[llvm] [SPIR-V] Avoid exponential path enumeration in findPathsToMatch (PR #212933)

[Juan Manuel Martinez Caamaño via llvm-commits]

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@@ -213,29 +214,52 @@ class ConvergenceRegionAnalyzer {
   SmallPtrSet<BasicBlock *, 0>
   findPathsToMatch(LoopInfo &LI, BasicBlock *From,
                    std::function<bool(const BasicBlock *)> isMatch) const {
-    SmallPtrSet<BasicBlock *, 0> Output;
-
-    if (isMatch(From))
-      Output.insert(From);
+    // Compute the postorder of the blocks forward-reachable from |From|,
+    // ignoring back edges. Successors therefore always appear before their
+    // predecessors in the resulting list.
+    SmallVector<BasicBlock *, 16> PostOrder;
+    SmallPtrSet<BasicBlock *, 16> Visited;
+    SmallVector<std::pair<BasicBlock *, unsigned>, 16> Stack;
+
+    Visited.insert(From);
+    Stack.push_back({From, 0});
+    while (!Stack.empty()) {
+      auto &[BB, NextSuccessor] = Stack.back();
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jmmartinez wrote:

`NextSuccessor` is always 0 ?

https://github.com/llvm/llvm-project/pull/212933