add quantized fast_hadamard_transform_28N (#5285)

Summary:
Pull Request resolved: #5285

ghstack-source-id: 242230779
exported-using-ghexport

Reviewed By: kimishpatel

Differential Revision: D60943029

fbshipit-source-id: a961d24508e7b9a87bdc65dc7ad3ae59dccc250e

Author: swolchok

extension/llm/custom_ops/spinquant/fast_hadamard_transform.cpp

@@ -11,6 +11,43 @@
 #include <algorithm>
 
 namespace executorch {
+namespace {
+// Normalization step: divide by sqrt(1 << log2_vec_size). Similar
+// to fast_sqrt above, if N is even, then the maximum-precision way
+// to do this is right-shift by log2_vec_size / 2. If N is odd, we
+// still do the right-shift, and then we have an extra division by
+// sqrt(2) that we perform by making use of a sufficiently accurate
+// rational approximation. Our initial idea was to divide by sqrt(2)
+// by adjusting the quantization scale, but that would cause this
+// function to tend to increase the magnitude of the elements of
+// vec, which would resulting in clipping and therefore accuracy
+// loss, especially compounded over 30+ transformer layers.
+void quantized_normalize_after_fht(
+    const int32_t* tmp,
+    int16_t* out,
+    int log2_vec_size,
+    int vec_size) {
+  const int log2_sqrt_vec_size = log2_vec_size / 2;
+  constexpr int32_t qmin = -(1 << 15) + 1;
+  constexpr int32_t qmax = -qmin;
+  if (log2_vec_size % 2 != 0) {
+    // 408 / 577 - 1.0 / sqrt(2) ~= 1.062e-0.6, which should be close enough.
+    static const int32_t inv_sqrt_2_numerator = 408;
+    static const int32_t inv_sqrt_2_denominator = 577;
+    for (int ii = 0; ii < vec_size; ++ii) {
+      const auto val_over_sqrt_vec_size =
+          (tmp[ii] * inv_sqrt_2_numerator / inv_sqrt_2_denominator) >>
+          log2_sqrt_vec_size;
+      out[ii] = std::clamp(val_over_sqrt_vec_size, qmin, qmax);
+    }
+  } else {
+    for (int ii = 0; ii < vec_size; ++ii) {
+      out[ii] = std::clamp(tmp[ii] >> log2_sqrt_vec_size, qmin, qmax);
+    }
+  }
+}
+} // namespace
+
 void fast_hadamard_transform_symmetric_quantized_s16(
     int16_t* vec,
     int log2_vec_size) {
@@ -27,42 +64,38 @@ void fast_hadamard_transform_symmetric_quantized_s16(
   auto tmp = std::make_unique<int32_t[]>(vec_size);
   std::copy(vec, vec + vec_size, tmp.get());
 
-  // Per the function-level comment in the header, we can ignore the
+  // Per the function-level comment above, we can ignore the
   // quantization scale, so we just delegate to the usual unnormalized
   // implementation.
   // NOTE: if we need this to be fast on CPU, we can use FFHT to
   // generate fht_uint32 similar to fht_float.
   internal::fast_hadamard_transform_unnormalized_simple_impl(
       tmp.get(), log2_vec_size);
 
-  // Normalization step: divide by sqrt(1 << log2_vec_size). Similar
-  // to fast_sqrt, if N is even, then the maximum-precision way
-  // to do this is right-shift by log2_vec_size / 2. If N is odd, we
-  // still do the right-shift, and then we have an extra division by
-  // sqrt(2) that we perform by making use of a sufficiently accurate
-  // rational approximation. (Our initial idea was to divide by sqrt(2)
-  // by adjusting the quantization scale, but that would cause this
-  // function to tend to increase the magnitude of the elements of
-  // vec, which would resulting in clipping and therefore accuracy
-  // loss, especially compounded over 30+ transformer layers.)
-  const int log2_sqrt_vec_size = log2_vec_size / 2;
-  constexpr int32_t qmin = -(1 << 15) + 1;
-  constexpr int32_t qmax = -qmin;
-  if (log2_vec_size % 2 != 0) {
-    // 408 / 577 - 1.0 / sqrt(2) ~= 1.062e-0.6, which should be close enough.
-    static const int32_t inv_sqrt_2_numerator = 408;
-    static const int32_t inv_sqrt_2_denominator = 577;
-    for (int ii = 0; ii < vec_size; ++ii) {
-      const auto val_over_sqrt_vec_size =
-          (tmp[ii] * inv_sqrt_2_numerator / inv_sqrt_2_denominator) >>
-          log2_sqrt_vec_size;
-      vec[ii] = std::clamp(val_over_sqrt_vec_size, qmin, qmax);
-    }
-  } else {
-    for (int ii = 0; ii < vec_size; ++ii) {
-      vec[ii] = std::clamp(tmp[ii] >> log2_sqrt_vec_size, qmin, qmax);
-    }
+  quantized_normalize_after_fht(tmp.get(), vec, log2_vec_size, vec_size);
+}
+
+void fast_hadamard_transform_symmetric_quantized_s16_28N(
+    int16_t* vec,
+    int log2_vec_size) {
+  if (log2_vec_size == 0) {
+    return;
   }
-  return;
+  const int vec_size = (1 << log2_vec_size);
+
+  auto tmp = std::make_unique<int32_t[]>(vec_size * 28);
+  std::copy(vec, vec + vec_size * 28, tmp.get());
+
+  for (int ii = 0; ii < 28; ++ii) {
+    internal::fast_hadamard_transform_unnormalized_simple_impl(
+        &tmp[ii * vec_size], log2_vec_size);
+  }
+
+  for (int ii = 0; ii < vec_size; ++ii) {
+    hadamard_mult_28_strided(&tmp[ii], vec_size);
+  }
+
+  quantized_normalize_after_fht(tmp.get(), vec, log2_vec_size, vec_size * 28);
 }
+
 } // namespace executorch

extension/llm/custom_ops/spinquant/fast_hadamard_transform.h

@@ -112,4 +112,11 @@ void fast_hadamard_transform_28N(T* vec, int log2_vec_size) {
   }
 }
 
+// We don't need the quantization scale; see the function-level
+// comment on fast_hadamard_transform_symmetric_quantized_s16 for
+// details.
+void fast_hadamard_transform_symmetric_quantized_s16_28N(
+    int16_t* vec,
+    int log2_vec_size);
+
 } // namespace executorch