[Kernel] AQ AZP 3/4: Asymmetric quantization kernels (vllm-project#7270)

Signed-off-by: Amit Garg <[email protected]>

Author: ProExpertProg

csrc/cpu/quant.cpp

@@ -257,11 +257,13 @@ void int8_scaled_mm(torch::Tensor& c,               // [M, OC], row-major
 // static-per-tensor quantization.
 void static_scaled_int8_quant(torch::Tensor& out,          // [..., hidden_size]
                               const torch::Tensor& input,  // [..., hidden_size]
-                              const torch::Tensor& scale) {
+                              const torch::Tensor& scale,
+                              c10::optional<torch::Tensor> const& azp) {
   CPU_KERNEL_GUARD_IN(static_scaled_int8_quant)
   TORCH_CHECK(input.is_contiguous());
   TORCH_CHECK(out.is_contiguous());
   TORCH_CHECK(scale.numel() == 1);
+  TORCH_CHECK(!azp.has_value(), "Zero point is not supported on CPU.");
 
   const int hidden_size = input.size(-1);
   const int num_tokens = input.numel() / hidden_size;
@@ -277,11 +279,12 @@ void static_scaled_int8_quant(torch::Tensor& out,          // [..., hidden_size]
 void dynamic_scaled_int8_quant(
     torch::Tensor& out,          // [..., hidden_size]
     const torch::Tensor& input,  // [..., hidden_size]
-    torch::Tensor& scale         // [..., 1]
-) {
+    torch::Tensor& scale,        // [..., 1]
+    c10::optional<torch::Tensor> const& azp) {
   CPU_KERNEL_GUARD_IN(dynamic_scaled_int8_quant)
   TORCH_CHECK(input.is_contiguous());
   TORCH_CHECK(out.is_contiguous());
+  TORCH_CHECK(!azp.has_value(), "Zero point is not supported on CPU.");
 
   int const hidden_size = input.size(-1);
   int const num_tokens = input.numel() / hidden_size;

csrc/cpu/torch_bindings.cpp

@@ -94,13 +94,14 @@ TORCH_LIBRARY_EXPAND(TORCH_EXTENSION_NAME, ops) {
 #ifdef __AVX512F__
   // Compute int8 quantized tensor for given scaling factor.
   ops.def(
-      "static_scaled_int8_quant(Tensor! out, Tensor input, Tensor scale) -> "
-      "()");
+      "static_scaled_int8_quant(Tensor! out, Tensor input, Tensor scale,"
+      "Tensor? azp) -> ()");
   ops.impl("static_scaled_int8_quant", torch::kCPU, &static_scaled_int8_quant);
+
   // Compute int8 quantized tensor and scaling factor
   ops.def(
-      "dynamic_scaled_int8_quant(Tensor! out, Tensor input, Tensor! scale) -> "
-      "()");
+      "dynamic_scaled_int8_quant(Tensor! out, Tensor input, Tensor! scale, "
+      "Tensor!? azp) -> ()");
   ops.impl("dynamic_scaled_int8_quant", torch::kCPU,
            &dynamic_scaled_int8_quant);
   // W8A8 GEMM, supporting symmetric per-tensor or per-row/column

csrc/ops.h

@@ -184,10 +184,12 @@ torch::Tensor marlin_qqq_gemm(torch::Tensor const& a,
 #endif
 
 void static_scaled_int8_quant(torch::Tensor& out, torch::Tensor const& input,
-                              torch::Tensor const& scale);
+                              torch::Tensor const& scale,
+                              c10::optional<torch::Tensor> const& azp);
 
 void dynamic_scaled_int8_quant(torch::Tensor& out, torch::Tensor const& input,
-                               torch::Tensor& scales);
+                               torch::Tensor& scales,
+                               c10::optional<torch::Tensor> const& azp);
 
 torch::Tensor gptq_gemm(torch::Tensor a, torch::Tensor b_q_weight,
                         torch::Tensor b_gptq_qzeros,

