[Kernel] Faster pre-processing time for W4A8 (vllm-project#23972)

Signed-off-by: czhu-cohere <conway.zhu@cohere.com>

Author: czhu-cohere

csrc/quantization/cutlass_w4a8/w4a8_mm_entry.cu

@@ -25,6 +25,8 @@
 #include "cutlass_extensions/common.hpp"
 #include "cutlass_extensions/epilogue/scaled_mm_epilogues_c3x.hpp"
 
+#include <cuda_runtime.h>
+
 namespace vllm::cutlass_w4a8 {
 
 using namespace cute;
@@ -393,6 +395,71 @@ torch::Tensor pack_scale_fp8(torch::Tensor const& scales) {
   return packed_scales;
 }
 
+/*
+  GPU-accelerated implementation of cutlass::unified_encode_int4b.
+  Constructs a lookup table in constant memory to map 8 bits
+  (two 4-bit values) at a time. Assumes memory is contiguous
+  and pointers are 16-byte aligned.
+*/
+__constant__ uint8_t kNibbleLUT[256];
+
+__global__ void unified_encode_int4b_device(const uint8_t* in, uint8_t* out,
+                                            size_t nbytes) {
+  constexpr size_t V = sizeof(uint4);  // 16 bytes
+  const size_t tid = blockIdx.x * blockDim.x + threadIdx.x;
+  const size_t nthreads = size_t(gridDim.x) * blockDim.x;
+  const size_t nvec = nbytes / V;
+
+  // 1-D grid-stride loop over 16-byte chunks
+  for (size_t vec = tid; vec < nvec; vec += nthreads) {
+    uint4 v = reinterpret_cast<const uint4*>(in)[vec];
+    uint8_t* b = reinterpret_cast<uint8_t*>(&v);
+#pragma unroll
+    for (int i = 0; i < int(V); ++i) b[i] = kNibbleLUT[b[i]];
+    reinterpret_cast<uint4*>(out)[vec] = v;
+  }
+}
+
+static bool upload_lut() {
+  std::array<uint8_t, 256> lut{};
+  auto map_nib = [](uint8_t v) -> uint8_t {
+    // 1..7 -> (8 - v); keep 0 and 8..15
+    return (v == 0 || (v & 0x8)) ? v : uint8_t(8 - v);
+  };
+  for (int b = 0; b < 256; ++b) {
+    uint8_t lo = b & 0xF;
+    uint8_t hi = (b >> 4) & 0xF;
+    lut[b] = uint8_t((map_nib(hi) << 4) | map_nib(lo));
+  }
+  cudaError_t e = cudaMemcpyToSymbol(kNibbleLUT, lut.data(), lut.size(),
+                                     /*offset=*/0, cudaMemcpyHostToDevice);
+
+  return (e == cudaSuccess);
+}
+
+static bool unified_encode_int4b(cutlass::int4b_t const* in,
+                                 cutlass::int4b_t* out, size_t num_int4_elems) {
+  // Build/upload LUT
+  if (!upload_lut()) return false;
+
+  static_assert(sizeof(typename cutlass::int4b_t::Storage) == 1,
+                "int4 storage must be 1 byte");
+  const size_t nbytes = num_int4_elems >> 1;
+
+  auto* in_bytes = reinterpret_cast<uint8_t const*>(in);
+  auto* out_bytes = reinterpret_cast<uint8_t*>(out);
+
+  // kernel launch params
+  constexpr int block = 256;
+  const size_t nvec = nbytes / sizeof(uint4);  // # of 16B vectors
+  int grid = int((nvec + block - 1) / block);
+  if (grid == 0) grid = 1;  // ensure we still cover the tail in the kernel
+
+  unified_encode_int4b_device<<<grid, block>>>(in_bytes, out_bytes, nbytes);
+  cudaError_t err = cudaGetLastError();
+  return (err == cudaSuccess);
+}
+
 torch::Tensor encode_and_reorder_int4b(torch::Tensor const& B) {
   TORCH_CHECK(B.dtype() == torch::kInt32);
   TORCH_CHECK(B.dim() == 2);
@@ -401,6 +468,7 @@ torch::Tensor encode_and_reorder_int4b(torch::Tensor const& B) {
 
   int k = B.size(0) * PackFactor;  // logical k
   int n = B.size(1);
+  TORCH_CHECK((n * k) % 32 == 0, "need multiples of 32 int4s for 16B chunks");
 
   auto B_ptr = static_cast<QuantType const*>(B.const_data_ptr());
   auto B_packed_ptr = static_cast<QuantType*>(B_packed.data_ptr());
@@ -409,7 +477,9 @@ torch::Tensor encode_and_reorder_int4b(torch::Tensor const& B) {
   LayoutB_Reordered layout_B_reordered =
       cute::tile_to_shape(LayoutAtomQuant{}, shape_B);
 
-  cutlass::unified_encode_int4b(B_ptr, B_packed_ptr, n * k);
+  bool ok =
+      vllm::cutlass_w4a8::unified_encode_int4b(B_ptr, B_packed_ptr, n * k);
+  TORCH_CHECK(ok, "unified_encode_int4b failed");
   cutlass::reorder_tensor(B_packed_ptr, layout_B, layout_B_reordered);
 
   return B_packed;