[llvm] 3610d31 - [NFC] Fix quadratic LexicalScopes::constructScopeNest

[Jeremy Morse via llvm-commits]

Author: Jan-Willem Maessen
Date: 2020-06-08T18:40:56+01:00
New Revision: 3610d31e7a366da27901313e03089736ec65c91d

URL: https://github.com/llvm/llvm-project/commit/3610d31e7a366da27901313e03089736ec65c91d
DIFF: https://github.com/llvm/llvm-project/commit/3610d31e7a366da27901313e03089736ec65c91d.diff

LOG: [NFC] Fix quadratic LexicalScopes::constructScopeNest

We sometimes have functions with large numbers of sibling basic
blocks (usually with an error path exit from each one). This was
triggering the qudratic behavior in this function - after visiting
each child llvm would re-scan the parent from the beginning again. We
modify the work stack to record the next index to be worked on
alongside the pointer. This avoids the need to linearly search for
the next unfinished child.

Differential Revision: https://reviews.llvm.org/D80029

Added: 
    

Modified: 
    llvm/lib/CodeGen/LexicalScopes.cpp

Removed: 
    


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diff  --git a/llvm/lib/CodeGen/LexicalScopes.cpp b/llvm/lib/CodeGen/LexicalScopes.cpp
index c5d786b13f90..690b429832a5 100644
--- a/llvm/lib/CodeGen/LexicalScopes.cpp
+++ b/llvm/lib/CodeGen/LexicalScopes.cpp
@@ -230,24 +230,24 @@ LexicalScopes::getOrCreateAbstractScope(const DILocalScope *Scope) {
   return &I->second;
 }
 
-/// constructScopeNest
+/// constructScopeNest - Traverse the Scope tree depth-first, storing
+/// traversal state in WorkStack and recording the depth-first
+/// numbering (setDFSIn, setDFSOut) for edge classification.
 void LexicalScopes::constructScopeNest(LexicalScope *Scope) {
   assert(Scope && "Unable to calculate scope dominance graph!");
-  SmallVector<LexicalScope *, 4> WorkStack;
-  WorkStack.push_back(Scope);
+  SmallVector<std::pair<LexicalScope *, size_t>, 4> WorkStack;
+  WorkStack.push_back(std::make_pair(Scope, 0));
   unsigned Counter = 0;
   while (!WorkStack.empty()) {
-    LexicalScope *WS = WorkStack.back();
+    auto &ScopePosition = WorkStack.back();
+    LexicalScope *WS = ScopePosition.first;
+    size_t ChildNum = ScopePosition.second++;
     const SmallVectorImpl<LexicalScope *> &Children = WS->getChildren();
-    bool visitedChildren = false;
-    for (auto &ChildScope : Children)
-      if (!ChildScope->getDFSOut()) {
-        WorkStack.push_back(ChildScope);
-        visitedChildren = true;
-        ChildScope->setDFSIn(++Counter);
-        break;
-      }
-    if (!visitedChildren) {
+    if (ChildNum < Children.size()) {
+      auto &ChildScope = Children[ChildNum];
+      WorkStack.push_back(std::make_pair(ChildScope, 0));
+      ChildScope->setDFSIn(++Counter);
+    } else {
       WorkStack.pop_back();
       WS->setDFSOut(++Counter);
     }