csrc/quantization/compressed_tensors/int8_quant_kernels.cu

@@ -14,12 +14,17 @@
 
 static inline __device__ int8_t float_to_int8_rn(float x) {
 #ifdef USE_ROCM
-  static const float i8_min =
+  static constexpr auto i8_min =
       static_cast<float>(std::numeric_limits<int8_t>::min());
-  static const float i8_max =
+  static constexpr auto i8_max =
       static_cast<float>(std::numeric_limits<int8_t>::max());
-  // round
+
+  // To match the rounding mode of CUDA, we use nearbyint.
+  // It uses the current rounding mode, which is always FE_TONEAREST on HIP.
+  // If that changes in the future, we may need to set the rounding mode
+  // explicitly, either at runtime or compile time.
   float dst = std::nearbyint(x);
+
   // saturate
   dst = std::clamp(dst, i8_min, i8_max);
   return static_cast<int8_t>(dst);
@@ -31,6 +36,59 @@ static inline __device__ int8_t float_to_int8_rn(float x) {
 #endif
 }
 
+static inline __device__ int32_t float_to_int32_rn(float x) {
+#ifdef USE_ROCM
+  // int32_max is not exactly representable as float.
+  // Therefore, we need to be careful and manually return int32_max on overflow.
+  // For symmetry, we also do the same for int32_min, even though it is exactly
+  // representable as float and the conversion should be exact.
+  static constexpr auto i32_min = std::numeric_limits<int32_t>::min();
+  static constexpr auto i32_min_f = static_cast<float>(i32_min);
+  static constexpr auto i32_max = std::numeric_limits<int32_t>::max();
+  static constexpr auto i32_max_f = static_cast<float>(i32_max);
+
+  // To match the rounding mode of CUDA, we use nearbyint.
+  // It uses the current rounding mode, which is always FE_TONEAREST on HIP.
+  // If that changes in the future, we may need to set the rounding mode
+  // explicitly, either at runtime or compile time.
+  float dst = std::nearbyint(x);
+
+  // saturate on the higher end.
+  if (dst >= i32_max_f) {
+    return i32_max;
+  }
+  // saturate on the lower end.
+  if (dst <= i32_min_f) {
+    return i32_min;
+  }
+
+  return static_cast<int32_t>(dst);
+#else
+  // CUDA path
+  uint32_t dst;
+  asm volatile("cvt.rni.sat.s32.f32 %0, %1;" : "=r"(dst) : "f"(x));
+  return reinterpret_cast<const int32_t&>(dst);
+#endif
+}
+
+static inline __device__ int8_t int32_to_int8(int32_t x) {
+#ifdef USE_ROCM
+  static constexpr auto i8_min =
+      static_cast<int32_t>(std::numeric_limits<int8_t>::min());
+  static constexpr auto i8_max =
+      static_cast<int32_t>(std::numeric_limits<int8_t>::max());
+
+  // saturate
+  int32_t dst = std::clamp(x, i8_min, i8_max);
+  return static_cast<int8_t>(dst);
+#else
+  // CUDA path
+  uint32_t dst;
+  asm volatile("cvt.sat.s8.s32 %0, %1;" : "=r"(dst) : "r"(x));
+  return reinterpret_cast<const int8_t&>(dst);
+#endif
+}
+
 namespace vllm {
 
 template <typename scalar_t, typename scale_type>
@@ -47,6 +105,23 @@ __global__ void static_scaled_int8_quant_kernel(
   }
 }
 
+template <typename scalar_t, typename scale_type, typename azp_type>
+__global__ void static_scaled_int8_azp_quant_kernel(
+    scalar_t const* __restrict__ input, int8_t* __restrict__ out,
+    scale_type const* scale_ptr, azp_type const* azp_ptr,
+    const int hidden_size) {
+  int const tid = threadIdx.x;
+  int const token_idx = blockIdx.x;
+  scale_type const scale = *scale_ptr;
+  azp_type const azp = *azp_ptr;
+
+  for (int i = tid; i < hidden_size; i += blockDim.x) {
+    auto const val = static_cast<float>(input[token_idx * hidden_size + i]);
+    auto const quant_val = int32_to_int8(float_to_int32_rn(val / scale) + azp);
+    out[token_idx * hidden_size + i] = quant_val;
+  }
+}
+
 template <typename scalar_t, typename scale_type>
 __global__ void dynamic_scaled_int8_quant_kernel(
     scalar_t const* __restrict__ input, int8_t* __restrict__ out,
@@ -80,14 +155,68 @@ __global__ void dynamic_scaled_int8_quant_kernel(
   }
 }
 
