/*This code from NVIDIA Megatron:
* with minor changes. */
#pragma once
#include <assert.h>
#include <cuda_fp16.h>
#include <cfloat>
#include <limits>
#include <stdint.h>
#include <cuda_fp16.h>
#include <c10/macros/Macros.h>
namespace {
template <typename Datatype, int ELEMENTS_PER_LDG>
__device__ __inline__ void copy_vector(Datatype *dst, const Datatype *src);
template <>
__device__ __inline__ void copy_vector<c10::BFloat16, 1>(c10::BFloat16 *dst, const c10::BFloat16 *src) { *dst = *src; }
template <>
__device__ __inline__ void copy_vector<c10::BFloat16, 4>(c10::BFloat16 *dst, const c10::BFloat16 *src) { *((float2*) dst) = *((float2*) src); }
template <>
__device__ __inline__ void copy_vector<c10::Half, 1>(c10::Half *dst, const c10::Half *src) { *dst = *src; }
template <>
__device__ __inline__ void copy_vector<c10::Half, 4>(c10::Half *dst, const c10::Half *src) { *((float2*) dst) = *((float2*) src); }
template <>
__device__ __inline__ void copy_vector<uint8_t, 1>(uint8_t *dst, const uint8_t *src) { *dst = *src; }
template <>
__device__ __inline__ void copy_vector<uint8_t, 4>(uint8_t *dst, const uint8_t *src) {*((half2*) dst) = *((half2*) src); }
int log2_ceil(int value) {
int log2_value = 0;
while ((1 << log2_value) < value) ++log2_value;
return log2_value;
}
template<typename T>
struct Add {
__device__ __forceinline__ T operator()(T a, T b) const {
return a + b;
}
};
template<typename T>
struct Max {
__device__ __forceinline__ T operator()(T a, T b) const {
return a < b ? b : a;
}
};
template <typename T>
__device__ __forceinline__ T WARP_SHFL_XOR_NATIVE(T value, int laneMask, int width = warpSize, unsigned int mask = 0xffffffff)
{
#if CUDA_VERSION >= 9000
return __shfl_xor_sync(mask, value, laneMask, width);
#else
return __shfl_xor(value, laneMask, width);
#endif
}
template <typename acc_t, int WARP_BATCH, int WARP_SIZE, template<typename> class ReduceOp>
__device__ __forceinline__ void warp_reduce(acc_t* sum) {
ReduceOp<acc_t> r;
#pragma unroll
for (int offset = WARP_SIZE / 2; offset > 0; offset /= 2) {
#pragma unroll
for (int i = 0; i < WARP_BATCH; ++i) {
acc_t b = WARP_SHFL_XOR_NATIVE(sum[i], offset, WARP_SIZE);
sum[i] = r(sum[i], b);
}
}
}
/*
* Extended softmax (from native aten pytorch) with following additional features
* 1) input scaling
* 2) Explicit masking
*/
template <typename input_t, typename output_t, typename acc_t, int log2_elements>
__global__ void scaled_masked_softmax_warp_forward(
output_t *dst,
const input_t *src,
const uint8_t *mask,
const acc_t scale,
int micro_batch_size,
int element_count,
int pad_batches)
{
// WARP_SIZE and WARP_BATCH must match the return values batches_per_warp and
// warp_size of method warp_softmax_forward_kernel.
constexpr int next_power_of_two = 1 << log2_elements;
constexpr int WARP_SIZE = (next_power_of_two < C10_WARP_SIZE) ? next_power_of_two : C10_WARP_SIZE;
constexpr int WARP_ITERATIONS = next_power_of_two / WARP_SIZE;
constexpr int WARP_BATCH = (next_power_of_two <= 128) ? 2 : 1;
constexpr int ELEMENTS_PER_LDG_STG = (WARP_ITERATIONS < 4) ? 1 : 4;
// blockDim/threadIdx = (WARP_SIZE, WARPS_PER_BLOCK, )
// gridDim/blockIdx = (seq_len, attn_heads, batches)
int first_batch = (blockDim.y * (blockIdx.x + gridDim.x * (blockIdx.y + gridDim.y * blockIdx.z))+ threadIdx.y) * WARP_BATCH;
int pad_first_batch = 0;
if (pad_batches != 1) { // bert style
pad_first_batch = (blockDim.y * (blockIdx.x + gridDim.x * blockIdx.z) + threadIdx.y) * WARP_BATCH;
} else { // gpt2 style
pad_first_batch = (blockDim.y * blockIdx.x + threadIdx.y) * WARP_BATCH;
}
// micro_batch_size might not be a multiple of WARP_BATCH. Check how
// many batches have to computed within this WARP.
