Make StringTableBuilder to cache hash values.

This change seems to speed up LLD a bit if it has a lot of mergeable
sections. The number is below. It's not too bad for a small patch.

Time to link Clang (debug build):

w/o patch 6.3696 seconds
w/patch   6.2746 seconds (-1.5%)

Differential Revision: http://reviews.llvm.org/D19933

llvm-svn: 268698

2 files changed, 9 insertions, 6 deletions

diff --git a/llvm/include/llvm/MC/StringTableBuilder.h b/llvm/include/llvm/MC/StringTableBuilder.h
index 5a5e518f976..f2b8ecd2d99 100644
--- a/llvm/include/llvm/MC/StringTableBuilder.h
+++ b/llvm/include/llvm/MC/StringTableBuilder.h
@@ -23,7 +23,7 @@ public:
 
 private:
   SmallString<256> StringTable;
-  DenseMap<StringRef, size_t> StringIndexMap;
+  DenseMap<CachedHash<StringRef>, size_t> StringIndexMap;
   size_t Size = 0;
   Kind K;
   unsigned Alignment;
@@ -57,7 +57,10 @@ public:
   /// after the table is finalized.
   size_t getOffset(StringRef S) const;
 
-  const DenseMap<StringRef, size_t> &getMap() const { return StringIndexMap; }
+  const DenseMap<CachedHash<StringRef>, size_t> &getMap() const {
+    return StringIndexMap;
+  }
+
   size_t getSize() const { return Size; }
   void clear();
 
diff --git a/llvm/lib/MC/StringTableBuilder.cpp b/llvm/lib/MC/StringTableBuilder.cpp
index 6118b17f248..9d95952a6d3 100644
--- a/llvm/lib/MC/StringTableBuilder.cpp
+++ b/llvm/lib/MC/StringTableBuilder.cpp
@@ -34,11 +34,11 @@ StringTableBuilder::StringTableBuilder(Kind K, unsigned Alignment)
   }
 }
 
-typedef std::pair<StringRef, size_t> StringPair;
+typedef std::pair<CachedHash<StringRef>, size_t> StringPair;
 
 // Returns the character at Pos from end of a string.
 static int charTailAt(StringPair *P, size_t Pos) {
-  StringRef S = P->first;
+  StringRef S = P->first.Val;
   if (Pos >= S.size())
     return -1;
   return (unsigned char)S[S.size() - Pos - 1];
@@ -86,7 +86,7 @@ void StringTableBuilder::finalizeInOrder() {
 }
 
 void StringTableBuilder::finalizeStringTable(bool Optimize) {
-  typedef std::pair<StringRef, size_t> StringOffsetPair;
+  typedef std::pair<CachedHash<StringRef>, size_t> StringOffsetPair;
   std::vector<StringOffsetPair *> Strings;
   Strings.reserve(StringIndexMap.size());
   for (StringOffsetPair &P : StringIndexMap)
@@ -121,7 +121,7 @@ void StringTableBuilder::finalizeStringTable(bool Optimize) {
 
   StringRef Previous;
   for (StringOffsetPair *P : Strings) {
-    StringRef S = P->first;
+    StringRef S = P->first.Val;
     if (K == WinCOFF)
       assert(S.size() > COFF::NameSize && "Short string in COFF string table!");