kv-cells : fix tracking of seq_pos (#14339)

* kv-cells : fix tracking of seq_pos during cache reuse

ggml-ci

* cont : improve error message

ggml-ci

* cont : add more comments

src/llama-kv-cells.h

@@ -7,6 +7,7 @@
 #include <cassert>
 #include <vector>
 #include <set>
+#include <map>
 
 // meta information about KV cells that can be part of multiple sequences at the same time
 // TODO: add unit tests
@@ -164,7 +165,7 @@ public:
         assert(seq_id >= 0);
 
         seq[i].reset(seq_id);
-        seq_pos[seq_id].erase(pos[i]);
+        seq_pos_dec(seq_id, pos[i]);
 
         if (seq[i].none()) {
             pos[i] = -1;
@@ -187,7 +188,7 @@ public:
             seq[i].reset();
 
             seq[i].set(seq_id);
-            seq_pos[seq_id].insert(pos[i]);
+            seq_pos_inc(seq_id, pos[i]);
 
             return false;
         }
@@ -232,7 +233,7 @@ public:
         assert(!seq[i].test(seq_id));
 
         seq[i].set(seq_id);
-        seq_pos[seq_id].insert(pos[i]);
+        seq_pos_inc(seq_id, pos[i]);
     }
 
     // return the sequence id of this cell
@@ -259,7 +260,9 @@ public:
             return -1;
         }
 
-        return *seq_pos[seq_id].begin();
+        assert(seq_pos[seq_id].begin()->second > 0);
+
+        return seq_pos[seq_id].begin()->first;
     }
 
     // the maximum position of sequence seq_id currently present in any of the cells
@@ -272,7 +275,9 @@ public:
             return -1;
         }
 
-        return *seq_pos[seq_id].rbegin();
+        assert(seq_pos[seq_id].rbegin()->second > 0);
+
+        return seq_pos[seq_id].rbegin()->first;
     }
 
     // note: call only if the cell is not empty
@@ -389,17 +394,36 @@ private:
     // the bitset seq[i] tells us which sequences are currently occupying the i-th cell
     std::vector<seq_set_t> seq;
 
-    // the set seq_pos[s] tells us which positions are currently present for sequence s
+    // the set seq_pos[s][p] tells us how many times the position p is currently present for sequence s
+    // if the position p is not present, seq_pos[s][p] is not set
     // this way seq_pos[s].begin() and seq_pos[s].rbegin() give us the min/max positions currently in the cache
-    std::set<llama_pos> seq_pos[LLAMA_MAX_SEQ];
+    //
+    // note that we cannot a use an std::set because in some cases a position can occur more than once for the same seq:
+    //  - during performing a cache reuse via (rm + add)
+    //  - some vision models have input embeddings with repeating positions
+    //
+    std::map<llama_pos, int> seq_pos[LLAMA_MAX_SEQ];
 
     // helper functions for updating `seq_pos`, once cell at a time:
 
+    void seq_pos_dec(llama_seq_id s, llama_pos p) {
+        auto it = seq_pos[s].find(p);
+        assert(it != seq_pos[s].end());
+
+        if (--it->second == 0) {
+            seq_pos[s].erase(it);
+        }
+    }
+
+    void seq_pos_inc(llama_seq_id s, llama_pos p) {
+        seq_pos[s][p]++;
+    }
+
     // remove cell i
     void seq_pos_rm(uint32_t i) {
         for (int s = 0; s < LLAMA_MAX_SEQ; ++s) {
             if (seq[i].test(s)) {
-                seq_pos[s].erase(pos[i]);
+                seq_pos_dec(s, pos[i]);
             }
         }
     }
@@ -408,7 +432,7 @@ private:
     void seq_pos_add(uint32_t i) {
         for (int s = 0; s < LLAMA_MAX_SEQ; ++s) {
             if (seq[i].test(s)) {
-                seq_pos[s].insert(pos[i]);
+                seq_pos_inc(s, pos[i]);
             }
         }
     }