int local_batches = micro_batch_size - first_batch;
if (local_batches > WARP_BATCH)
local_batches = WARP_BATCH;
// there might be multiple batches per warp. compute the index within the batch
int local_idx = threadIdx.x;
src += first_batch * element_count + ELEMENTS_PER_LDG_STG * local_idx;
dst += first_batch * element_count + ELEMENTS_PER_LDG_STG * local_idx;
mask += pad_first_batch * element_count + ELEMENTS_PER_LDG_STG * local_idx;
// load data from global memory
acc_t elements[WARP_BATCH][WARP_ITERATIONS];
input_t temp_data[ELEMENTS_PER_LDG_STG];
uint8_t temp_mask[ELEMENTS_PER_LDG_STG];
#pragma unroll
for (int i = 0; i < WARP_BATCH; ++i) {
int batch_element_count = (i >= local_batches) ? 0 : element_count;
#pragma unroll
for (int it = 0; it < WARP_ITERATIONS; it+=ELEMENTS_PER_LDG_STG) {
int element_index = ELEMENTS_PER_LDG_STG * local_idx + it * WARP_SIZE;
if (element_index < batch_element_count) {
int itr_idx = i*element_count+it*WARP_SIZE;
copy_vector<input_t, ELEMENTS_PER_LDG_STG>(temp_data, src + itr_idx);
copy_vector<uint8_t, ELEMENTS_PER_LDG_STG>(temp_mask, mask + itr_idx);
#pragma unroll
for (int element = 0; element < ELEMENTS_PER_LDG_STG; ++element) {
if (temp_mask[element] != 1) {
elements[i][it + element] = (acc_t)temp_data[element] * scale;
} else {
elements[i][it + element] = -10000.0;
}
}
} else {
#pragma unroll
for (int element = 0; element < ELEMENTS_PER_LDG_STG; ++element) {
elements[i][it + element] = -std::numeric_limits<acc_t>::infinity();
}
}
}
}
// compute max_value
acc_t max_value[WARP_BATCH];
#pragma unroll
for (int i = 0; i < WARP_BATCH; ++i) {
max_value[i] = elements[i][0];
#pragma unroll
for (int it = 1; it < WARP_ITERATIONS; ++it) {
max_value[i] = (max_value[i] > elements[i][it]) ? max_value[i] : elements[i][it];
}
}
warp_reduce<acc_t, WARP_BATCH, WARP_SIZE, Max>(max_value);
acc_t sum[WARP_BATCH] { 0.0f };
#pragma unroll
for (int i = 0; i < WARP_BATCH; ++i) {
#pragma unroll
for (int it = 0; it < WARP_ITERATIONS; ++it) {
elements[i][it] = std::exp((elements[i][it] - max_value[i]));
sum[i] += elements[i][it];
}
}
warp_reduce<acc_t, WARP_BATCH, WARP_SIZE, Add>(sum);
// store result
output_t out[ELEMENTS_PER_LDG_STG];
#pragma unroll
for (int i = 0; i < WARP_BATCH; ++i) {
if (i >= local_batches)
break;
#pragma unroll
for (int it = 0; it < WARP_ITERATIONS; it+=ELEMENTS_PER_LDG_STG) {
int element_index = ELEMENTS_PER_LDG_STG * local_idx + it * WARP_SIZE;
if (element_index < element_count) {
#pragma unroll
for (int element = 0; element < ELEMENTS_PER_LDG_STG; ++element) {
out[element] = elements[i][it + element] / sum[i];
}
copy_vector<output_t, ELEMENTS_PER_LDG_STG>(dst + i * element_count + it * WARP_SIZE, out);
} else {
break;
}
}
}
}
template <typename input_t, typename output_t, typename acc_t, int log2_elements>
__global__ void scaled_masked_softmax_warp_backward(
output_t *gradInput,
input_t *grad,
const input_t *output,
acc_t scale,
int micro_batch_size,
int element_count)
{
// WARP_SIZE and WARP_BATCH must match the return values batches_per_warp and
// warp_size of method warp_softmax_backward_kernel.
