[llvm] [SSAUpdater] Don't use large SmallSets for IDFcalc (PR #97823)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-support

Author: Jeremy Morse (jmorse)

<details>
<summary>Changes</summary>

As per the LLVM programmers manual, SmallSets do linear scans on insertion and then turn into a hash-table if the set gets big. Here in the IDFCalculator, the SmallSets have been configured to have 32 elements in each static allocation... which means that we linearly scan for all problems with up to 32 elements, which I feel is quite a large N.

Replace these SmallSets with a plain DenseSet (and use reserve to avoid any repeated allocations). Doing this yields a 0.13% compile-time improvement for debug-info builds, as we hit IDFCalculator pretty hard in InstrRefBasedLDV. Adding @<!-- -->nikic as this could affect more than debug-info things, although apparently not in a visible way on CTMark.

http://llvm-compile-time-tracker.com/compare.php?from=4ce6cc488083a931e795dafe30e69a0e4a96a3e4&to=a9f1d377013b1d208ac58f6f75548f1f24d174d2&stat=instructions%3Au

(tracker link is across a few commits, with this patch being the summary).

---
Full diff: https://github.com/llvm/llvm-project/pull/97823.diff


1 Files Affected:

- (modified) llvm/include/llvm/Support/GenericIteratedDominanceFrontier.h (+6-2) 


``````````diff
diff --git a/llvm/include/llvm/Support/GenericIteratedDominanceFrontier.h b/llvm/include/llvm/Support/GenericIteratedDominanceFrontier.h
index 0dc58e37c821df..c218d33c3eb303 100644
--- a/llvm/include/llvm/Support/GenericIteratedDominanceFrontier.h
+++ b/llvm/include/llvm/Support/GenericIteratedDominanceFrontier.h
@@ -144,8 +144,12 @@ void IDFCalculatorBase<NodeTy, IsPostDom>::calculate(
   DT.updateDFSNumbers();
 
   SmallVector<DomTreeNodeBase<NodeTy> *, 32> Worklist;
-  SmallPtrSet<DomTreeNodeBase<NodeTy> *, 32> VisitedPQ;
-  SmallPtrSet<DomTreeNodeBase<NodeTy> *, 32> VisitedWorklist;
+  SmallDenseSet<DomTreeNodeBase<NodeTy> *, 16> VisitedPQ;
+  SmallDenseSet<DomTreeNodeBase<NodeTy> *, 16> VisitedWorklist;
+  if (useLiveIn) {
+    VisitedPQ.reserve(LiveInBlocks->size());
+    VisitedWorklist.reserve(LiveInBlocks->size());
+  }
 
   for (NodeTy *BB : *DefBlocks)
     if (DomTreeNodeBase<NodeTy> *Node = DT.getNode(BB)) {

``````````

</details>


https://github.com/llvm/llvm-project/pull/97823