[IR] Use SmallDenseSet (NFC)

[kazutakahirata]

The use of SmallDenseSet saves 0.39% of heap allocations during the
compilation of a large preprocessed file, namely X86ISelLowering.cpp,
for the X86 target. During the experiment, WL.size() was 2 or less
99.9% of the time. The inline size of 4 should accommodate up to 2
entries at the 3/4 occupancy rate.

[llvmbot]

@llvm/pr-subscribers-llvm-ir

Author: Kazu Hirata (kazutakahirata)

Changes

The use of SmallDenseSet saves 0.39% of heap allocations during the
compilation of a large preprocessed file, namely X86ISelLowering.cpp,
for the X86 target. During the experiment, WL.size() was 2 or less
99.9% of the time. The inline size of 4 should accommodate up to 2
entries at the 3/4 occupancy rate.

---

Full diff: https://github.com/llvm/llvm-project/pull/79556.diff

1 Files Affected:

• (modified) llvm/lib/IR/Instruction.cpp (+1-3)

diff --git a/llvm/lib/IR/Instruction.cpp b/llvm/lib/IR/Instruction.cpp
index f9a38e48166c9a..904ce17fb0e7c9 100644
--- a/llvm/lib/IR/Instruction.cpp
+++ b/llvm/lib/IR/Instruction.cpp
@@ -1167,9 +1167,7 @@ void Instruction::copyMetadata(const Instruction &SrcInst,
   if (!SrcInst.hasMetadata())
     return;
 
-  DenseSet<unsigned> WLS;
-  for (unsigned M : WL)
-    WLS.insert(M);
+  SmallDenseSet<unsigned, 4> WLS(WL.begin(), WL.end());
 
   // Otherwise, enumerate and copy over metadata from the old instruction to the
   // new one.

[nikic]

LGTM