Index: lib/Transforms/Utils/SimplifyIndVar.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyIndVar.cpp
+++ lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -555,6 +555,26 @@
       return false;
   }
 
+  auto HighestBitsMatch = [&](Value *X, Value *Y) {
+    const SCEV *SCEVX = SE->getSCEV(X);
+    const SCEV *SCEVY = SE->getSCEV(X);
+    return (SE->isKnownNonNegative(SCEVX) && SE->isKnownNonNegative(SCEVY)) ||
+           (SE->isKnownNegative(SCEVX) && SE->isKnownNegative(SCEVY));
+  };
+  auto CanUseZExt = [&](ICmpInst *ICI) {
+    // Unsigned comparison can be widened as unsigned.
+    if (ICI->isUnsigned())
+      return true;
+    // Is it profitable to do zext?
+    if (!DoesZExtCollapse)
+      return false;
+    // For equality, we can safely zext both parts.
+    if (ICI->isEquality())
+      return true;
+    // Otherwise we can only use zext when comparing two non-negative or two
+    // negative values.
+    return HighestBitsMatch(ICI->getOperand(0), ICI->getOperand(1));
+  };
   // Replace all comparisons against trunc with comparisons against IV.
   for (auto *ICI : ICmpUsers) {
     auto *Op1 = ICI->getOperand(1);
@@ -565,17 +585,20 @@
     // then prefer zext as a more canonical form.
     // TODO: If we see a signed comparison which can be turned into unsigned,
     // we can do it here for canonicalization purposes.
-    if (ICI->isUnsigned() || (ICI->isEquality() && DoesZExtCollapse)) {
+    ICmpInst::Predicate Pred = ICI->getPredicate();
+    if (CanUseZExt(ICI)) {
       assert(DoesZExtCollapse && "Unprofitable zext?");
       Ext = new ZExtInst(Op1, IVTy, "zext", ICI);
+      Pred = ICmpInst::getUnsignedPredicate(Pred);
     } else {
       assert(DoesSExtCollapse && "Unprofitable sext?");
       Ext = new SExtInst(Op1, IVTy, "sext", ICI);
+      assert(Pred == ICmpInst::getSignedPredicate(Pred) && "Must be signed!");
     }
     bool Changed;
     L->makeLoopInvariant(Ext, Changed);
     (void)Changed;
-    ICmpInst *NewICI = new ICmpInst(ICI, ICI->getPredicate(), IV, Ext);
+    ICmpInst *NewICI = new ICmpInst(ICI, Pred, IV, Ext);
     ICI->replaceAllUsesWith(NewICI);
     DeadInsts.emplace_back(ICI);
   }
Index: test/Transforms/IndVarSimplify/eliminate-trunc.ll
===================================================================
--- test/Transforms/IndVarSimplify/eliminate-trunc.ll
+++ test/Transforms/IndVarSimplify/eliminate-trunc.ll
@@ -36,12 +36,12 @@
 ;
 ; CHECK-LABEL: @test_01(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[SEXT:%.*]] = sext i32 [[N:%.*]] to i64
+; CHECK-NEXT:    [[ZEXT:%.*]] = zext i32 [[N:%.*]] to i64
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
-; CHECK-NEXT:    [[TMP0:%.*]] = icmp slt i64 [[IV]], [[SEXT]]
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i64 [[IV]], [[ZEXT]]
 ; CHECK-NEXT:    br i1 [[TMP0]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
@@ -63,12 +63,12 @@
 ;
 ; CHECK-LABEL: @test_02(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[SEXT:%.*]] = sext i32 [[N:%.*]] to i64
+; CHECK-NEXT:    [[ZEXT:%.*]] = zext i32 [[N:%.*]] to i64
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 2147483646, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
-; CHECK-NEXT:    [[TMP0:%.*]] = icmp slt i64 [[IV]], [[SEXT]]
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i64 [[IV]], [[ZEXT]]
 ; CHECK-NEXT:    br i1 [[TMP0]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
@@ -90,9 +90,11 @@
 ;
 ; CHECK-LABEL: @test_03(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[ZEXT:%.*]] = zext i32 [[N:%.*]] to i64
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    br i1 false, label [[LOOP]], label [[EXIT:%.*]]
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i64 2147483647, [[ZEXT]]
+; CHECK-NEXT:    br i1 [[TMP0]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;
@@ -403,12 +405,12 @@
 ; CHECK-LABEL: @test_08(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[ZEXT:%.*]] = zext i32 [[N:%.*]] to i64
-; CHECK-NEXT:    [[SEXT:%.*]] = sext i32 [[N]] to i64
+; CHECK-NEXT:    [[ZEXT1:%.*]] = zext i32 [[N]] to i64
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 1, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
-; CHECK-NEXT:    [[TMP0:%.*]] = icmp slt i64 [[IV]], [[SEXT]]
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i64 [[IV]], [[ZEXT1]]
 ; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult i64 [[IV]], [[ZEXT]]
 ; CHECK-NEXT:    [[CMP:%.*]] = and i1 [[TMP0]], [[TMP1]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP]], label [[EXIT:%.*]]