp >
__device__ __forceinline__ T Reduce (T input, ReductionOp reduction_op, int valid_items)
Computes a partially-full warp-wide reduction in the calling warp using the specified binary reduction functor. The output is valid in warp lane0. More...

template<typename ReductionOp , typename FlagT >
Computes a segmented reduction in the calling warp where segments are defined by head-flags. The reduction of each segment is returned to the first lane in that segment (which always includes lane0). More...

template<typename ReductionOp , typename FlagT >
__device__ __forceinline__ T TailSegmentedReduce (T input, FlagT tail_flag, ReductionOp reduction_op)
Computes a segmented reduction in the calling warp where segments are defined by tail-flags. The reduction of each segment is returned to the first lane in that segment (which always includes lane0). More...

## Private Types

typedef InternalWarpReduce::TempStorage _TempStorage
Shared memory storage layout type for WarpReduce.

## Private Attributes

_TempStoragetemp_storage
Shared storage reference.

## ◆ anonymous enum

 anonymous enum
private
Enumerator
IS_ARCH_WARP

Whether the logical warp size and the PTX warp size coincide.

IS_POW_OF_TWO

Whether the logical warp size is a power-of-two.

Definition at line 149 of file warp_reduce.cuh.

## ◆ WarpReduce()

 __device__ __forceinline__ cub::WarpReduce< T, LOGICAL_WARP_THREADS, PTX_ARCH >::WarpReduce ( TempStorage & temp_storage )
inline

Collective constructor using the specified memory allocation as temporary storage. Logical warp and lane identifiers are constructed from threadIdx.x.

Parameters
 [in] temp_storage Reference to memory allocation having layout type TempStorage

Definition at line 203 of file warp_reduce.cuh.

## Member Function Documentation

template<typename ReductionOp , typename FlagT >
inline

Computes a segmented reduction in the calling warp where segments are defined by head-flags. The reduction of each segment is returned to the first lane in that segment (which always includes lane0).

Supports non-commutative reduction operators

\smemreuse

Snippet
The code snippet below illustrates a head-segmented warp max reduction within a block of 32 threads (one warp).
#include <cub/cub.cuh>
__global__ void ExampleKernel(...)
{
// Specialize WarpReduce for type int
// Allocate WarpReduce shared memory for one warp
__shared__ typename WarpReduce::TempStorage temp_storage;
// Obtain one input item and flag per thread
// Return the warp-wide reductions to each lane0
Suppose the set of input thread_data and head_flag across the block of threads is {0, 1, 2, 3, ..., 31 and is {1, 0, 0, 0, 1, 0, 0, 0, ..., 1, 0, 0, 0, respectively. The corresponding output aggregate in threads 0, 4, 8, etc. will be 3, 7, 11, etc. (and is undefined in other threads).
Template Parameters
 ReductionOp [inferred] Binary reduction operator type having member T operator()(const T &a, const T &b)
Parameters
 [in] input Calling thread's input [in] head_flag Head flag denoting whether or not input is the start of a new segment [in] reduction_op Reduction operator

Definition at line 545 of file warp_reduce.cuh.

template<typename FlagT >
inline

Computes a segmented sum in the calling warp where segments are defined by head-flags. The sum of each segment is returned to the first lane in that segment (which always includes lane0).

\smemreuse

Snippet
The code snippet below illustrates a head-segmented warp sum reduction within a block of 32 threads (one warp).
#include <cub/cub.cuh>
__global__ void ExampleKernel(...)
{
// Specialize WarpReduce for type int
// Allocate WarpReduce shared memory for one warp
__shared__ typename WarpReduce::TempStorage temp_storage;
// Obtain one input item and flag per thread
// Return the warp-wide sums to each lane0
Suppose the set of input thread_data and head_flag across the block of threads is {0, 1, 2, 3, ..., 31 and is {1, 0, 0, 0, 1, 0, 0, 0, ..., 1, 0, 0, 0, respectively. The corresponding output aggregate in threads 0, 4, 8, etc. will be 6, 22, 38, etc. (and is undefined in other threads).
Template Parameters
 ReductionOp [inferred] Binary reduction operator type having member T operator()(const T &a, const T &b)
Parameters
 [in] input Calling thread's input [in] head_flag Head flag denoting whether or not input is the start of a new segment

Definition at line 344 of file warp_reduce.cuh.

