[Mem2Reg] Fix nondeterministic corner case

JosephTremoulet · JosephTremoulet · commit 27b1e3bd4f7a · 2018-11-30T19:20:02.000Z
Summary: When mem2reg inserts phi nodes in blocks with unreachable predecessors, it adds undef operands for those incoming edges. When there are multiple such predecessors, the order is currently based on the address of the BasicBlocks. This change fixes that by using the BBNumbers in the sort/search predicates, as is done elsewhere in mem2reg to ensure determinism. Also adds a testcase with a bunch of unreachable preds, which (nodeterministically) fails without the fix. Reviewers: majnemer Reviewed By: majnemer Subscribers: mgrang, llvm-commits Differential Revision: https://reviews.llvm.org/D55077 llvm-svn: 348024
diff --git a/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp b/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
@@ -751,14 +751,18 @@ void PromoteMem2Reg::run() {
     // Ok, now we know that all of the PHI nodes are missing entries for some
     // basic blocks.  Start by sorting the incoming predecessors for efficient
     // access.
-    llvm::sort(Preds);
+    auto CompareBBNumbers = [this](BasicBlock *A, BasicBlock *B) {
+      return BBNumbers.lookup(A) < BBNumbers.lookup(B);
+    };
+    llvm::sort(Preds, CompareBBNumbers);
 
     // Now we loop through all BB's which have entries in SomePHI and remove
     // them from the Preds list.
     for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) {
       // Do a log(n) search of the Preds list for the entry we want.
       SmallVectorImpl<BasicBlock *>::iterator EntIt = std::lower_bound(
-          Preds.begin(), Preds.end(), SomePHI->getIncomingBlock(i));
+          Preds.begin(), Preds.end(), SomePHI->getIncomingBlock(i),
+          CompareBBNumbers);
       assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i) &&
              "PHI node has entry for a block which is not a predecessor!");
 
diff --git a/llvm/test/Transforms/Mem2Reg/undef-order.ll b/llvm/test/Transforms/Mem2Reg/undef-order.ll
@@ -0,0 +1,53 @@
+;RUN: opt -mem2reg -S < %s | FileCheck %s
+
+declare i1 @cond()
+
+define i32 @foo() {
+Entry:
+    %val = alloca i32
+    %c1 = call i1 @cond()
+    br i1 %c1, label %Store1, label %Store2
+Block1:
+    br label %Join
+Block2:
+    br label %Join
+Block3:
+    br label %Join
+Block4:
+    br label %Join
+Block5:
+    br label %Join
+Store1:
+    store i32 1, i32* %val
+    br label %Join
+Block6:
+    br label %Join
+Block7:
+    br label %Join
+Block8:
+    br label %Join
+Block9:
+    br label %Join
+Block10:
+    br label %Join
+Store2:
+    store i32 2, i32* %val
+    br label %Join
+Block11:
+    br label %Join
+Block12:
+    br label %Join
+Block13:
+    br label %Join
+Block14:
+    br label %Join
+Block15:
+    br label %Join
+Block16:
+    br label %Join
+Join:
+; Phi inserted here should have operands appended deterministically
+; CHECK: %val.0 = phi i32 [ 1, %Store1 ], [ 2, %Store2 ], [ undef, %Block1 ], [ undef, %Block2 ], [ undef, %Block3 ], [ undef, %Block4 ], [ undef, %Block5 ], [ undef, %Block6 ], [ undef, %Block7 ], [ undef, %Block8 ], [ undef, %Block9 ], [ undef, %Block10 ], [ undef, %Block11 ], [ undef, %Block12 ], [ undef, %Block13 ], [ undef, %Block14 ], [ undef, %Block15 ], [ undef, %Block16 ]
+    %result = load i32, i32* %val
+    ret i32 %result
+}
