[WebAssembly] Fix a non-determinism problem in FixIrreducibleControlFlow

Summary: We already sorted the blocks when fixing up a set of mutual loop entries, however, there can be multiple sets of such mutual loop entries, and the order we encounter them should not be random, so sort them too. Fixes https://bugs.llvm.org/show_bug.cgi?id=44982 Patch by Alon Zakai (kripken) Reviewers: aheejin, sbc100, dschuff Subscribers: mgrang, sunfish, hiraditya, jgravelle-google, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D74999
rust-lang · cuviper · Jul 9, 2020 · Feb 22, 2020 · Jul 9, 2020 · 4ae9fa5659ae38b4df3298b1500b8d450a3b9826
commit 4ae9fa5659ae38b4df3298b1500b8d450a3b9826
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyFixIrreducibleControlFlow.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyFixIrreducibleControlFlow.cpp
@@ -66,6 +66,17 @@ namespace {
 using BlockVector = SmallVector<MachineBasicBlock *, 4>;
 using BlockSet = SmallPtrSet<MachineBasicBlock *, 4>;
 
+static BlockVector getSortedEntries(const BlockSet &Entries) {
+  BlockVector SortedEntries(Entries.begin(), Entries.end());
+  llvm::sort(SortedEntries,
+             [](const MachineBasicBlock *A, const MachineBasicBlock *B) {
+               auto ANum = A->getNumber();
+               auto BNum = B->getNumber();
+               return ANum < BNum;
+             });
+  return SortedEntries;
+}
+
 // Calculates reachability in a region. Ignores branches to blocks outside of
 // the region, and ignores branches to the region entry (for the case where
 // the region is the inner part of a loop).
@@ -241,20 +252,25 @@ class WebAssemblyFixIrreducibleControlFlow final : public MachineFunctionPass {
 bool WebAssemblyFixIrreducibleControlFlow::processRegion(
     MachineBasicBlock *Entry, BlockSet &Blocks, MachineFunction &MF) {
   bool Changed = false;
-
   // Remove irreducibility before processing child loops, which may take
   // multiple iterations.
   while (true) {
     ReachabilityGraph Graph(Entry, Blocks);
 
     bool FoundIrreducibility = false;
 
-    for (auto *LoopEntry : Graph.getLoopEntries()) {
+    for (auto *LoopEntry : getSortedEntries(Graph.getLoopEntries())) {
       // Find mutual entries - all entries which can reach this one, and
       // are reached by it (that always includes LoopEntry itself). All mutual
       // entries must be in the same loop, so if we have more than one, then we
       // have irreducible control flow.
       //
+      // (Note that we need to sort the entries here, as otherwise the order can
+      // matter: being mutual is a symmetric relationship, and each set of
+      // mutuals will be handled properly no matter which we see first. However,
+      // there can be multiple disjoint sets of mutuals, and which we process
+      // first changes the output.)
+      //
       // Note that irreducibility may involve inner loops, e.g. imagine A
       // starts one loop, and it has B inside it which starts an inner loop.
       // If we add a branch from all the way on the outside to B, then in a
@@ -325,13 +341,7 @@ void WebAssemblyFixIrreducibleControlFlow::makeSingleEntryLoop(
   assert(Entries.size() >= 2);
 
   // Sort the entries to ensure a deterministic build.
-  BlockVector SortedEntries(Entries.begin(), Entries.end());
-  llvm::sort(SortedEntries,
-             [&](const MachineBasicBlock *A, const MachineBasicBlock *B) {
-               auto ANum = A->getNumber();
-               auto BNum = B->getNumber();
-               return ANum < BNum;
-             });
+  BlockVector SortedEntries = getSortedEntries(Entries);
 
 #ifndef NDEBUG
   for (auto Block : SortedEntries)