constexpr int next_power_of_two = 1 << log2_elements;
constexpr int WARP_SIZE = (next_power_of_two < C10_WARP_SIZE) ? next_power_of_two : C10_WARP_SIZE;
constexpr int WARP_ITERATIONS = next_power_of_two / WARP_SIZE;
constexpr int WARP_BATCH = (next_power_of_two <= 128) ? 2 : 1;
constexpr int ELEMENTS_PER_LDG_STG = (WARP_ITERATIONS < 4) ? 1 : 4;
// blockDim/threadIdx = (WARP_SIZE, WARPS_PER_BLOCK, )
// gridDim/blockIdx = (seq_len, attn_heads, batches)
int first_batch = (blockDim.y * blockIdx.x + threadIdx.y) * WARP_BATCH;
// micro_batch_size might not be a multiple of WARP_BATCH. Check how
// many batches have to computed within this WARP.
int local_batches = micro_batch_size - first_batch;
if (local_batches > WARP_BATCH)
local_batches = WARP_BATCH;
// there might be multiple batches per warp. compute the index within the batch
int local_idx = threadIdx.x;
// the first element to process by the current thread
int thread_offset = first_batch * element_count + ELEMENTS_PER_LDG_STG * local_idx;
grad += thread_offset;
output += thread_offset;
gradInput += thread_offset;
// load data from global memory
acc_t grad_reg[WARP_BATCH][WARP_ITERATIONS] { 0.0f };
acc_t output_reg[WARP_BATCH][WARP_ITERATIONS] { 0.0f };
input_t temp_grad[ELEMENTS_PER_LDG_STG];
input_t temp_output[ELEMENTS_PER_LDG_STG];
#pragma unroll
for (int i = 0; i < WARP_BATCH; ++i) {
int batch_element_count = (i >= local_batches) ? 0 : element_count;
#pragma unroll
for (int it = 0; it < WARP_ITERATIONS; it+=ELEMENTS_PER_LDG_STG) {
int element_index = ELEMENTS_PER_LDG_STG * local_idx + it * WARP_SIZE;
if (element_index < batch_element_count) {
copy_vector<input_t, ELEMENTS_PER_LDG_STG>(temp_grad, grad + i * element_count + it * WARP_SIZE);
copy_vector<input_t, ELEMENTS_PER_LDG_STG>(temp_output, output + i * element_count + it * WARP_SIZE);
#pragma unroll
for (int element = 0; element < ELEMENTS_PER_LDG_STG; ++element) {
output_reg[i][it + element] = (acc_t)temp_output[element];
}
#pragma unroll
for (int element = 0; element < ELEMENTS_PER_LDG_STG; ++element) {
grad_reg[i][it + element] = (acc_t)temp_grad[element] * output_reg[i][it + element];
}
}
}
}
acc_t sum[WARP_BATCH];
#pragma unroll
for (int i = 0; i < WARP_BATCH; ++i) {
sum[i] = grad_reg[i][0];
#pragma unroll
for (int it = 1; it < WARP_ITERATIONS; ++it) {
sum[i] += grad_reg[i][it];
}
}
warp_reduce<acc_t, WARP_BATCH, WARP_SIZE, Add>(sum);
// store result
#pragma unroll
for (int i = 0; i < WARP_BATCH; ++i) {
if (i >= local_batches)
break;
#pragma unroll
for (int it = 0; it < WARP_ITERATIONS; it+=ELEMENTS_PER_LDG_STG) {
int element_index = ELEMENTS_PER_LDG_STG * local_idx + it * WARP_SIZE;
if (element_index < element_count) {
// compute gradients
output_t out[ELEMENTS_PER_LDG_STG];
#pragma unroll
for (int element = 0; element < ELEMENTS_PER_LDG_STG; ++element) {
out[element] = (output_t)(scale * (grad_reg[i][it + element] - output_reg[i][it + element] * sum[i]));
}
copy_vector<output_t, ELEMENTS_PER_LDG_STG>(gradInput + i * element_count + it * WARP_SIZE, out);
}
}
}
}
} // end of anonymous namespace
int get_batch_per_block(int query_seq_len, int key_seq_len, int batches, int attn_heads){
int log2_elements = log2_ceil(key_seq_len);
const int next_power_of_two = 1 << log2_elements;
int warp_size = (next_power_of_two < C10_WARP_SIZE) ? next_power_of_two : C10_WARP_SIZE;
int batches_per_warp = (next_power_of_two <= 128) ? 2 : 1;
constexpr int threads_per_block = 128;
int warps_per_block = (threads_per_block / warp_size);
int batches_per_block = warps_per_block * batches_per_warp;
return batches_per_block;
}
template<typename input_t, typename output_t, typename acc_t>
void dispatch_scaled_masked_softmax_forward(
output_t *dst,
const input_t *src,
const uint8_t *mask,
const input_t scale,
int query_seq_len,
int key_seq_len,
int batches,
int attn_heads,
int pad_batches)
{
TORCH_INTERNAL_ASSERT(key_seq_len >= 0 && key_seq_len <= 2048 );
if (key_seq_len == 0) {
return;
} else {
int log2_elements = log2_ceil(key_seq_len);
