Don't rely on the DepCands iteration order when constructing checking pointer groups

Summary:
The checking pointer group construction algorithm relied on the iteration on DepCands.
We would need the same leaders across runs and the same iteration order over the underlying std::set for determinism.

This changes the algorithm to process the pointers in the order in which they were added to the runtime check, which is deterministic.
We need to update the tests, since the order in which pointers appear has changed.

No new tests were added, since it is impossible to test for non-determinism.

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11064

llvm-svn: 241809

1 files changed, 26 insertions, 4 deletions

diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 35431f62a6c..cd30915ac0a 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -222,18 +222,40 @@ void LoopAccessInfo::RuntimePointerCheck::groupChecks(
   for (unsigned Pointer = 0; Pointer < Pointers.size(); ++Pointer)
     PositionMap[Pointers[Pointer]] = Pointer;
 
+  // We need to keep track of what pointers we've already seen so we
+  // don't process them twice.
+  SmallSet<unsigned, 2> Seen;
+
   // Go through all equivalence classes, get the the "pointer check groups"
-  // and add them to the overall solution.
-  for (auto DI = DepCands.begin(), DE = DepCands.end(); DI != DE; ++DI) {
-    if (!DI->isLeader())
+  // and add them to the overall solution. We use the order in which accesses
+  // appear in 'Pointers' to enforce determinism.
+  for (unsigned I = 0; I < Pointers.size(); ++I) {
+    // We've seen this pointer before, and therefore already processed
+    // its equivalence class.
+    if (Seen.count(I))
       continue;
 
+    MemoryDepChecker::MemAccessInfo Access(Pointers[I], IsWritePtr[I]);
+
     SmallVector<CheckingPtrGroup, 2> Groups;
+    auto LeaderI = DepCands.findValue(DepCands.getLeaderValue(Access));
+
+    SmallVector<unsigned, 2> MemberIndices;
 
-    for (auto MI = DepCands.member_begin(DI), ME = DepCands.member_end();
+    // Get all indeces of the members of this equivalence class and sort them.
+    // This will allow us to process all accesses in the order in which they
+    // were added to the RuntimePointerCheck.
+    for (auto MI = DepCands.member_begin(LeaderI), ME = DepCands.member_end();
          MI != ME; ++MI) {
       unsigned Pointer = PositionMap[MI->getPointer()];
+      MemberIndices.push_back(Pointer);
+    }
+    std::sort(MemberIndices.begin(), MemberIndices.end());
+
+    for (unsigned Pointer : MemberIndices) {
       bool Merged = false;
+      // Mark this pointer as seen.
+      Seen.insert(Pointer);
 
       // Go through all the existing sets and see if we can find one
       // which can include this pointer.