[ADT] Teach DenseMap to support StringRef keys.

While often you want to use something specialized like StringMap, when
the strings already have persistent storage a normal densemap over them
can be more efficient.

This can't go into StringRef.h because of really obnoxious header chains
from the hashing code to the endian detection code to CPU feature
detection code to StringMap.

llvm-svn: 240528

llvm/include/llvm/ADT/DenseMapInfo.h

@@ -14,6 +14,8 @@
 #ifndef LLVM_ADT_DENSEMAPINFO_H
 #define LLVM_ADT_DENSEMAPINFO_H
 
+#include "llvm/ADT/Hashing.h"
+#include "llvm/ADT/StringRef.h"
 #include "llvm/Support/PointerLikeTypeTraits.h"
 #include "llvm/Support/type_traits.h"
 
@@ -163,6 +165,31 @@ struct DenseMapInfo<std::pair<T, U> > {
   }
 };
 
+// Provide DenseMapInfo for StringRefs.
+template <> struct DenseMapInfo<StringRef> {
+  static inline StringRef getEmptyKey() {
+    return StringRef(reinterpret_cast<const char *>(~static_cast<uintptr_t>(0)),
+                     0);
+  }
+  static inline StringRef getTombstoneKey() {
+    return StringRef(reinterpret_cast<const char *>(~static_cast<uintptr_t>(1)),
+                     0);
+  }
+  static unsigned getHashValue(StringRef Val) {
+    assert(Val.data() != getEmptyKey().data() && "Cannot hash the empty key!");
+    assert(Val.data() != getTombstoneKey().data() &&
+           "Cannot hash the tombstone key!");
+    return (unsigned)(hash_value(Val));
+  }
+  static bool isEqual(StringRef LHS, StringRef RHS) {
+    if (RHS.data() == getEmptyKey().data())
+      return LHS.data() == getEmptyKey().data();
+    if (RHS.data() == getTombstoneKey().data())
+      return LHS.data() == getTombstoneKey().data();
+    return LHS == RHS;
+  }
+};
+
 } // end namespace llvm
 
 #endif

llvm/unittests/ADT/DenseMapTest.cpp

@@ -323,6 +323,31 @@ TYPED_TEST(DenseMapTest, ConstIteratorTest) {
   EXPECT_TRUE(cit == cit2);
 }
 
+// Make sure DenseMap works with StringRef keys.
+TEST(DenseMapCustomTest, StringRefTest) {
+  DenseMap<StringRef, int> M;
+
+  M["a"] = 1;
+  M["b"] = 2;
+  M["c"] = 3;
+
+  EXPECT_EQ(3u, M.size());
+  EXPECT_EQ(1, M.lookup("a"));
+  EXPECT_EQ(2, M.lookup("b"));
+  EXPECT_EQ(3, M.lookup("c"));
+
+  EXPECT_EQ(0, M.lookup("q"));
+
+  // Test the empty string, spelled various ways.
+  EXPECT_EQ(0, M.lookup(""));
+  EXPECT_EQ(0, M.lookup(StringRef()));
+  EXPECT_EQ(0, M.lookup(StringRef("a", 0)));
+  M[""] = 42;
+  EXPECT_EQ(42, M.lookup(""));
+  EXPECT_EQ(42, M.lookup(StringRef()));
+  EXPECT_EQ(42, M.lookup(StringRef("a", 0)));
+}
+
 // Key traits that allows lookup with either an unsigned or char* key;
 // In the latter case, "a" == 0, "b" == 1 and so on.
 struct TestDenseMapInfo {