const int next_power_of_two = 1 << log2_elements;
int batch_count = batches * attn_heads * query_seq_len;
// This value must match the WARP_SIZE constexpr value computed inside softmax_warp_forward.
int warp_size = (next_power_of_two < C10_WARP_SIZE) ? next_power_of_two : C10_WARP_SIZE;
// This value must match the WARP_BATCH constexpr value computed inside softmax_warp_forward.
int batches_per_warp = (next_power_of_two <= 128) ? 2 : 1;
// use 128 threads per block to maximimize gpu utilization
constexpr int threads_per_block = 128;
int warps_per_block = (threads_per_block / warp_size);
int batches_per_block = warps_per_block * batches_per_warp;
TORCH_INTERNAL_ASSERT(query_seq_len%batches_per_block == 0);
dim3 blocks(query_seq_len/batches_per_block, attn_heads, batches);
dim3 threads(warp_size, warps_per_block, 1);
// Launch code would be more elegant if C++ supported FOR CONSTEXPR
switch (log2_elements) {
case 0: // 1
scaled_masked_softmax_warp_forward<input_t, output_t, acc_t, 0>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(dst, src, mask, scale, batch_count, key_seq_len, pad_batches);
break;
case 1: // 2
scaled_masked_softmax_warp_forward<input_t, output_t, acc_t, 1>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(dst, src, mask, scale, batch_count, key_seq_len, pad_batches);
break;
case 2: // 4
scaled_masked_softmax_warp_forward<input_t, output_t, acc_t, 2>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(dst, src, mask, scale, batch_count, key_seq_len, pad_batches);
break;
case 3: // 8
scaled_masked_softmax_warp_forward<input_t, output_t, acc_t, 3>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(dst, src, mask, scale, batch_count, key_seq_len, pad_batches);
break;
case 4: // 16
scaled_masked_softmax_warp_forward<input_t, output_t, acc_t, 4>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(dst, src, mask, scale, batch_count, key_seq_len, pad_batches);
break;
case 5: // 32
scaled_masked_softmax_warp_forward<input_t, output_t, acc_t, 5>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(dst, src, mask, scale, batch_count, key_seq_len, pad_batches);
break;
case 6: // 64
scaled_masked_softmax_warp_forward<input_t, output_t, acc_t, 6>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(dst, src, mask, scale, batch_count, key_seq_len, pad_batches);
break;
case 7: // 128
scaled_masked_softmax_warp_forward<input_t, output_t, acc_t, 7>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(dst, src, mask, scale, batch_count, key_seq_len, pad_batches);
break;
case 8: // 256
scaled_masked_softmax_warp_forward<input_t, output_t, acc_t, 8>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(dst, src, mask, scale, batch_count, key_seq_len, pad_batches);
break;
case 9: // 512
scaled_masked_softmax_warp_forward<input_t, output_t, acc_t, 9>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(dst, src, mask, scale, batch_count, key_seq_len, pad_batches);
break;
case 10: // 1024
scaled_masked_softmax_warp_forward<input_t, output_t, acc_t, 10>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(dst, src, mask, scale, batch_count, key_seq_len, pad_batches);
break;
case 11: // 2048
scaled_masked_softmax_warp_forward<input_t, output_t, acc_t, 11>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(dst, src, mask, scale, batch_count, key_seq_len, pad_batches);
break;
default:
break;
}
}
}
template<typename input_t, typename output_t, typename acc_t>
void dispatch_scaled_masked_softmax_backward(
output_t *grad_input,
input_t *grad,
const input_t *output,
const acc_t scale,
int query_seq_len,
int key_seq_len,
int batches,
int attn_heads)
{
TORCH_INTERNAL_ASSERT( key_seq_len >= 0 && key_seq_len <= 2048 );
if (key_seq_len == 0) {
return;
} else {
int log2_elements = log2_ceil(key_seq_len);
const int next_power_of_two = 1 << log2_elements;
int batch_count = batches * attn_heads * query_seq_len;
// This value must match the WARP_SIZE constexpr value computed inside softmax_warp_backward.