+template <typename scalar_t, typename scale_type, typename azp_type>
+__global__ void dynamic_scaled_int8_azp_quant_kernel(
+    scalar_t const* __restrict__ input, int8_t* __restrict__ out,
+    scale_type* scale, azp_type* azp, const int hidden_size) {
+  int const token_idx = blockIdx.x;
+
+  // Scan for the min and max value for this token
+  float max_val = std::numeric_limits<float>::min();
+  float min_val = std::numeric_limits<float>::max();
+  for (int i = threadIdx.x; i < hidden_size; i += blockDim.x) {
+    auto val = static_cast<float>(input[token_idx * hidden_size + i]);
+    max_val = std::max(max_val, val);
+    min_val = std::min(min_val, val);
+  }
+
+  // Reduce the max and min values across the block
+  using BlockReduce = cub::BlockReduce<float, 1024>;
+  __shared__ typename BlockReduce::TempStorage reduceStorage;
+  max_val = BlockReduce(reduceStorage).Reduce(max_val, cub::Max{}, blockDim.x);
+  __syncthreads();  // Make sure min doesn't mess with max shared memory
+  min_val = BlockReduce(reduceStorage).Reduce(min_val, cub::Min{}, blockDim.x);
+
+  __shared__ scale_type scale_sh;
+  __shared__ azp_type azp_sh;
+
+  // Compute the scale and zero point and store them, only on the first thread
+  if (threadIdx.x == 0) {
+    float const scale_val = (max_val - min_val) / 255.0f;
+    // Use rounding to even (same as torch.round)
+    auto const azp_float = std::nearbyint(-128.0f - min_val / scale_val);
+    auto const azp_val = static_cast<azp_type>(azp_float);
+
+    // Store the scale and azp into shared and global
+    scale[token_idx] = scale_sh = scale_val;
+    azp[token_idx] = azp_sh = azp_val;
+  }
+
+  // Wait for the scale and azp to be computed
+  __syncthreads();
+
+  float const scale_val = scale_sh;
+  azp_type const azp_val = azp_sh;
+
+  // Quantize the values
+  for (int i = threadIdx.x; i < hidden_size; i += blockDim.x) {
+    auto const val = static_cast<float>(input[token_idx * hidden_size + i]);
+    auto const quant_val =
+        int32_to_int8(float_to_int32_rn(val / scale_val) + azp_val);
+    out[token_idx * hidden_size + i] = quant_val;
+  }
+}
+
 }  // namespace vllm
 
 void static_scaled_int8_quant(torch::Tensor& out,          // [..., hidden_size]
                               torch::Tensor const& input,  // [..., hidden_size]
-                              torch::Tensor const& scale) {
+                              torch::Tensor const& scale,
+                              c10::optional<torch::Tensor> const& azp) {
   TORCH_CHECK(input.is_contiguous());
   TORCH_CHECK(out.is_contiguous());
   TORCH_CHECK(scale.numel() == 1);
+  TORCH_CHECK(!azp || azp->numel() == 1);
 