## ◆ Reduce() [1/2]

template<typename ReductionOp >
 __device__ __forceinline__ T cub::WarpReduce< T, LOGICAL_WARP_THREADS, PTX_ARCH >::Reduce ( T input, ReductionOp reduction_op )
inline

Computes a warp-wide reduction in the calling warp using the specified binary reduction functor. The output is valid in warp lane0.

Supports non-commutative reduction operators

\smemreuse

Snippet
The code snippet below illustrates four concurrent warp max reductions within a block of 128 threads (one per each of the 32-thread warps).
#include <cub/cub.cuh>
__global__ void ExampleKernel(...)
{
// Specialize WarpReduce for type int
// Allocate WarpReduce shared memory for 4 warps
__shared__ typename WarpReduce::TempStorage temp_storage[4];
// Obtain one input item per thread
// Return the warp-wide reductions to each lane0
int warp_id = threadIdx.x / 32;
int aggregate = WarpReduce(temp_storage[warp_id]).Reduce(
Suppose the set of input thread_data across the block of threads is {0, 1, 2, 3, ..., 127}. The corresponding output aggregate in threads 0, 32, 64, and 96 will 31, 63, 95, and 127, respectively (and is undefined in other threads).
Template Parameters
 ReductionOp [inferred] Binary reduction operator type having member T operator()(const T &a, const T &b)
Parameters
 [in] input Calling thread's input [in] reduction_op Binary reduction operator

Definition at line 445 of file warp_reduce.cuh.

## ◆ Reduce() [2/2]

template<typename ReductionOp >
 __device__ __forceinline__ T cub::WarpReduce< T, LOGICAL_WARP_THREADS, PTX_ARCH >::Reduce ( T input, ReductionOp reduction_op, int valid_items )
inline

Computes a partially-full warp-wide reduction in the calling warp using the specified binary reduction functor. The output is valid in warp lane0.

All threads across the calling warp must agree on the same value for valid_items. Otherwise the result is undefined.

Supports non-commutative reduction operators

\smemreuse

Snippet
The code snippet below illustrates a max reduction within a single, partially-full block of 32 threads (one warp).
#include <cub/cub.cuh>
__global__ void ExampleKernel(int *d_data, int valid_items)
{
// Specialize WarpReduce for type int
// Allocate WarpReduce shared memory for one warp
__shared__ typename WarpReduce::TempStorage temp_storage;
// Obtain one input item per thread if in range
// Return the warp-wide reductions to each lane0
Suppose the input d_data is {0, 1, 2, 3, 4, ... and valid_items is 4. The corresponding output aggregate in thread0 is 3 (and is undefined in other threads).
Template Parameters
 ReductionOp [inferred] Binary reduction operator type having member T operator()(const T &a, const T &b)
Parameters
 [in] input Calling thread's input [in] reduction_op Binary reduction operator [in] valid_items Total number of valid items in the calling thread's logical warp (may be less than LOGICAL_WARP_THREADS)

Definition at line 494 of file warp_reduce.cuh.

## ◆ Sum() [1/2]

 __device__ __forceinline__ T cub::WarpReduce< T, LOGICAL_WARP_THREADS, PTX_ARCH >::Sum ( T input )
inline

Computes a warp-wide sum in the calling warp. The output is valid in warp lane0.

\smemreuse

Snippet
The code snippet below illustrates four concurrent warp sum reductions within a block of 128 threads (one per each of the 32-thread warps).
#include <cub/cub.cuh>
__global__ void ExampleKernel(...)
{
// Specialize WarpReduce for type int
// Allocate WarpReduce shared memory for 4 warps
__shared__ typename WarpReduce::TempStorage temp_storage[4];
// Obtain one input item per thread
// Return the warp-wide sums to each lane0
int warp_id = threadIdx.x / 32;
Suppose the set of input thread_data across the block of threads is {0, 1, 2, 3, ..., 127}. The corresponding output aggregate in threads 0, 32, 64, and 96 will 496, 1520, 2544, and 3568, respectively (and is undefined in other threads).
Parameters

Definition at line 251 of file warp_reduce.cuh.

## ◆ Sum() [2/2]

 __device__ __forceinline__ T cub::WarpReduce< T, LOGICAL_WARP_THREADS, PTX_ARCH >::Sum ( T input, int valid_items )
inline

Computes a partially-full warp-wide sum in the calling warp. The output is valid in warp lane0.

