i#6893: Delete unused cacheline snoop objects. (#6906)

The snoop cache tracks sharing for each cacheline accessed. The prior
code never deleted these tracking structures once they were allocated,
causing the tracking data to grow with every new cacheline touched. This
change deletes cacheline trackers once the cacheline is no longer
present in any tracked cache. It also changes the container used for
tracking from a vector of bools to a set of cache IDs, to simplify
snooped cache lookup and iteration.

Fixes: #6893

clients/drcachesim/simulator/snoop_filter.cpp

@@ -74,14 +74,8 @@ void
 snoop_filter_t::snoop(addr_t tag, int id, bool is_write)
 {
     coherence_table_entry_t *coherence_entry = &coherence_table_[tag];
-    // Initialize new snoop filter entry.
-    if (coherence_entry->sharers.empty()) {
-        coherence_entry->sharers.resize(num_snooped_caches_, false);
-        coherence_entry->dirty = false;
-    }
 
-    auto num_sharers = std::count(coherence_entry->sharers.begin(),
-                                  coherence_entry->sharers.end(), true);
+    size_t num_sharers = coherence_entry->sharers.size();
 
     // Check that cache id is valid.
     assert(id >= 0 && id < num_snooped_caches_);
@@ -91,7 +85,8 @@ snoop_filter_t::snoop(addr_t tag, int id, bool is_write)
     assert(!coherence_entry->dirty || num_sharers == 1);
 
     // Check if this request causes a writeback.
-    if (!coherence_entry->sharers[id] && coherence_entry->dirty) {
+    if (coherence_entry->sharers.find(id) == coherence_entry->sharers.end() &&
+        coherence_entry->dirty) {
         num_writebacks_++;
         coherence_entry->dirty = false;
     }
@@ -101,16 +96,18 @@ snoop_filter_t::snoop(addr_t tag, int id, bool is_write)
         coherence_entry->dirty = true;
         if (num_sharers > 0) {
             // Writes will invalidate other caches_.
-            for (int i = 0; i < num_snooped_caches_; i++) {
-                if (coherence_entry->sharers[i] && id != i) {
-                    caches_[i]->invalidate(tag, INVALIDATION_COHERENCE);
-                    num_invalidates_++;
-                    coherence_entry->sharers[i] = false;
-                }
+            auto it = coherence_entry->sharers.begin();
+            while (it != coherence_entry->sharers.end()) {
+                int i = *it++;
+                if (i == id)
+                    continue;
+                caches_[i]->invalidate(tag, INVALIDATION_COHERENCE);
+                num_invalidates_++;
+                coherence_entry->sharers.erase(i);
             }
         }
     }
-    coherence_entry->sharers[id] = true;
+    coherence_entry->sharers.insert(id);
 }
 
 /* This function is called whenever a coherent cache evicts a line. */
@@ -120,20 +117,23 @@ snoop_filter_t::snoop_eviction(addr_t tag, int id)
     coherence_table_entry_t *coherence_entry = &coherence_table_[tag];
 
     // Check if sharer list is initialized.
-    assert(coherence_entry->sharers.size() == (uint64_t)num_snooped_caches_);
+    assert(!coherence_entry->sharers.empty());
     // Check that cache id is valid.
     assert(id >= 0 && id < num_snooped_caches_);
     // Check that tag is valid.
     assert(tag != TAG_INVALID);
     // Check that we currently have this cache marked as a sharer.
-    assert(coherence_entry->sharers[id]);
+    assert(coherence_entry->sharers.find(id) != coherence_entry->sharers.end());
 
     if (coherence_entry->dirty) {
         num_writebacks_++;
         coherence_entry->dirty = false;
     }
 
-    coherence_entry->sharers[id] = false;
+    coherence_entry->sharers.erase(id);
+    if (coherence_entry->sharers.empty()) {
+        coherence_table_.erase(tag);
+    }
 }
 
 void

clients/drcachesim/simulator/snoop_filter.h

@@ -36,6 +36,7 @@
 #include <stdint.h>
 
 #include <unordered_map>
+#include <unordered_set>
 #include <vector>
 
 #include "cache.h"
@@ -45,8 +46,8 @@ namespace dynamorio {
 namespace drmemtrace {
 
 struct coherence_table_entry_t {
-    std::vector<bool> sharers;
-    bool dirty;
+    std::unordered_set<int> sharers; // IDs of caches sharing this line.
+    bool dirty = false;
 };
 
 class snoop_filter_t {