Quantized SDPA

.gitignore

@@ -27,6 +27,7 @@ parts/
 sdist/
 var/
 venv/
+.venv/
 wheels/
 share/python-wheels/
 *.egg-info/

mlx/backend/metal/kernels/fp_quantized.h

@@ -5,6 +5,8 @@
 
 #include "mlx/backend/metal/kernels/fp4.h"
 #include "mlx/backend/metal/kernels/fp8.h"
+#include "mlx/backend/metal/kernels/steel/gemm/loader.h"
+#include "mlx/backend/metal/kernels/steel/gemm/mma.h"
 
 constant bool align_M [[function_constant(200)]];
 constant bool align_N [[function_constant(201)]];

mlx/backend/metal/kernels/quantized_utils.h

@@ -1,8 +1,183 @@
 // Copyright © 2023-2024 Apple Inc.
 
+#pragma once
+
 #include <metal_simdgroup>
 #include <metal_stdlib>
 
+#include "mlx/backend/metal/kernels/fp4.h"
+#include "mlx/backend/metal/kernels/fp8.h"
+
+enum class QuantMode { Affine, Mxfp4, Mxfp8, Nvfp4 };
+
+template <typename OutT, typename EncodedT>
+struct DecodeValue {
+  [[clang::always_inline]] OutT operator()(uint8_t v) const {
+    return OutT(*(thread EncodedT*)(&v));
+  }
+};
+
+// Specialization for Affine (plain integer cast)
+template <typename OutT>
+struct DecodeValue<OutT, void> {
+  [[clang::always_inline]] OutT operator()(uint8_t v) const {
+    return OutT(v);
+  }
+};
+
+template <QuantMode mode>
+struct QuantConfig;
+
+template <>
+struct QuantConfig<QuantMode::Affine> {
+  static constant constexpr bool has_bias = true;
+
+  using value_type = void;
+  using scale_type = void;
+
+  template <typename T>
+  using scale_storage_t = T;
+};
+
+template <>
+struct QuantConfig<QuantMode::Mxfp4> {
+  static constant constexpr bool has_bias = false;
+
+  using value_type = fp4_e2m1;
+  using scale_type = fp8_e8m0;
+
+  template <typename T>
+  using scale_storage_t = uint8_t;
+};
+
+template <>
+struct QuantConfig<QuantMode::Nvfp4> {
+  static constant constexpr bool has_bias = false;
+
+  using value_type = fp4_e2m1;
+  using scale_type = fp8_e4m3;
+
+  template <typename T>
+  using scale_storage_t = uint8_t;
+};
+
+template <>
+struct QuantConfig<QuantMode::Mxfp8> {
+  static constant constexpr bool has_bias = false;
+
+  using value_type = fp8_e4m3;
+  using scale_type = fp8_e8m0;
+
+  template <typename T>
+  using scale_storage_t = uint8_t;
+};
+
+template <QuantMode mode, typename T>
+struct Dequant {
+  using Cfg = QuantConfig<mode>;
+
+  [[clang::always_inline]] T raw(uint8_t v) const {
+    return DecodeValue<T, typename Cfg::value_type>{}(v);
+  }
+
+  [[clang::always_inline]] T scale(
+      typename Cfg::template scale_storage_t<T> s) const {
+    if constexpr (metal::is_same_v<typename Cfg::scale_type, void>) {
+      return s;
+    } else {
+      return DecodeValue<T, typename Cfg::scale_type>{}(s);
+    }
+  }
+
+  [[clang::always_inline]] T operator()(uint8_t v, T s, T bias) const {
+    if constexpr (Cfg::has_bias) {
+      return fma(s, raw(v), bias);
+    } else {
+      return s * raw(v);
+    }
+  }
+};
+
+// Pack metadata and unpackers for arbitrary bit-widths (wsize fixed at 32 bits)
+template <int bits>
+struct PackInfo {
+  static_assert(
+      bits == 2 || bits == 3 || bits == 4 || bits == 5 || bits == 6 ||
+          bits == 8,
+      "PackInfo only supports bits in {2,3,4,5,6,8}");
+
+  static constant constexpr int pack_factor =
+      (bits == 3 || bits == 5) ? 8 : (bits == 6 ? 4 : 32 / bits);
+  static constant constexpr int bytes_per_pack =
+      ((bits & (bits - 1)) == 0) ? 4 : (bits == 5 ? 5 : 3);
