extracted from 3490: implements DotProductSSE() for FAST_FLOAT

stweil · Jul 13, 2021 · a220999 · a220999
1 parent e2529dd
commit a220999
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Showing 2 changed files with 58 additions and 7 deletions.
diff --git a/src/arch/dotproductsse.cpp b/src/arch/dotproductsse.cpp
@@ -32,14 +32,65 @@ namespace tesseract {
 // Uses Intel SSE intrinsics to access the SIMD instruction set.
 #if defined(FAST_FLOAT)
 TFloat DotProductSSE(const TFloat *u, const TFloat *v, int n) {
-  TFloat total = 0.0;
-  for (int k = 0; k < n; ++k) {
-    total += u[k] * v[k];
-  }
-  return total;
+	int max_offset = n - 4;
+	int offset = 0;
+	// Accumulate a set of 4 sums in sum, by loading pairs of 4 values from u and
+	// v, and multiplying them together in parallel.
+	__m128 sum = _mm_setzero_ps();
+	if (offset <= max_offset) {
+		offset = 4;
+		// Aligned load is reputedly faster but requires 16 byte aligned input.
+		if ((reinterpret_cast<uintptr_t>(u) & 15) == 0 && (reinterpret_cast<uintptr_t>(v) & 15) == 0) {
+			// Use aligned load.
+			sum = _mm_load_ps(u);
+			__m128 floats2 = _mm_load_ps(v);
+			// Multiply.
+			sum = _mm_mul_ps(sum, floats2);
+			while (offset <= max_offset) {
+				__m128 floats1 = _mm_load_ps(u + offset);
+				floats2 = _mm_load_ps(v + offset);
+				floats1 = _mm_mul_ps(floats1, floats2);
+				sum = _mm_add_ps(sum, floats1);
+				offset += 4;
+			}
+		}
+		else {
+			// Use unaligned load.
+			sum = _mm_loadu_ps(u);
+			__m128 floats2 = _mm_loadu_ps(v);
+			// Multiply.
+			sum = _mm_mul_ps(sum, floats2);
+			while (offset <= max_offset) {
+				__m128 floats1 = _mm_loadu_ps(u + offset);
+				floats2 = _mm_loadu_ps(v + offset);
+				floats1 = _mm_mul_ps(floats1, floats2);
+				sum = _mm_add_ps(sum, floats1);
+				offset += 4;
+			}
+		}
+	}
+	// Add the 4 sums in sum horizontally.
+#if 0
+	alignas(32) TFloat tmp[4];
+	_mm_store_ps(tmp, sum);
+	float result = tmp[0] + tmp[1] + tmp[2] + tmp[3];
+#else
+	__m128 zero = _mm_setzero_ps();
+	// https://www.felixcloutier.com/x86/haddps
+	sum = _mm_hadd_ps(sum, zero);
+	sum = _mm_hadd_ps(sum, zero);
+	// Extract the low result.
+	float result = _mm_cvtss_f32(sum);
+#endif
+	// Add on any left-over products.
+	while (offset < n) {
+		result += u[offset] * v[offset];
+		++offset;
+	}
+	return result;
 }
 #else
-double DotProductSSE(const double *u, const double *v, int n) {
+TFloat DotProductSSE(const TFloat *u, const TFloat *v, int n) {
   int max_offset = n - 2;
   int offset = 0;
   // Accumulate a set of 2 sums in sum, by loading pairs of 2 values from u and

diff --git a/src/arch/intsimdmatrix.h b/src/arch/intsimdmatrix.h
@@ -115,7 +115,7 @@ struct TESS_API IntSimdMatrix {
   static const IntSimdMatrix *intSimdMatrix;
   // Only available with NEON.
   static const IntSimdMatrix *intSimdMatrixNEON;
-  // Only available with AVX2 / SSE.
+  // Only available with AVX2 / AVX / FMA / SSE.
   static const IntSimdMatrix *intSimdMatrixAVX2;
   static const IntSimdMatrix *intSimdMatrixSSE;
 };