calculate the freq_cis online, no need to write/read them to/from checkpoints

Author: karpathy

run.c

@@ -43,7 +43,7 @@ typedef struct {
     float* w3; // (layer, hidden_dim, dim)
     // final rmsnorm
     float* rms_final_weight; // (dim,)
-    // freq_cis for RoPE relatively positional embeddings
+    // freq_cis for RoPE relatively positional embeddings (not used anymore)
     float* freq_cis_real; // (seq_len, head_size/2)
     float* freq_cis_imag; // (seq_len, head_size/2)
     // (optional) classifier weights for the logits, on the last layer
@@ -214,10 +214,6 @@ void transformer(int token, int pos, Config* p, RunState* s, TransformerWeights*
     float* content_row = &(w->token_embedding_table[token * dim]);
     memcpy(x, content_row, dim*sizeof(*x));
 
-    // pluck out the "pos" row of freq_cis_real and freq_cis_imag
-    float* freq_cis_real_row = w->freq_cis_real + pos * head_size / 2;
-    float* freq_cis_imag_row = w->freq_cis_imag + pos * head_size / 2;
-
     // forward all the layers
     for(int l = 0; l < p->n_layers; l++) {
 
@@ -229,15 +225,18 @@ void transformer(int token, int pos, Config* p, RunState* s, TransformerWeights*
         matmul(s->k, s->xb, w->wk + l*dim*kv_dim, dim, kv_dim);
         matmul(s->v, s->xb, w->wv + l*dim*kv_dim, dim, kv_dim);
 
-        // RoPE relative positional encoding: complex-valued rotate q and k by freq_cis in each head
-        for (int v = 0; v < 2; v++) {
-            float* vec   = v == 0 ? s->q : s->k;    // the vector to rotate (query or key)
-            int vec_size = v == 0 ? dim  : kv_dim;  // the size of the vector
-            for (int i = 0; i < vec_size; i+=2) {
+        // RoPE relative positional encoding: complex-valued rotate q and k in each head
+        for (int i = 0; i < dim; i+=2) {
+            int head_dim = i % head_size;
+            float freq = 1.0f / powf(10000.0f, head_dim / (float)head_size);
+            float val = pos * freq;
+            float fcr = cosf(val);
+            float fci = sinf(val);
+            int rotn = i < kv_dim ? 2 : 1; // how many vectors? 2 = q & k, 1 = q only
+            for (int v = 0; v < rotn; v++) {
+                float* vec = v == 0 ? s->q : s->k; // the vector to rotate (query or key)
                 float v0 = vec[i];
                 float v1 = vec[i+1];
-                float fcr = freq_cis_real_row[(i % head_size) / 2];
-                float fci = freq_cis_imag_row[(i % head_size) / 2];
                 vec[i]   = v0 * fcr - v1 * fci;
                 vec[i+1] = v0 * fci + v1 * fcr;
             }