All threads across the calling warp must agree on the same value for valid_items. Otherwise the result is undefined.

\smemreuse

Snippet
The code snippet below illustrates a sum reduction within a single, partially-full block of 32 threads (one warp).
#include <cub/cub.cuh>
__global__ void ExampleKernel(int *d_data, int valid_items)
{
// Specialize WarpReduce for type int
// Allocate WarpReduce shared memory for one warp
__shared__ typename WarpReduce::TempStorage temp_storage;
// Obtain one input item per thread if in range
// Return the warp-wide sums to each lane0
Suppose the input d_data is {0, 1, 2, 3, 4, ... and valid_items is 4. The corresponding output aggregate in thread0 is 6 (and is undefined in other threads).
Parameters
 [in] input Calling thread's input [in] valid_items Total number of valid items in the calling thread's logical warp (may be less than LOGICAL_WARP_THREADS)

Definition at line 295 of file warp_reduce.cuh.

## ◆ TailSegmentedReduce()

template<typename ReductionOp , typename FlagT >
 __device__ __forceinline__ T cub::WarpReduce< T, LOGICAL_WARP_THREADS, PTX_ARCH >::TailSegmentedReduce ( T input, FlagT tail_flag, ReductionOp reduction_op )
inline

Computes a segmented reduction in the calling warp where segments are defined by tail-flags. The reduction of each segment is returned to the first lane in that segment (which always includes lane0).

Supports non-commutative reduction operators

\smemreuse

Snippet
The code snippet below illustrates a tail-segmented warp max reduction within a block of 32 threads (one warp).
#include <cub/cub.cuh>
__global__ void ExampleKernel(...)
{
// Specialize WarpReduce for type int
// Allocate WarpReduce shared memory for one warp
__shared__ typename WarpReduce::TempStorage temp_storage;
// Obtain one input item and flag per thread
int tail_flag = ...
// Return the warp-wide reductions to each lane0
int aggregate = WarpReduce(temp_storage).TailSegmentedReduce(
Suppose the set of input thread_data and tail_flag across the block of threads is {0, 1, 2, 3, ..., 31 and is {0, 0, 0, 1, 0, 0, 0, 1, ..., 0, 0, 0, 1, respectively. The corresponding output aggregate in threads 0, 4, 8, etc. will be 3, 7, 11, etc. (and is undefined in other threads).
Template Parameters
 ReductionOp [inferred] Binary reduction operator type having member T operator()(const T &a, const T &b)
Parameters
 [in] input Calling thread's input [in] tail_flag Tail flag denoting whether or not input is the end of the current segment [in] reduction_op Reduction operator

Definition at line 596 of file warp_reduce.cuh.

## ◆ TailSegmentedSum()

template<typename FlagT >
 __device__ __forceinline__ T cub::WarpReduce< T, LOGICAL_WARP_THREADS, PTX_ARCH >::TailSegmentedSum ( T input, FlagT tail_flag )
inline

Computes a segmented sum in the calling warp where segments are defined by tail-flags. The sum of each segment is returned to the first lane in that segment (which always includes lane0).

\smemreuse

Snippet
The code snippet below illustrates a tail-segmented warp sum reduction within a block of 32 threads (one warp).
#include <cub/cub.cuh>
__global__ void ExampleKernel(...)
{
// Specialize WarpReduce for type int
// Allocate WarpReduce shared memory for one warp
__shared__ typename WarpReduce::TempStorage temp_storage;
// Obtain one input item and flag per thread
int tail_flag = ...
// Return the warp-wide sums to each lane0
int aggregate = WarpReduce(temp_storage).TailSegmentedSum(
Suppose the set of input thread_data and tail_flag across the block of threads is {0, 1, 2, 3, ..., 31 and is {0, 0, 0, 1, 0, 0, 0, 1, ..., 0, 0, 0, 1, respectively. The corresponding output aggregate in threads 0, 4, 8, etc. will be 6, 22, 38, etc. (and is undefined in other threads).
 ReductionOp [inferred] Binary reduction operator type having member T operator()(const T &a, const T &b)
 [in] input Calling thread's input [in] tail_flag Head flag denoting whether or not input is the start of a new segment