[llvm] 0a1e832 - [ADT] Exit doFind on an empty map, not just an unallocated one (#220294)

[via llvm-commits]

Author: Matsu
Date: 2026-09-01T16:53:34-07:00
New Revision: 0a1e8326ea877739c921ef4434b469442de73066

URL: https://github.com/llvm/llvm-project/commit/0a1e8326ea877739c921ef4434b469442de73066
DIFF: https://github.com/llvm/llvm-project/commit/0a1e8326ea877739c921ef4434b469442de73066.diff

LOG: [ADT] Exit doFind on an empty map, not just an unallocated one (#220294)

`doFind` exits early only when no buckets were ever allocated. A map that
had entries and lost them keeps its bucket array. `NumBuckets` is then not
zero, so every lookup hashes the key and probes.

For a `SmallDenseMap` in small mode, `NumBuckets` is the template parameter
`InlineBuckets`, a nonzero constant. The existing check can never fire for
those maps. An empty one hashes and probes on every lookup.

`getNumEntries() == 0` covers both cases. It also subsumes the old check.
There are no entries without buckets, so `Mask = NumBuckets - 1` is still
safe. It is one test either way, so non-empty lookups are unchanged. The
check goes before `getRep()`. Keeping it after costs 0.158% on clang, so
those loads are not sunk past the branch.

| workload | instructions:u |
|---|---:|
| clang compiling 600 LLVM/Clang/MLIR translation units | **-0.028%** |
| `mlir-opt` canonicalize/cse over 1060 `mlir/test` files >4KB |
**-0.291%** |

Three interleaved runs each. Ranges do not overlap. Exit codes are
identical on all 1448 candidate `mlir/test` files.

Adding `LLVM_UNLIKELY` to the branch is a regression, not a win. It takes
clang from -0.133% to -0.007%. Once this is the first check, an empty map
is the common case, not the rare one.

Added: 
    

Modified: 
    llvm/include/llvm/ADT/DenseMap.h

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/ADT/DenseMap.h b/llvm/include/llvm/ADT/DenseMap.h
index 6d036d4a0729f..3073ab0ec9712 100644
--- a/llvm/include/llvm/ADT/DenseMap.h
+++ b/llvm/include/llvm/ADT/DenseMap.h
@@ -721,9 +721,9 @@ class DenseMapBase : public DebugEpochBase {
 
   template <typename LookupKeyT>
   const BucketT *doFind(const LookupKeyT &Val) const {
-    auto [BucketsPtr, U, NumBuckets] = getRep();
-    if (NumBuckets == 0)
+    if (empty())
       return nullptr;
+    auto [BucketsPtr, U, NumBuckets] = getRep();
 
     const unsigned Mask = NumBuckets - 1;
     unsigned BucketNo = KeyInfoT::getHashValue(Val) & Mask;