Index: lib/Analysis/ValueTracking.cpp
===================================================================
--- lib/Analysis/ValueTracking.cpp
+++ lib/Analysis/ValueTracking.cpp
@@ -795,6 +795,14 @@
     computeKnownBits(I->getOperand(0), KnownZero, KnownOne, Depth + 1, Q);
     KnownZero = KZF(KnownZero, ShiftAmt);
     KnownOne  = KOF(KnownOne, ShiftAmt);
+    // If there is conflict between KnownZero and KnownOne, this must be an 
+    // overflowing left shift, so the shift result is undefined. Clear KnownZero
+    // and KnownOne bits so that other code could propagate this undef.
+    if ((KnownZero & KnownOne) != 0) {
+      KnownZero.clearAllBits();
+      KnownOne.clearAllBits();
+    }
+    
     return;
   }
 
@@ -1065,13 +1073,23 @@
   }
   case Instruction::Shl: {
     // (shl X, C1) & C2 == 0   iff   (X & C2 >>u C1) == 0
-    auto KZF = [BitWidth](const APInt &KnownZero, unsigned ShiftAmt) {
-      return (KnownZero << ShiftAmt) |
-             APInt::getLowBitsSet(BitWidth, ShiftAmt); // Low bits known 0.
+    bool NSW = cast<OverflowingBinaryOperator>(I)->hasNoSignedWrap();
+    auto KZF = [BitWidth, NSW](const APInt &KnownZero, unsigned ShiftAmt) {
+      APInt KZResult = 
+          (KnownZero << ShiftAmt) |
+          APInt::getLowBitsSet(BitWidth, ShiftAmt); // Low bits known 0.
+      // If this shift has "nsw" keyword, then the result is either a poison
+      // value or has the same sign bit as the first operand.
+      if (NSW && KnownZero.isNegative())
+        KZResult.setBit(BitWidth - 1);
+      return KZResult;
     };
 
-    auto KOF = [BitWidth](const APInt &KnownOne, unsigned ShiftAmt) {
-      return KnownOne << ShiftAmt;
+    auto KOF = [BitWidth, NSW](const APInt &KnownOne, unsigned ShiftAmt) {
+      APInt KOResult = KnownOne << ShiftAmt;
+      if (NSW && KnownOne.isNegative())
+        KOResult.setBit(BitWidth - 1);
+      return KOResult;
     };
 
     computeKnownBitsFromShiftOperator(I, KnownZero, KnownOne,
Index: test/Analysis/ValueTracking/known-signbit-shift.ll
===================================================================
--- test/Analysis/ValueTracking/known-signbit-shift.ll
+++ test/Analysis/ValueTracking/known-signbit-shift.ll
@@ -0,0 +1,55 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+; Result of left shifting a non-negative integer
+; with nsw flag should also be non-negative
+define i1 @test_shift_nonnegative(i32 %a) {
+; CHECK-LABEL: @test_shift_nonnegative(
+; CHECK-NEXT:    ret i1 true
+;
+  %b = lshr i32 %a, 2
+  %shift = shl nsw i32 %b, 3
+  %cmp = icmp sge i32 %shift, 0
+  ret i1 %cmp
+}
+
+; Result of left shifting a negative integer with
+; nsw flag should also be negative
+define i1 @test_shift_negative(i32 %a, i32 %b) {
+; CHECK-LABEL: @test_shift_negative(
+; CHECK-NEXT:    ret i1 true
+;
+  %c = or i32 %a, -2147483648
+  %d = and i32 %b, 7
+  %shift = shl nsw i32 %c, %d
+  %cmp = icmp slt i32 %shift, 0
+  ret i1 %cmp
+}
+
+; If sign bit is a known zero, it cannot be a known one.
+; This test should not crash opt.
+define i32 @test_no_sign_bit_conflict1(i1 %b) {
+; CHECK-LABEL: @test_no_sign_bit_conflict1(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 %b, i32 -2147221504, i32 -2147483648
+; CHECK-NEXT:    ret i32 [[SEL]]
+;
+entry:
+  %sel = select i1 %b, i32 8193, i32 8192
+  %mul = shl nsw i32 %sel, 18
+  ret i32 %mul
+}
+
+; If sign bit is a known one, it cannot be a known zero.
+; This test should not crash opt.
+define i32 @test_no_sign_bit_conflict2(i1 %b) {
+; CHECK-LABEL: @test_no_sign_bit_conflict2(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 %b, i32 2147221504, i32 2146959360
+; CHECK-NEXT:    ret i32 [[SEL]]
+;
+entry:
+  %sel = select i1 %b, i32 -8193, i32 -8194
+  %mul = shl nsw i32 %sel, 18
+  ret i32 %mul
+}