[IR] Use SmallSet with more inline elements in dropUnknownNonDebugMetadata (NFC)

[kazutakahirata]

SmallSet here often ends up allocating memory via std::set inside
SmallSet because KnownIDs.size() goes up to 17 on an x86 host. This
patch switches to SmallSet<unsigned, 32> to avoid memory
allocations.

The increased inline elements here save 0.57% of heap allocations during
the compilation of X86ISelLowering.cpp.ii, a preprocessed version of
X86ISelLowering.cpp.

[llvmbot]

@llvm/pr-subscribers-llvm-ir

Author: Kazu Hirata (kazutakahirata)

Changes

SmallSet here often ends up allocating memory via std::set inside
SmallSet because KnownIDs.size() goes up to 17 on an x86 host. This
patch switches to SmallDenseSet<unsigned, 32> to avoid memory
allocations.

The use of SmallDenseSet here saves 0.57% of heap allocations during
the compilation of X86ISelLowering.cpp.ii, a preprocessed version of
X86ISelLowering.cpp.

---

Full diff: https://github.com/llvm/llvm-project/pull/98853.diff

1 Files Affected:

• (modified) llvm/lib/IR/Metadata.cpp (+1-2)

diff --git a/llvm/lib/IR/Metadata.cpp b/llvm/lib/IR/Metadata.cpp
index 5f42ce22f72fe..a1bae38b07ab6 100644
--- a/llvm/lib/IR/Metadata.cpp
+++ b/llvm/lib/IR/Metadata.cpp
@@ -20,7 +20,6 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
-#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringRef.h"
@@ -1590,7 +1589,7 @@ void Instruction::dropUnknownNonDebugMetadata(ArrayRef<unsigned> KnownIDs) {
   if (!Value::hasMetadata())
     return; // Nothing to remove!
 
-  SmallSet<unsigned, 4> KnownSet;
+  SmallDenseSet<unsigned, 32> KnownSet;
   KnownSet.insert(KnownIDs.begin(), KnownIDs.end());
 
   // A DIAssignID attachment is debug metadata, don't drop it.

[kazutakahirata]

Please take a look. Thanks!

[nikic]

LGTM