Add AVX2 implementation of dequantize_row_q4_0 (#467)

Author: slaren

Diff for: ggml.c

@@ -771,6 +771,40 @@ void dequantize_row_q4_0(const void * restrict x, float * restrict y, int k) {
     const uint8_t * restrict pd = ((const uint8_t *)x + 0*bs);
     const uint8_t * restrict pb = ((const uint8_t *)x + 0*bs + sizeof(float));
 
+#if defined(__AVX2__) && QK % 32 == 0
+    for (int i = 0; i < nb; i++) {
+        // scale factor
+        const __m256 d_v = _mm256_broadcast_ss((const float *) (pd + i*bs));
+
+        const uint8_t * restrict pp = pb + i*bs;
+
+        for (int l = 0; l < QK; l += 32) {
+            // Load 32x4-bit integers into 32x8-bit integers
+            __m256i vx8 = bytesFromNibbles(pp+l/2);
+
+            // Subtract 8 from the integers
+            vx8 = _mm256_sub_epi8(vx8, _mm256_set1_epi8(8));
+
+            // Convert to 16-bit int
+            const __m256i vx16_lo = _mm256_cvtepi8_epi16(_mm256_extracti128_si256(vx8, 0));
+            const __m256i vx16_hi = _mm256_cvtepi8_epi16(_mm256_extracti128_si256(vx8, 1));
+
+            // Convert to 32-bit int -> float 32
+            const __m256 vf[4] = {
+                _mm256_cvtepi32_ps(_mm256_cvtepi16_epi32(_mm256_extracti128_si256(vx16_lo, 0))),
+                _mm256_cvtepi32_ps(_mm256_cvtepi16_epi32(_mm256_extracti128_si256(vx16_lo, 1))),
+                _mm256_cvtepi32_ps(_mm256_cvtepi16_epi32(_mm256_extracti128_si256(vx16_hi, 0))),
+                _mm256_cvtepi32_ps(_mm256_cvtepi16_epi32(_mm256_extracti128_si256(vx16_hi, 1)))
+            };
+
+            // Scale and store
+            for (int j = 0; j < 4; j++) {
+                __m256 result = _mm256_mul_ps(vf[j], d_v);
+                _mm256_storeu_ps(y + i * QK + l + j*8, result);
+            }
+        }
+    }
+#else
     // scalar
     for (int i = 0; i < nb; i++) {
         const float d = *(const float *) (pd + i*bs);
@@ -795,6 +829,7 @@ void dequantize_row_q4_0(const void * restrict x, float * restrict y, int k) {
             assert(!isnan(y[i*QK + l + 1]));
         }
     }
+#endif
 }
 
 void dequantize_row_q4_1(const void * restrict x, float * restrict y, int k) {