[PATCH] D82771: [ASTMatcher] Fix a performance regression: memorize the child match.

[Haojian Wu via Phabricator via cfe-commits]

hokein created this revision.
hokein added reviewers: klimek, sammccall.
Herald added a project: clang.

D80025 <https://reviews.llvm.org/D80025> introduced a performance regression: in some cases, it makes
clang-tidy readability-container-size-empty ~80x slower (running on an internal
huge TU, before that patch 12s vs after 950s).

after this patch, we go back to 12s.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D82771

Files:
  clang/lib/ASTMatchers/ASTMatchFinder.cpp


Index: clang/lib/ASTMatchers/ASTMatchFinder.cpp
===================================================================
--- clang/lib/ASTMatchers/ASTMatchFinder.cpp
+++ clang/lib/ASTMatchers/ASTMatchFinder.cpp
@@ -45,7 +45,9 @@
 
 enum class MatchType {
   Ancestors,
-  Descendants
+
+  Descendants,
+  Child,
 };
 
 // We use memoization to avoid running the same matcher on the same
@@ -452,8 +454,7 @@
                                   BoundNodesTreeBuilder *Builder, int MaxDepth,
                                   TraversalKind Traversal, BindKind Bind) {
     // For AST-nodes that don't have an identity, we can't memoize.
-    // When doing a single-level match, we don't need to memoize
-    if (!Node.getMemoizationData() || !Builder->isComparable() || MaxDepth == 1)
+    if (!Node.getMemoizationData() || !Builder->isComparable())
       return matchesRecursively(Node, Matcher, Builder, MaxDepth, Traversal,
                                 Bind);
 
@@ -463,7 +464,8 @@
     // Note that we key on the bindings *before* the match.
     Key.BoundNodes = *Builder;
     Key.Traversal = Ctx.getParentMapContext().getTraversalKind();
-    Key.Type = MatchType::Descendants;
+    // Memorize result even doing a single-level match, it might be expensive.
+    Key.Type = MaxDepth == 1 ? MatchType::Child : MatchType::Descendants;
     MemoizationMap::iterator I = ResultCache.find(Key);
     if (I != ResultCache.end()) {
       *Builder = I->second.Nodes;
@@ -700,8 +702,10 @@
                                             BoundNodesTreeBuilder *Builder,
                                             AncestorMatchMode MatchMode) {
     // For AST-nodes that don't have an identity, we can't memoize.
-    // When doing a single-level match, we don't need to memoize
-    if (!Builder->isComparable() || MatchMode == AncestorMatchMode::AMM_ParentOnly)
+    // When doing a single-level match, we don't need to memoize because
+    // ParentMap (in ASTContext) already memorizes the result.
+    if (!Builder->isComparable() ||
+        MatchMode == AncestorMatchMode::AMM_ParentOnly)
       return matchesAncestorOfRecursively(Node, Ctx, Matcher, Builder,
                                           MatchMode);
 


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D82771.274123.patch
Type: text/x-patch
Size: 2219 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20200629/e62df0f6/attachment-0001.bin>