iq4_nl: faster quantization (#76)

Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>

Author: 2 people

ggml/src/ggml-quants.c

@@ -15051,15 +15051,21 @@ size_t quantize_iq1_m(const float * restrict src, void * restrict dst, int64_t n
 
 // ============================ 4-bit non-linear quants
 
-static inline int best_index_int8(int n, const int8_t * val, float x) {
-    if (x <= val[0]) return 0;
-    if (x >= val[n-1]) return n-1;
-    int ml = 0, mu = n-1;
-    while (mu-ml > 1) {
-        int mav = (ml+mu)/2;
-        if (x < val[mav]) mu = mav; else ml = mav;
-    }
-    return x - val[mu-1] < val[mu] - x ? mu-1 : mu;
+static const int8_t iq4nl_index[241] = {
+     0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0, 16, 16,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1,
+     1, 17, 17,  2,  2,  2,  2,  2,  2,  2,  2,  2,  2,  2,  2,  2,  2,  2,  2,  2,  2, 18,  3,  3,  3,  3,  3,  3,  3,  3,  3,  3,
+     3,  3,  3,  3,  3,  3, 19,  4,  4,  4,  4,  4,  4,  4,  4,  4,  4,  4,  4,  4,  4, 20,  5,  5,  5,  5,  5,  5,  5,  5,  5,  5,
+     5,  5, 21, 21,  6,  6,  6,  6,  6,  6,  6,  6,  6,  6,  6, 22,  7,  7,  7,  7,  7,  7,  7,  7,  7,  7, 23, 23,  8,  8,  8,  8,
+     8,  8,  8,  8,  8,  8, 24,  9,  9,  9,  9,  9,  9,  9,  9,  9,  9,  9, 25, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 26, 26,
+    11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 27, 27, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 28, 13, 13, 13,
+    13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 29, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
+    14, 14, 14, 14, 30, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15
+};
+static inline int best_index_iq4nl(const int8_t * values, float x) {
+    int ix = (int)x - values[0];
+    if (ix < 0 || ix >= 241) return ix < 0 ? 0 : 15;
+    ix = iq4nl_index[ix];
+    return ix < 16 ? ix : x - values[ix-16] < values[ix-15] - x ? ix-16 : ix-15;
 }
 
 static void quantize_row_iq4_nl_impl(const int super_block_size, const int block_size, const float * restrict x,
@@ -15102,7 +15108,7 @@ static void quantize_row_iq4_nl_impl(const int super_block_size, const int block
         float sumqx = 0, sumq2 = 0;
         for (int j = 0; j < block_size; ++j) {
             float al = id*xb[j];
-            int l = best_index_int8(16, values, al);
+            int l = best_index_iq4nl(values, al);
             Lb[j] = l;
             float q = values[l];
             float w = weight[j];
@@ -15116,7 +15122,7 @@ static void quantize_row_iq4_nl_impl(const int super_block_size, const int block
             sumqx = sumq2 = 0;
             for (int j = 0; j < block_size; ++j) {
                 float al = id*xb[j];
-                int l = best_index_int8(16, values, al);
+                int l = best_index_iq4nl(values, al);
                 float q = values[l];
                 float w = weight[j];
                 sumqx += w*q*xb[j];
@@ -15147,7 +15153,7 @@ static void quantize_row_iq4_nl_impl(const int super_block_size, const int block
             uint8_t * Lb = L + ib*block_size;
             const float * xb = x + ib*block_size;
             for (int j = 0; j < block_size; ++j) {
-                Lb[j] = best_index_int8(16, values, idl*xb[j]);
+                Lb[j] = best_index_iq4nl(values, idl*xb[j]);
             }
             l += 32;
             uint8_t l_l = l & 0xf;
@@ -15161,7 +15167,7 @@ static void quantize_row_iq4_nl_impl(const int super_block_size, const int block
         if (ntry > 0) {
             float id = scales[0] ? 1/scales[0] : 0;
             for (int j = 0; j < super_block_size; ++j) {
-                L[j] = best_index_int8(16, values, id*x[j]);
+                L[j] = best_index_iq4nl(values, id*x[j]);
             }
         }
     }