diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -1324,8 +1324,21 @@
   // shl nuw i8 C, %x  ->  C  iff C has sign bit set.
   if (isNUW && match(Op0, m_Negative()))
     return Op0;
-  // NOTE: could use computeKnownBits() / LazyValueInfo,
-  // but the cost-benefit analysis suggests it isn't worth it.
+
+  // check for nsw shl leading to a poison value.
+  if (isNSW) {
+    KnownBits KnownVal = computeKnownBits(Op0, Q.DL, 0, Q.AC, Q.CxtI, Q.DT);
+    KnownBits KnownAmt = computeKnownBits(Op1, Q.DL, 0, Q.AC, Q.CxtI, Q.DT);
+    KnownBits KnownShl = KnownBits::shl(KnownVal, KnownAmt);
+
+    if (KnownVal.Zero.isSignBitSet())
+      KnownShl.Zero.setSignBit();
+    if (KnownVal.One.isSignBitSet())
+      KnownShl.One.setSignBit();
+
+    if (KnownShl.hasConflict())
+      return PoisonValue::get(Op0->getType());
+  }
 
   return nullptr;
 }
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -994,30 +994,25 @@
   bool ShiftAmtIsConstant = Known.isConstant();
   bool MaxShiftAmtIsOutOfRange = Known.getMaxValue().uge(BitWidth);
 
-  if (ShiftAmtIsConstant) {
-    Known = KF(Known2, Known);
-
-    // If the known bits conflict, this must be an overflowing left shift, so
-    // the shift result is poison. We can return anything we want. Choose 0 for
-    // the best folding opportunity.
-    if (Known.hasConflict())
-      Known.setAllZero();
+  // Use the KF callback to get an initial knownbits approximation.
+  Known = KF(Known2, Known);
 
+  // If the known bits conflict, this must be an overflowing left shift, so
+  // the shift result is poison.
+  if (Known.hasConflict()) {
+    Known.resetAll();
     return;
   }
 
+  if (ShiftAmtIsConstant)
+    return;
+
   // If the shift amount could be greater than or equal to the bit-width of the
   // LHS, the value could be poison, but bail out because the check below is
   // expensive.
   // TODO: Should we just carry on?
-  if (MaxShiftAmtIsOutOfRange) {
-    Known.resetAll();
+  if (MaxShiftAmtIsOutOfRange)
     return;
-  }
-
-  // It would be more-clearly correct to use the two temporaries for this
-  // calculation. Reusing the APInts here to prevent unnecessary allocations.
-  Known.resetAll();
 
   // If we know the shifter operand is nonzero, we can sometimes infer more
   // known bits. However this is expensive to compute, so be lazy about it and
@@ -1232,10 +1227,6 @@
     };
     computeKnownBitsFromShiftOperator(I, DemandedElts, Known, Known2, Depth, Q,
                                       KF);
-    // Trailing zeros of a right-shifted constant never decrease.
-    const APInt *C;
-    if (match(I->getOperand(0), m_APInt(C)))
-      Known.Zero.setLowBits(C->countTrailingZeros());
     break;
   }
   case Instruction::LShr: {
@@ -1244,10 +1235,6 @@
     };
     computeKnownBitsFromShiftOperator(I, DemandedElts, Known, Known2, Depth, Q,
                                       KF);
-    // Leading zeros of a left-shifted constant never decrease.
-    const APInt *C;
-    if (match(I->getOperand(0), m_APInt(C)))
-      Known.Zero.setHighBits(C->countLeadingZeros());
     break;
   }
   case Instruction::AShr: {
diff --git a/llvm/test/Transforms/InstCombine/known-signbit-shift.ll b/llvm/test/Transforms/InstCombine/known-signbit-shift.ll
--- a/llvm/test/Transforms/InstCombine/known-signbit-shift.ll
+++ b/llvm/test/Transforms/InstCombine/known-signbit-shift.ll
@@ -31,7 +31,7 @@
 ; This test should not crash opt. The shift produces poison.
 define i32 @test_no_sign_bit_conflict1(i1 %b) {
 ; CHECK-LABEL: @test_no_sign_bit_conflict1(
-; CHECK-NEXT:    ret i32 undef
+; CHECK-NEXT:    ret i32 poison
 ;
   %sel = select i1 %b, i32 8193, i32 8192
   %mul = shl nsw i32 %sel, 18
@@ -42,7 +42,7 @@
 ; This test should not crash opt. The shift produces poison.
 define i32 @test_no_sign_bit_conflict2(i1 %b) {
 ; CHECK-LABEL: @test_no_sign_bit_conflict2(
-; CHECK-NEXT:    ret i32 undef
+; CHECK-NEXT:    ret i32 poison
 ;
   %sel = select i1 %b, i32 -8193, i32 -8194
   %mul = shl nsw i32 %sel, 18
diff --git a/llvm/test/Transforms/InstSimplify/icmp-constant.ll b/llvm/test/Transforms/InstSimplify/icmp-constant.ll
--- a/llvm/test/Transforms/InstSimplify/icmp-constant.ll
+++ b/llvm/test/Transforms/InstSimplify/icmp-constant.ll
@@ -772,7 +772,7 @@
 
 define i1 @ne_shl_by_constant_produces_poison(i8 %x) {
 ; CHECK-LABEL: @ne_shl_by_constant_produces_poison(
-; CHECK-NEXT:    ret i1 true
+; CHECK-NEXT:    ret i1 poison
 ;
   %zx = zext i8 %x to i16      ; zx  = 0x00xx
   %xor = xor i16 %zx, 32767    ; xor = 0x7fyy
@@ -784,7 +784,7 @@
 
 define i1 @eq_shl_by_constant_produces_poison(i8 %x) {
 ; CHECK-LABEL: @eq_shl_by_constant_produces_poison(
-; CHECK-NEXT:    ret i1 false
+; CHECK-NEXT:    ret i1 poison
 ;
   %clear_high_bit = and i8 %x, 127                 ; 0x7f
   %set_next_high_bits = or i8 %clear_high_bit, 112 ; 0x70