ValueMapper: Stop memoizing ConstantAsMetadata

Stop memoizing ConstantAsMetadata in ValueMapper::mapMetadata.  Now we
have to recompute it, but these metadata aren't particularly common, and
it restricts the lifetime of the Metadata map unnecessarily.

(The motivation is that I have a patch which uses a single Metadata map
for the lifetime of IRMover.  Mehdi profiled r266446 with the patch
applied and we saw a pretty big speedup in lib/Linker.)

llvm-svn: 266513

1 files changed, 5 insertions, 4 deletions

diff --git a/llvm/unittests/Transforms/Utils/ValueMapperTest.cpp b/llvm/unittests/Transforms/Utils/ValueMapperTest.cpp
index 2c6d45a..34b62bb 100644
--- a/llvm/unittests/Transforms/Utils/ValueMapperTest.cpp
+++ b/llvm/unittests/Transforms/Utils/ValueMapperTest.cpp
@@ -238,13 +238,14 @@ TEST(ValueMapperTest, mapMetadataConstantAsMetadata) {
 
   auto *CAM = ConstantAsMetadata::get(F.get());
   {
+    // ConstantAsMetadata shouldn't be memoized.
     ValueToValueMapTy VM;
     EXPECT_EQ(CAM, ValueMapper(VM).mapMetadata(*CAM));
-    EXPECT_TRUE(VM.MD().count(CAM));
-    VM.MD().erase(CAM);
+    EXPECT_FALSE(VM.MD().count(CAM));
     EXPECT_EQ(CAM, ValueMapper(VM, RF_IgnoreMissingLocals).mapMetadata(*CAM));
-    EXPECT_TRUE(VM.MD().count(CAM));
+    EXPECT_FALSE(VM.MD().count(CAM));
 
+    // But it should respect a mapping that gets seeded.
     auto *N = MDTuple::get(C, None);
     VM.MD()[CAM].reset(N);
     EXPECT_EQ(N, ValueMapper(VM).mapMetadata(*CAM));
@@ -256,7 +257,7 @@ TEST(ValueMapperTest, mapMetadataConstantAsMetadata) {
   ValueToValueMapTy VM;
   VM[F.get()] = F2.get();
   auto *F2MD = ValueMapper(VM).mapMetadata(*CAM);
-  EXPECT_TRUE(VM.MD().count(CAM));
+  EXPECT_FALSE(VM.MD().count(CAM));
   EXPECT_TRUE(F2MD);
   EXPECT_EQ(F2.get(), cast<ConstantAsMetadata>(F2MD)->getValue());
 }