int warp_size = (next_power_of_two < C10_WARP_SIZE) ? next_power_of_two : C10_WARP_SIZE;
// This value must match the WARP_BATCH constexpr value computed inside softmax_warp_backward.
int batches_per_warp = (next_power_of_two <= 128) ? 2 : 1;
// use 128 threads per block to maximimize gpu utilization
constexpr int threads_per_block = 128;
int warps_per_block = (threads_per_block / warp_size);
int batches_per_block = warps_per_block * batches_per_warp;
int blocks = batch_count/batches_per_block;
dim3 threads(warp_size, warps_per_block, 1);
// Launch code would be more elegant if C++ supported FOR CONSTEXPR
switch (log2_elements) {
case 0: // 1
scaled_masked_softmax_warp_backward<input_t, output_t, acc_t, 0>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(grad_input, grad, output, scale, batch_count, key_seq_len);
break;
case 1: // 2
scaled_masked_softmax_warp_backward<input_t, output_t, acc_t, 1>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(grad_input, grad, output, scale, batch_count, key_seq_len);
break;
case 2: // 4
scaled_masked_softmax_warp_backward<input_t, output_t, acc_t, 2>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(grad_input, grad, output, scale, batch_count, key_seq_len);
break;
case 3: // 8
scaled_masked_softmax_warp_backward<input_t, output_t, acc_t, 3>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(grad_input, grad, output, scale, batch_count, key_seq_len);
break;
case 4: // 16
scaled_masked_softmax_warp_backward<input_t, output_t, acc_t, 4>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(grad_input, grad, output, scale, batch_count, key_seq_len);
break;
case 5: // 32
scaled_masked_softmax_warp_backward<input_t, output_t, acc_t, 5>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(grad_input, grad, output, scale, batch_count, key_seq_len);
break;
case 6: // 64
scaled_masked_softmax_warp_backward<input_t, output_t, acc_t, 6>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(grad_input, grad, output, scale, batch_count, key_seq_len);
break;
case 7: // 128
scaled_masked_softmax_warp_backward<input_t, output_t, acc_t, 7>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(grad_input, grad, output, scale, batch_count, key_seq_len);
break;
case 8: // 256
scaled_masked_softmax_warp_backward<input_t, output_t, acc_t, 8>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(grad_input, grad, output, scale, batch_count, key_seq_len);
break;
case 9: // 512
scaled_masked_softmax_warp_backward<input_t, output_t, acc_t, 9>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(grad_input, grad, output, scale, batch_count, key_seq_len);
break;
case 10: // 1024
scaled_masked_softmax_warp_backward<input_t, output_t, acc_t, 10>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(grad_input, grad, output, scale, batch_count, key_seq_len);
break;
case 11: // 2048
scaled_masked_softmax_warp_backward<input_t, output_t, acc_t, 11>
<<<blocks, threads, 0, at::cuda::getCurrentCUDAStream()>>>(grad_input, grad, output, scale, batch_count, key_seq_len);
break;
default:
break;
}
}
}