Index: llvm/trunk/lib/Transforms/Scalar/MergeICmps.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/MergeICmps.cpp
+++ llvm/trunk/lib/Transforms/Scalar/MergeICmps.cpp
@@ -571,6 +571,19 @@
       DEBUG(dbgs() << "skip: several non-constant values\n");
       return false;
     }
+    if (!isa<ICmpInst>(Phi.getIncomingValue(I)) ||
+        cast<ICmpInst>(Phi.getIncomingValue(I))->getParent() !=
+            Phi.getIncomingBlock(I)) {
+      // Non-constant incoming value is not from a cmp instruction or not
+      // produced by the last block. We could end up processing the value
+      // producing block more than once.
+      //
+      // This is an uncommon case, so we bail.
+      DEBUG(
+          dbgs()
+          << "skip: non-constant value not from cmp or not from last block.\n");
+      return false;
+    }
     LastBlock = Phi.getIncomingBlock(I);
   }
   if (!LastBlock) {
Index: llvm/trunk/test/Transforms/MergeICmps/X86/last-block-produce-no-value.ll
===================================================================
--- llvm/trunk/test/Transforms/MergeICmps/X86/last-block-produce-no-value.ll
+++ llvm/trunk/test/Transforms/MergeICmps/X86/last-block-produce-no-value.ll
@@ -0,0 +1,57 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -mergeicmps -mtriple=x86_64-unknown-unknown -S | FileCheck %s
+
+%"struct.std::pair" = type { i32, i32, i32 }
+
+; Last block does not produce the non-constant value into the phi.
+; We could handle this case, but an easier way would be to allow other transformations such as
+; SimplifyCFG to remove %land.rhs.i.2 and turn the terminator of %land.rhs.i into an unconditional
+; branch.
+
+define zeroext i1 @opeq1(
+; CHECK-LABEL: @opeq1(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[FIRST_I:%.*]] = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* [[A:%.*]], i64 0, i32 0
+; CHECK-NEXT:    [[TMP0:%.*]] = load i32, i32* [[FIRST_I]], align 4
+; CHECK-NEXT:    [[FIRST1_I:%.*]] = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* [[B:%.*]], i64 0, i32 0
+; CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* [[FIRST1_I]], align 4
+; CHECK-NEXT:    [[CMP_I:%.*]] = icmp eq i32 [[TMP0]], [[TMP1]]
+; CHECK-NEXT:    br i1 [[CMP_I]], label [[LAND_RHS_I:%.*]], label [[OPEQ1_EXIT:%.*]]
+; CHECK:       land.rhs.i:
+; CHECK-NEXT:    [[SECOND_I:%.*]] = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* [[A]], i64 0, i32 1
+; CHECK-NEXT:    [[TMP2:%.*]] = load i32, i32* [[SECOND_I]], align 4
+; CHECK-NEXT:    [[SECOND2_I:%.*]] = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* [[B]], i64 0, i32 1
+; CHECK-NEXT:    [[TMP3:%.*]] = load i32, i32* [[SECOND2_I]], align 4
+; CHECK-NEXT:    [[CMP3_I:%.*]] = icmp eq i32 [[TMP2]], [[TMP3]]
+; CHECK-NEXT:    br i1 [[CMP3_I]], label [[LAND_RHS_I:%.*]], label [[OPEQ1_EXIT:%.*]]
+; CHECK:       land.rhs.i.2:
+; CHECK-NEXT:    br label [[OPEQ1_EXIT]]
+; CHECK:       opeq1.exit:
+; CHECK-NEXT:    [[TMP4:%.*]] = phi i1 [ false, [[ENTRY:%.*]] ], [ [[CMP3_I]], [[LAND_RHS_I]] ]
+; CHECK-NEXT:    ret i1 [[TMP4]]
+;
+  %"struct.std::pair"* nocapture readonly dereferenceable(12) %a,
+  %"struct.std::pair"* nocapture readonly dereferenceable(12) %b) local_unnamed_addr #0 {
+entry:
+  %first.i = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* %a, i64 0, i32 0
+  %0 = load i32, i32* %first.i, align 4
+  %first1.i = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* %b, i64 0, i32 0
+  %1 = load i32, i32* %first1.i, align 4
+  %cmp.i = icmp eq i32 %0, %1
+  br i1 %cmp.i, label %land.rhs.i, label %opeq1.exit
+
+land.rhs.i:
+  %second.i = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* %a, i64 0, i32 1
+  %2 = load i32, i32* %second.i, align 4
+  %second2.i = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* %b, i64 0, i32 1
+  %3 = load i32, i32* %second2.i, align 4
+  %cmp3.i = icmp eq i32 %2, %3
+  br i1 %cmp3.i, label %land.rhs.i.2, label %opeq1.exit
+
+land.rhs.i.2:
+  br label %opeq1.exit
+
+opeq1.exit:
+  %4 = phi i1 [ false, %entry ], [ false,  %land.rhs.i], [ %cmp3.i, %land.rhs.i.2 ]
+  ret i1 %4
+}