+};
+
+template <int bits>
+struct PackReader {
+  static constant constexpr int pack_factor = PackInfo<bits>::pack_factor;
+  static constant constexpr int bytes_per_pack = PackInfo<bits>::bytes_per_pack;
+  static constant constexpr uint64_t mask = (uint64_t(1) << bits) - 1;
+
+  [[gnu::always_inline]] static void load(
+      const device uint8_t* p,
+      thread uint8_t (&out)[pack_factor]) {
+    uint64_t packed = load_packed(p);
+#pragma clang loop unroll(full)
+    for (int i = 0; i < pack_factor; ++i) {
+      out[i] = static_cast<uint8_t>((packed >> (bits * i)) & mask);
+    }
+  }
+
+ private:
+  [[gnu::always_inline]] static uint64_t load_packed(const device uint8_t* p) {
+    if constexpr (bytes_per_pack == 4) {
+      return static_cast<uint64_t>(
+          *(reinterpret_cast<const device uint32_t*>(p)));
+    } else {
+      uint64_t packed = 0;
+#pragma clang loop unroll(full)
+      for (int i = 0; i < bytes_per_pack; ++i) {
+        packed |= static_cast<uint64_t>(p[i]) << (8 * i);
+      }
+      return packed;
+    }
+  }
+};
+
+// Pointer wrapper for quantized data that handles byte-level addressing
+// correctly for all bit widths. For non-4-byte-aligned packs (3, 5, 6-bit),
+template <int bits>
+class QuantDataPtr {
+  const device uint8_t* byte_ptr_;
+
+ public:
+  static constant constexpr int pack_factor = PackInfo<bits>::pack_factor;
+  static constant constexpr int bytes_per_pack = PackInfo<bits>::bytes_per_pack;
+
+  // Initialize from base pointer, head stride (in uint32 units), head index,
+  // and element index
+  [[clang::always_inline]] QuantDataPtr(
+      const device uint32_t* base,
+      size_t head_stride,
+      int head_idx,
+      int elem_idx) {
+    int packed_idx = elem_idx / pack_factor;
+    byte_ptr_ = reinterpret_cast<const device uint8_t*>(base) +
+        head_idx * head_stride * 4 + // head_stride is in uint32 units
+        packed_idx * bytes_per_pack;
+  }
+
+  // Advance by number of elements
+  [[clang::always_inline]] void advance(int num_elements) {
+    byte_ptr_ += num_elements * bits / 8;
+  }
+
+  // Get raw pointer for passing to dot/accumulate functions
+  [[clang::always_inline]] const device uint32_t* ptr() const {
+    return reinterpret_cast<const device uint32_t*>(byte_ptr_);
+  }
+};
+
 template <typename T, typename mma_t, typename loader_a_t, typename loader_b_t>
 METAL_FUNC void gemm_loop_aligned(
     threadgroup T* As,

mlx/backend/metal/kernels/scaled_dot_product_attention.metal

@@ -41,4 +41,20 @@ using namespace metal;
 instantiate_sdpa_vector_heads(float)
 instantiate_sdpa_vector_heads(bfloat16_t)
 instantiate_sdpa_vector_heads(float16_t)
+
+#define instantiate_quant_sdpa_vector(type, qk_dim, value_dim)   \
+  instantiate_kernel(                                             \
+      "quant_sdpa_vector_2pass_1_" #type "_" #qk_dim "_" #value_dim, \
+      quant_sdpa_vector_2pass_1,                                  \
+      type,                                                       \
+      qk_dim)
+
+#define instantiate_quant_sdpa_vector_heads(type) \
+  instantiate_quant_sdpa_vector(type, 64, 64)     \
+  instantiate_quant_sdpa_vector(type, 128, 128)   \
+  instantiate_quant_sdpa_vector(type, 256, 256)
+
+instantiate_quant_sdpa_vector_heads(float)
+instantiate_quant_sdpa_vector_heads(bfloat16_t)
+instantiate_quant_sdpa_vector_heads(float16_t)
     // clang-format on