Switch a std::map to a DenseMap in CodeGenRegisters

[resistor]

Summary

As far as I can tell, the keys of the map are unique by pointer address, so there's no need to use the llvm::less comparator. This allows us to use DenseMap instead, which reduces tblgen time by 20% on my stress test.

[resistor]

Added checking of uniqueness to one loop that would potentially change iteration order here. I'd prefer *not* to add checking to computeSubRegs() in CodeGenRegisters.cpp because the checking would be quite heavyweight, the method is very hot, and I am explicitly trying to improve LLVM build times in Release+Asserts configs here.

[resistor]

Ping?

[resistor]

r230769

utils/TableGen/CodeGenRegisters.h

     std::string getQualifiedName() const;
 
     // Map of composite subreg indices.
-    typedef std::map<CodeGenSubRegIndex *, CodeGenSubRegIndex *,
-                     deref<llvm::less>> CompMap;
+    typedef DenseMap<CodeGenSubRegIndex *, CodeGenSubRegIndex *> CompMap;
 
     // Returns the subreg index that results from composing this with Idx.
     // Returns NULL if this and Idx don't compose.

utils/TableGen/RegisterInfoEmitter.cpp

 static bool combine(const CodeGenSubRegIndex *Idx,
                     SmallVectorImpl<CodeGenSubRegIndex*> &Vec) {
   const CodeGenSubRegIndex::CompMap &Map = Idx->getComposites();
-  for (CodeGenSubRegIndex::CompMap::const_iterator
-       I = Map.begin(), E = Map.end(); I != E; ++I) {
-    CodeGenSubRegIndex *&Entry = Vec[I->first->EnumValue - 1];
-    if (Entry && Entry != I->second)
+  for (const auto &I : Map) {
+    CodeGenSubRegIndex *&Entry = Vec[I.first->EnumValue - 1];
+    if (Entry && Entry != I.second)
       return false;
   }
 
   // All entries are compatible. Make it so.
-  for (CodeGenSubRegIndex::CompMap::const_iterator
-       I = Map.begin(), E = Map.end(); I != E; ++I)
-    Vec[I->first->EnumValue - 1] = I->second;
+  for (const auto &I : Map) {
+    auto *&Entry = Vec[I.first->EnumValue - 1];
+    assert((!Entry || Entry == I.second) &&
+           "Expected EnumValue to be unique");
+    Entry = I.second;
+  }
   return true;
 }