[Utils] Avoid a hash table lookup in salvageDI, NFC

According to the current coverage report salvageDebugInfo() is called
5.12 million times during testing and almost always returns early.

The early return depends on LocalAsMetadata::getIfExists returning null,
which involves a DenseMap lookup in an LLVMContextImpl. We can probably
speed this up by simply checking the IsUsedByMD bit in Value.

llvm-svn: 325738

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index e75127f77e8c6ecec27154d9b5fcf1780479ac03..7573fc39b9d852f23ee3b1ba95b9e646a21ec6f0 100644 (file)
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1486,6 +1486,11 @@ void llvm::replaceDbgValueForAlloca(AllocaInst *AI, Value *NewAllocaAddress,
 }
 
 void llvm::salvageDebugInfo(Instruction &I) {
+  // This function is hot. An early check to determine whether the instruction
+  // has any metadata to save allows it to return earlier on average.
+  if (!I.isUsedByMetadata())
+    return;
+
   SmallVector<DbgInfoIntrinsic *, 1> DbgUsers;
   findDbgUsers(DbgUsers, &I);
   if (DbgUsers.empty())