   int const hidden_size = input.size(-1);
   int const num_tokens = input.numel() / hidden_size;
@@ -96,19 +225,29 @@ void static_scaled_int8_quant(torch::Tensor& out,          // [..., hidden_size]
   const cudaStream_t stream = at::cuda::getCurrentCUDAStream();
   VLLM_DISPATCH_FLOATING_TYPES(
       input.scalar_type(), "static_scaled_int8_quant_kernel", [&] {
-        vllm::static_scaled_int8_quant_kernel<scalar_t, float>
-            <<<grid, block, 0, stream>>>(input.data_ptr<scalar_t>(),
-                                         out.data_ptr<int8_t>(),
-                                         scale.data_ptr<float>(), hidden_size);
+        if (!azp) {
+          vllm::static_scaled_int8_quant_kernel<scalar_t, float>
+              <<<grid, block, 0, stream>>>(
+                  input.data_ptr<scalar_t>(), out.data_ptr<int8_t>(),
+                  scale.data_ptr<float>(), hidden_size);
+        } else {
+          vllm::static_scaled_int8_azp_quant_kernel<scalar_t, float, int32_t>
+              <<<grid, block, 0, stream>>>(
+                  input.data_ptr<scalar_t>(), out.data_ptr<int8_t>(),
+                  scale.data_ptr<float>(), azp->data_ptr<int32_t>(),
+                  hidden_size);
+        }
       });
 }
 
 void dynamic_scaled_int8_quant(
     torch::Tensor& out,          // [..., hidden_size]
     torch::Tensor const& input,  // [..., hidden_size]
-    torch::Tensor& scales) {
+    torch::Tensor& scales, c10::optional<torch::Tensor> const& azp) {
   TORCH_CHECK(input.is_contiguous());
   TORCH_CHECK(out.is_contiguous());
+  TORCH_CHECK(scales.is_contiguous());
+  TORCH_CHECK(!azp || azp->is_contiguous());
 
   int const hidden_size = input.size(-1);
   int const num_tokens = input.numel() / hidden_size;
@@ -117,9 +256,17 @@ void dynamic_scaled_int8_quant(
   const cudaStream_t stream = at::cuda::getCurrentCUDAStream();
   VLLM_DISPATCH_FLOATING_TYPES(
       input.scalar_type(), "dynamic_scaled_int8_quant_kernel", [&] {
-        vllm::dynamic_scaled_int8_quant_kernel<scalar_t, float>
-            <<<grid, block, 0, stream>>>(input.data_ptr<scalar_t>(),
-                                         out.data_ptr<int8_t>(),
-                                         scales.data_ptr<float>(), hidden_size);
+        if (!azp) {
+          vllm::dynamic_scaled_int8_quant_kernel<scalar_t, float>
+              <<<grid, block, 0, stream>>>(
+                  input.data_ptr<scalar_t>(), out.data_ptr<int8_t>(),
+                  scales.data_ptr<float>(), hidden_size);
+        } else {
+          vllm::dynamic_scaled_int8_azp_quant_kernel<scalar_t, float, int32_t>
+              <<<grid, block, 0, stream>>>(
+                  input.data_ptr<scalar_t>(), out.data_ptr<int8_t>(),
+                  scales.data_ptr<float>(), azp->data_ptr<int32_t>(),
+                  hidden_size);
+        }
       });
 }

csrc/torch_bindings.cpp

@@ -336,14 +336,14 @@ TORCH_LIBRARY_EXPAND(TORCH_EXTENSION_NAME, ops) {
 
   // Compute int8 quantized tensor for given scaling factor.
   ops.def(
-      "static_scaled_int8_quant(Tensor! out, Tensor input, Tensor scale) -> "
-      "()");
+      "static_scaled_int8_quant(Tensor! out, Tensor input, Tensor scale,"
+      "Tensor? azp) -> ()");
   ops.impl("static_scaled_int8_quant", torch::kCUDA, &static_scaled_int8_quant);
 
   // Compute int8 quantized tensor and scaling factor
   ops.def(
-      "dynamic_scaled_int8_quant(Tensor! out, Tensor input, Tensor! scale) -> "
-      "()");
+      "dynamic_scaled_int8_quant(Tensor! out, Tensor input, Tensor! scale, "
+      "Tensor!? azp) -> ()");
   ops.impl("dynamic_scaled_int8_quant", torch::kCUDA,
            &dynamic_scaled_int8_quant);
 }