[ADT][StringMap] Add ability to precompute and reuse the string hash

Useful for lldb's const string pool, using the hash to determine which
string map to lock and query/insert.

Derived from https://reviews.llvm.org/D122974 by Luboš Luňák

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/unittests/ADT/StringMapTest.cpp b/llvm/unittests/ADT/StringMapTest.cpp
index f9b138e..c9ef3f8a 100644
--- a/llvm/unittests/ADT/StringMapTest.cpp
+++ b/llvm/unittests/ADT/StringMapTest.cpp
@@ -487,6 +487,17 @@ TEST_F(StringMapTest, NotEqualWithDifferentValues) {
   ASSERT_TRUE(B != A);
 }
 
+TEST_F(StringMapTest, PrecomputedHash) {
+  StringMap<int> A;
+  StringRef Key = "foo";
+  int Value = 42;
+  uint64_t Hash = StringMap<int>::hash(Key);
+  A.insert({"foo", Value}, Hash);
+  auto I = A.find(Key, Hash);
+  ASSERT_NE(I, A.end());
+  ASSERT_EQ(I->second, Value);
+}
+
 struct Countable {
   int &InstanceCount;
   int Number;