Fix OOB read in qmv when non-divisible by blocksize (#917)

mlx/backend/metal/kernels/quantized.metal

@@ -15,14 +15,6 @@ using namespace metal;
 
 MLX_MTL_CONST int SIMD_SIZE = 32;
 
-template <typename T> struct AccT {
-  typedef T acc_t;
-};
-
-template <> struct AccT<bfloat16_t> {
-  typedef float acc_t;
-};
-
 
 template <typename T, typename U, int values_per_thread, int bits>
 inline U load_vector(const device T *x, thread U *x_thread) {
@@ -60,6 +52,51 @@ inline U load_vector(const device T *x, thread U *x_thread) {
   return sum;
 }
 
+template <typename T, typename U, int values_per_thread, int bits>
+inline U load_vector_safe(const device T *x, thread U *x_thread, int N) {
+  static_assert(bits == 2 || bits == 4 || bits == 8, "Template undefined for bits not in {2, 4, 8}");
+
+  U sum = 0;
+
+  if (bits == 2) {
+    for (int i = 0; i < N; i += 4) {
+      sum += x[i] + x[i+1] + x[i+2] + x[i+3];
+      x_thread[i] = x[i];
+      x_thread[i+1] = x[i+1] / 4.0f;
+      x_thread[i+2] = x[i+2] / 16.0f;
+      x_thread[i+3] = x[i+3] / 64.0f;
+    }
+    for (int i=N; i<values_per_thread; i++) {
+      x_thread[i] = 0;
+    }
+  }
+
+  else if (bits == 4) {
+    for (int i = 0; i < N; i += 4) {
+      sum += x[i] + x[i+1] + x[i+2] + x[i+3];
+      x_thread[i] = x[i];
+      x_thread[i+1] = x[i+1] / 16.0f;
+      x_thread[i+2] = x[i+2] / 256.0f;
+      x_thread[i+3] = x[i+3] / 4096.0f;
+    }
+    for (int i=N; i<values_per_thread; i++) {
+      x_thread[i] = 0;
+    }
+  }
+
+  else if (bits == 8) {
+    for (int i = 0; i < N; i++) {
+      sum += x[i];
+      x_thread[i] = x[i];
+    }
+    for (int i=N; i<values_per_thread; i++) {
+      x_thread[i] = 0;
+    }
+  }
+
+  return sum;
+}
+
 template <typename U, int values_per_thread, int bits>
 inline U qdot(const device uint8_t* w, const thread U *x_thread, U scale, U bias, U sum) {
   static_assert(bits == 2 || bits == 4 || bits == 8, "Template undefined for bits not in {2, 4, 8}");
@@ -96,6 +133,42 @@ inline U qdot(const device uint8_t* w, const thread U *x_thread, U scale, U bias
   return scale * accum + sum * bias;
 }
 
+template <typename U, int values_per_thread, int bits>
+inline U qdot_safe(const device uint8_t* w, const thread U *x_thread, U scale, U bias, U sum, int N) {
+  static_assert(bits == 2 || bits == 4 || bits == 8, "Template undefined for bits not in {2, 4, 8}");
+
+  U accum = 0;
+
+  if (bits == 2) {
+    for (int i = 0; i < (N / 4); i++) {
+      accum += (
+          x_thread[4*i] * (w[i] & 0x03)
+          + x_thread[4*i+1] * (w[i] & 0x0c)
+          + x_thread[4*i+2] * (w[i] & 0x30)
+          + x_thread[4*i+3] * (w[i] & 0xc0));
+    }
+  }
+
+  else if (bits == 4) {
+    const device uint16_t* ws = (const device uint16_t*)w;
+    for (int i = 0; i < (N / 4); i++) {
+      accum += (
+          x_thread[4*i] * (ws[i] & 0x000f)
+          + x_thread[4*i+1] * (ws[i] & 0x00f0)
+          + x_thread[4*i+2] * (ws[i] & 0x0f00)
+          + x_thread[4*i+3] * (ws[i] & 0xf000));
+    }
+  }
+
+  else if (bits == 8) {
+    for (int i = 0; i < N; i++) {
+      accum += x_thread[i] * w[i];
+    }
+  }
+
+  return scale * accum + sum * bias;
+}
+
 template <typename U, int values_per_thread, int bits>
 inline void qouter(const thread uint8_t* w, U x, U scale, U bias, thread U* result) {
   static_assert(bits == 2 || bits == 4 || bits == 8, "Template undefined for bits not in {2, 4, 8}");
@@ -236,7 +309,8 @@ template <typename T, const int group_size, const int bits>
     x += tid.z * in_vec_size + simd_lid * values_per_thread;
     y += tid.z * out_vec_size + out_row;
 
-    for (int k = 0; k < in_vec_size; k += block_size) {
+    int k = 0;
+    for (; k < in_vec_size-block_size; k += block_size) {
       U sum = load_vector<T, U, values_per_thread, bits>(x, x_thread);
 
       for (int row = 0; out_row + row < out_vec_size; row++) {
@@ -254,6 +328,18 @@ template <typename T, const int group_size, const int bits>
       biases += block_size / group_size;
       x += block_size;
     }
+    const int remaining = clamp(static_cast<int>(in_vec_size - k - simd_lid * values_per_thread), 0, values_per_thread);
+    U sum = load_vector_safe<T, U, values_per_thread, bits>(x, x_thread, remaining);
+
+    for (int row = 0; out_row + row < out_vec_size; row++) {
+      const device uint8_t* wl = (const device uint8_t *)(w + row * in_vec_size_w);
+      const device T* sl = scales + row * in_vec_size_g;
+      const device T* bl = biases + row * in_vec_size_g;
+
+      U s = sl[0];
+      U b = bl[0];
+      result[row] += qdot<U, values_per_thread, bits>(wl, x_thread, s, b, sum);
+    }
 
     for (int row = 0; out_row + row < out_vec_size; row++) {
       result[row] = simd_sum(result[row]);
@@ -271,7 +357,8 @@ template <typename T, const int group_size, const int bits>
     x += tid.z * in_vec_size + simd_lid * values_per_thread;
     y += tid.z * out_vec_size + used_out_row;
 
-    for (int k = 0; k < in_vec_size; k += block_size) {
+    int k = 0;
+    for (; k < in_vec_size-block_size; k += block_size) {
       U sum = load_vector<T, U, values_per_thread, bits>(x, x_thread);
 
       for (int row = 0; row < results_per_simdgroup; row++) {
@@ -289,6 +376,18 @@ template <typename T, const int group_size, const int bits>
       biases += block_size / group_size;
       x += block_size;
     }
+    const int remaining = clamp(static_cast<int>(in_vec_size - k - simd_lid * values_per_thread), 0, values_per_thread);
+    U sum = load_vector_safe<T, U, values_per_thread, bits>(x, x_thread, remaining);
+
+    for (int row = 0; row < results_per_simdgroup; row++) {
+      const device uint8_t* wl = (const device uint8_t *)(w + row * in_vec_size_w);
+      const device T* sl = scales + row * in_vec_size_g;
+      const device T* bl = biases + row * in_vec_size_g;
+
+      U s = sl[0];
+      U b = bl[0];
+      result[row] += qdot_safe<U, values_per_thread, bits>(wl, x_thread, s, b, sum, remaining);
+    }
 
     for (int row = 0; row < results_per_simdgroup; row++) {
       result[row] = simd_sum(result[row]);