[clang] [clang][LifetimeSafety] Short-circuit joins of identical immutable containers (PR #208908)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-analysis

Author: Gábor Horváth (Xazax-hun)

<details>
<summary>Changes</summary>

The dataflow join unions two ImmutableSets / ImmutableMaps key-wise by re-inserting every entry of one into the other. At confluence points the two operands are frequently the same tree -- state carried through a branch that did not touch it -- and the union is then that tree. Return it directly when the roots are pointer-equal: O(1) instead of O(n log n). A dataflow join is idempotent, so the result is exact.

benchmark.py, LoanPropagation phase:

| Test                  | Before   | After    | Speedup |
|-----------------------|---------:|---------:|:-------:|
| Switch fan-out N=1000 |  71.3 ms |   1.7 ms | ~41x    |
| Switch fan-out N=2000 | 329.9 ms |   3.9 ms | ~85x    |
| Switch fan-out N=4000 |  1440 ms |   8.6 ms | ~168x   |
| Merge N=2000          | 297.0 ms | 236.8 ms | 1.25x   |
| Merge N=5000          |  2110 ms |  1680 ms | 1.26x   |
| Cycle N=300           |  1740 ms |  1670 ms | 1.04x   |

The speedup tracks how much branches perturb pointer state: switch fan-out merges identical maps (whole-map short-circuit), merge carries most origins unchanged (per-value short-circuit), and the cycle reseats every pointer each iteration so little is shared.

Assisted by: Claude Opus 4.8

---
Full diff: https://github.com/llvm/llvm-project/pull/208908.diff


1 Files Affected:

- (modified) clang/include/clang/Analysis/Analyses/LifetimeSafety/Utils.h (+4) 


``````````diff
diff --git a/clang/include/clang/Analysis/Analyses/LifetimeSafety/Utils.h b/clang/include/clang/Analysis/Analyses/LifetimeSafety/Utils.h
index 101ee083f0d95..543bbd045593c 100644
--- a/clang/include/clang/Analysis/Analyses/LifetimeSafety/Utils.h
+++ b/clang/include/clang/Analysis/Analyses/LifetimeSafety/Utils.h
@@ -38,6 +38,8 @@ template <typename Tag> struct ID {
 template <typename T>
 llvm::ImmutableSet<T> join(llvm::ImmutableSet<T> A, llvm::ImmutableSet<T> B,
                            typename llvm::ImmutableSet<T>::Factory &F) {
+  if (A.getRootWithoutRetain() == B.getRootWithoutRetain())
+    return A;
   if (A.getHeight() < B.getHeight())
     std::swap(A, B);
   for (const T &E : B)
@@ -70,6 +72,8 @@ llvm::ImmutableMap<K, V> join(const llvm::ImmutableMap<K, V> &A,
                               const llvm::ImmutableMap<K, V> &B,
                               typename llvm::ImmutableMap<K, V>::Factory &F,
                               Joiner JoinValues, JoinKind Kind) {
+  if (A.getRootWithoutRetain() == B.getRootWithoutRetain())
+    return A;
   if (A.getHeight() < B.getHeight())
     return join(B, A, F, JoinValues, Kind);
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/208908