10+% performance improvement of ggml_vec_dot_q4_0 on AVX2 (#654)

* Performance improvement of AVX2 code
* Fixed problem with MSVC compiler
* Reviewer comments: removed double semicolon, deleted empty line 1962

Author: SebastianApel

ggml.c

@@ -1962,42 +1962,71 @@ static void ggml_vec_dot_q4_0(const int n, float * restrict s, const void * rest
     // Initialize accumulator with zeros
     __m256 acc = _mm256_setzero_ps();
 
-    // Main loop
-    // TODO: figure a way to do this in a portable way
-    #ifdef __GNUC__
-    #pragma GCC unroll 16
-    #endif
-    for (int i = 0; i < nb; ++i) {
-        // Compute combined scale for the block
-        const __m256 d = _mm256_mul_ps( _mm256_broadcast_ss( &x[i].d ), _mm256_broadcast_ss( &y[i].d ) );
-
-        // Load 16 bytes, and unpack 4 bit fields into bytes, making 32 bytes
-        __m256i bx = bytesFromNibbles( x[i].qs );
-        __m256i by = bytesFromNibbles( y[i].qs );
-
-        // Now we have a vector with bytes in [ 0 .. 15 ] interval. Offset them into [ -8 .. +7 ] interval.
-        const __m256i off = _mm256_set1_epi8( 8 );
-        bx = _mm256_sub_epi8( bx, off );
-        by = _mm256_sub_epi8( by, off );
-
-        // Get absolute values of x vectors
-        const __m256i ax = _mm256_sign_epi8(bx, bx);
-
-        // Sign the values of the y vectors
-        const __m256i sy = _mm256_sign_epi8(by, bx);
-
-        // Perform multiplication and create 16-bit values
-        const __m256i dot = _mm256_maddubs_epi16(ax, sy);
-
-        const __m256i ones = _mm256_set1_epi16(1);
-        const __m256i i32 = _mm256_madd_epi16(ones, dot);
+    /* Prepare the constants we will need during execution */        
+    const __m256i lowMask = _mm256_set1_epi8( 0xF );
+    const __m256i offset_8 = _mm256_set1_epi16( 8 );
 
-        // Convert int32_t to float
-        const __m256 p = _mm256_cvtepi32_ps( i32 );
+#define UNROLL_COUNT 8
+    // make sure we only unroll multiples of the block count
+    assert(nb % UNROLL_COUNT == 0);
 
-        // Apply the scale, and accumulate
-        acc = _mm256_fmadd_ps( d, p, acc );
-    }
+    // Main loop
+    for (int i = 0; i < nb; i+=UNROLL_COUNT) {
+
+        // This loop will be unrolled by the compiler    
+        for (int u=0;u<UNROLL_COUNT;u++)  {
+            /* Compute combined scale for the block */ 
+            const __m256 scale = _mm256_mul_ps( 
+                    _mm256_broadcast_ss( &x[i+u].d ), 
+                    _mm256_broadcast_ss( &y[i+u].d ) ); 
+
+            /* get input from x 
+               Input: 32 Nibbles (16 bytes) at *x[i+u] 
+               Output: 2 vectors with 16 values of type int16_t (x_high_q, x_low_q) */             
+                                      
+            /* Load 16 bytes from memory */  
+            const __m128i tmp_x = _mm_loadu_si128( ( const __m128i* ) x[i+u].qs); 
+            /* Expand bytes into uint16_t values */                                 
+            const __m256i bytes_x = _mm256_cvtepu8_epi16(tmp_x); 
+            /* Unpack values into individual bytes */
+            __m256i x_low_q = _mm256_and_si256( lowMask, bytes_x );
+            const __m256i pre_shift_x_high_q = _mm256_andnot_si256( lowMask, bytes_x );
+            __m256i x_high_q = _mm256_srli_epi16( pre_shift_x_high_q, 4 );            
+            /* Now we have two vectors with bytes in [ 0 .. 15 ] interval.  Offset them into [ -8 .. +7 ] interval.  */
+            x_high_q = _mm256_sub_epi16( x_high_q, offset_8 ); 
+            x_low_q = _mm256_sub_epi16( x_low_q, offset_8 ); 
+
+            /* get input from y 
+               Input: 32 Nibbles (16 bytes) at *y[i+u] 
+               Output: 2 vectors with 16 values of type int16_t (y_high_q, y_low_q) */             
+
+            /* Load 16 bytes from memory */  
+            const __m128i tmp_y = _mm_loadu_si128( (const __m128i* ) y[i+u].qs); 
+            /* Expand bytes into uint16_t values */     
+            const __m256i bytes_y = _mm256_cvtepu8_epi16(tmp_y); 
+            /* Unpack values into individual bytes */
+            const __m256i pre_shift_y_high_q = _mm256_andnot_si256( lowMask, bytes_y ); 
+            __m256i y_high_q = _mm256_srli_epi16( pre_shift_y_high_q, 4 ); 
+            __m256i y_low_q = _mm256_and_si256( lowMask, bytes_y ); 
+            /* Now we have two vectors with bytes in [ 0 .. 15 ] interval.  Offset them into [ -8 .. +7 ] interval.  */
+            y_high_q = _mm256_sub_epi16( y_high_q, offset_8 ); 
+            y_low_q = _mm256_sub_epi16( y_low_q, offset_8 ); 
+
+            /* Compute products of int16_t integers, add pairwise, store as int32_t */     
+            __m256i xy_high_q = _mm256_madd_epi16( x_high_q, y_high_q ); 
+            __m256i xy_low_q = _mm256_madd_epi16( x_low_q, y_low_q ); 
+
+            /* Accumulate the products of int32_t integers -> we now have a vector of 8 int_32t */ 
+            __m256i xy_q = _mm256_add_epi32( xy_high_q, xy_low_q ); 
+
+            /* Convert to vectore of 8 int32_t to 8 floats */ 
+            __m256 q = _mm256_cvtepi32_ps( xy_q ); 
+
+            /* Multiply q with scale and accumulate */ 
+            acc = _mm256_fmadd_ps( scale, q, acc );    
+        }
+       
+    }   
 
     // Return horizontal sum of the acc vector
     __m128 res = _mm256_extractf128_ps( acc, 1 );
@@ -2026,7 +2055,7 @@ static void ggml_vec_dot_q4_0(const int n, float * restrict s, const void * rest
             bx = _mm_sub_epi8( bx, off );
             by = _mm_sub_epi8( by, off );
 
-	    // Get absolute values of x vectors
+            // Get absolute values of x vectors
             const __m128i ax = _mm_sign_epi8(bx, bx);
 
             // Sign the values of the y vectors