[DebugNames] Use hashes to quickly filter false positives

[felipepiovezan]

The current implementation of DebugNames is only using hashes to compute the bucket number. Once inside the bucket, it reverts back to string comparisons, even though not all hashes inside a bucket are identical.

This commit changes the behavior so that we check the hash before comparing strings. Such check is so important that it speeds up a simple benchmark by 20%. In other words, the following expression evaluation time goes from 1100ms to 850ms.

bin/lldb \
		--batch \
		-o "b CodeGenFunction::GenerateCode" \
		-o run \
		-o "expr Fn" \
		-- \
		clang++ -c -g test.cpp -o /dev/null &> output

(Note, these numbers are considering the usage of IDX_parent)

[llvmbot]

@llvm/pr-subscribers-debuginfo

Author: Felipe de Azevedo Piovezan (felipepiovezan)

Changes

The current implementation of DebugNames is only using hashes to compute the bucket number. Once inside the bucket, it reverts back to string comparisons, even though not all hashes inside a bucket are identical.

This commit changes the behavior so that we check the hash before comparing strings. Such check is so important that it speeds up a simple benchmark by 20%. In other words, the following expression evaluation time goes from 1100ms to 850ms.

bin/lldb \
		--batch \
		-o "b CodeGenFunction::GenerateCode" \
		-o run \
		-o "expr Fn" \
		-- \
		clang++ -c -g test.cpp -o /dev/null &> output

(Note, these numbers are considering the usage of IDX_parent)

---

Full diff: https://github.com/llvm/llvm-project/pull/79755.diff

1 Files Affected:

• (modified) llvm/lib/DebugInfo/DWARF/DWARFAcceleratorTable.cpp (+4-2)

diff --git a/llvm/lib/DebugInfo/DWARF/DWARFAcceleratorTable.cpp b/llvm/lib/DebugInfo/DWARF/DWARFAcceleratorTable.cpp
index 03ad5d133caddf..a1a1ac093aa5c1 100644
--- a/llvm/lib/DebugInfo/DWARF/DWARFAcceleratorTable.cpp
+++ b/llvm/lib/DebugInfo/DWARF/DWARFAcceleratorTable.cpp
@@ -937,9 +937,11 @@ DWARFDebugNames::ValueIterator::findEntryOffsetInCurrentIndex() {
     return std::nullopt; // Empty bucket
 
   for (; Index <= Hdr.NameCount; ++Index) {
-    uint32_t Hash = CurrentIndex->getHashArrayEntry(Index);
-    if (Hash % Hdr.BucketCount != Bucket)
+    uint32_t HashAtIndex = CurrentIndex->getHashArrayEntry(Index);
+    if (HashAtIndex % Hdr.BucketCount != Bucket)
       return std::nullopt; // End of bucket
+    if (HashAtIndex != Hash)
+      continue;
 
     NameTableEntry NTE = CurrentIndex->getNameTableEntry(Index);
     if (NTE.getString() == Key)

[adrian-prantl]

That's a solid improvement!

[JDevlieghere]

🥳

[dwblaikie]

Nice!