base, mem-cache: Remove IP dependency from the CacheEntry

We don't store a pointer to the indexing policy anymore.
Instead, we register a tag extractor callback when we
construct the CacheEntry

Change-Id: I06dc58e2f67e01f3f9bcd9f0c641505d3aec82ff
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/base/cache/cache_entry.hh

@@ -61,12 +61,12 @@ class CacheEntry : public ReplaceableEntry
   public:
     using IndexingPolicy = BaseIndexingPolicy;
     using KeyType = Addr;
+    using TagExtractor = std::function<Addr(Addr)>;
 
-    CacheEntry(BaseIndexingPolicy *ip) : indexingPolicy(ip) {}
-    ~CacheEntry() = default;
-    CacheEntry(const CacheEntry &rhs)
-      : indexingPolicy(rhs.indexingPolicy)
+    CacheEntry(TagExtractor ext)
+      : extractTag(ext), valid(false), tag(MaxAddr)
     {}
+    ~CacheEntry() = default;
 
     /**
      * Checks if the entry is valid.
@@ -138,24 +138,19 @@ class CacheEntry : public ReplaceableEntry
         valid = true;
     }
 
-    virtual Addr
-    extractTag(Addr addr) const
-    {
-        return indexingPolicy->extractTag(addr);
-    }
-
   private:
+    /** Callback used to extract the tag from the entry */
+    TagExtractor extractTag;
+
     /**
      * Valid bit. The contents of this entry are only valid if this bit is set.
      * @sa invalidate()
      * @sa insert()
      */
-    bool valid{false};
-
-    BaseIndexingPolicy *indexingPolicy{nullptr};
+    bool valid;
 
     /** The entry's tag. */
-    Addr tag{MaxAddr};
+    Addr tag;
 };
 
 } // namespace gem5

src/mem/cache/compressors/frequent_values.cc

@@ -53,7 +53,7 @@ FrequentValues::FrequentValues(const Params &p)
     VFT((name() + ".VFT").c_str(),
         p.vft_entries, p.vft_assoc, p.vft_replacement_policy,
         p.vft_indexing_policy,
-        VFTEntry(counterBits, p.vft_indexing_policy)),
+        VFTEntry(counterBits, genTagExtractor(p.vft_indexing_policy))),
     codeGenerationEvent([this]{ phase = COMPRESSING; }, name())
 {
     fatal_if((numVFTEntries - 1) > mask(chunkSizeBits),

src/mem/cache/compressors/frequent_values.hh

@@ -129,8 +129,8 @@ class FrequentValues : public Base
          */
         SatCounter32 counter;
 
-        VFTEntry(std::size_t num_bits, BaseIndexingPolicy *ip)
-          : CacheEntry(ip), value(0), counter(num_bits)
+        VFTEntry(std::size_t num_bits, TagExtractor ext)
+          : CacheEntry(ext), value(0), counter(num_bits)
         {
         }
 

src/mem/cache/prefetch/delta_correlating_prediction_tables.cc

@@ -44,7 +44,8 @@ DeltaCorrelatingPredictionTables::DeltaCorrelatingPredictionTables(
    table((name() + "DCPT").c_str(), p.table_entries,
          p.table_assoc, p.table_replacement_policy,
          p.table_indexing_policy,
-         DCPTEntry(p.deltas_per_entry, p.table_indexing_policy))
+         DCPTEntry(p.deltas_per_entry,
+                   genTagExtractor(p.table_indexing_policy)))
 {
 }
 

src/mem/cache/prefetch/delta_correlating_prediction_tables.hh

@@ -76,8 +76,8 @@ class DeltaCorrelatingPredictionTables : public SimObject
          * Constructor
          * @param num_deltas number of deltas stored in the entry
          */
-        DCPTEntry(unsigned int num_deltas, BaseIndexingPolicy *ip)
-          : CacheEntry(ip), lastAddress(0), deltas(num_deltas)
+        DCPTEntry(unsigned int num_deltas, TagExtractor ext)
+          : CacheEntry(ext), lastAddress(0), deltas(num_deltas)
         {
         }
 

src/mem/cache/tags/indexing_policies/base.hh

@@ -213,6 +213,20 @@ class AddrTypes
 using BaseIndexingPolicy = IndexingPolicyTemplate<AddrTypes>;
 template class IndexingPolicyTemplate<AddrTypes>;
 
+/**
+ * This helper generates an a tag extractor function object
+ * which will be typically used by Replaceable entries indexed
+ * with the BaseIndexingPolicy.
+ * It allows to "decouple" indexing from tagging. Those entries
+ * would call the functor without directly holding a pointer
+ * to the indexing policy which should reside in the cache.
+ */
+static constexpr auto
+genTagExtractor(BaseIndexingPolicy *ip)
+{
+    return [ip] (Addr addr) { return ip->extractTag(addr); };
+}
+
 } // namespace gem5
 
 #endif //__MEM_CACHE_INDEXING_POLICIES_BASE_HH__