Index: lib/Analysis/InstructionSimplify.cpp
===================================================================
--- lib/Analysis/InstructionSimplify.cpp
+++ lib/Analysis/InstructionSimplify.cpp
@@ -2405,12 +2405,13 @@
   if (!match(RHS, m_APInt(C)))
     return nullptr;
 
+  Type *CmpTy = GetCompareTy(RHS);
   // Rule out tautological comparisons (eg., ult 0 or uge 0).
   ConstantRange RHS_CR = ConstantRange::makeExactICmpRegion(Pred, *C);
   if (RHS_CR.isEmptySet())
-    return ConstantInt::getFalse(GetCompareTy(RHS));
+    return ConstantInt::getFalse(CmpTy);
   if (RHS_CR.isFullSet())
-    return ConstantInt::getTrue(GetCompareTy(RHS));
+    return ConstantInt::getTrue(CmpTy);
 
   // Find the range of possible values for binary operators.
   unsigned Width = C->getBitWidth();
@@ -2428,9 +2429,22 @@
 
   if (!LHS_CR.isFullSet()) {
     if (RHS_CR.contains(LHS_CR))
-      return ConstantInt::getTrue(GetCompareTy(RHS));
+      return ConstantInt::getTrue(CmpTy);
     if (RHS_CR.inverse().contains(LHS_CR))
-      return ConstantInt::getFalse(GetCompareTy(RHS));
+      return ConstantInt::getFalse(CmpTy);
+  }
+
+  // Shift-left doesn't easily conform to range reduction, but we can still
+  // check if the inserted zero bits make this comparison true or false.
+  const APInt *ShiftAmtC;
+  if (match(LHS, m_Shl(m_Value(), m_APInt(ShiftAmtC))) &&
+      C->lshr(*ShiftAmtC).shl(*ShiftAmtC) != *C) {
+    // icmp eq (shl X, ShiftAmtC), C --> false if any low bits of C are set
+    if (Pred == ICmpInst::ICMP_EQ)
+      return ConstantInt::getFalse(CmpTy);
+    // icmp ne (shl X, ShiftAmtC), C --> true if any low bits of C are set
+    if (Pred == ICmpInst::ICMP_NE)
+      return ConstantInt::getTrue(CmpTy);
   }
 
   return nullptr;
Index: test/Transforms/InstSimplify/icmp-constant.ll
===================================================================
--- test/Transforms/InstSimplify/icmp-constant.ll
+++ test/Transforms/InstSimplify/icmp-constant.ll
@@ -571,3 +571,29 @@
   ret <2 x i1> %cmp
 }
 
+; PR34838 - https://bugs.llvm.org/show_bug.cgi?id=34838
+; The shift is known to create poison, but that doesn't mean we can't simplify the cmp.
+
+define i1 @ne_shl_low_bits_set(i8 %x) {
+; CHECK-LABEL: @ne_shl_low_bits_set(
+; CHECK-NEXT:    ret i1 true
+;
+  %zx = zext i8 %x to i16      ; zx  = 0x00xx
+  %xor = xor i16 %zx, 32767    ; xor = 0x7fyy
+  %sub = sub nsw i16 %zx, %xor ; sub = 0x80zz  (the top bit is known one)
+  %sh = shl nsw i16 %sub, 2    ; oops! this shl can't be nsw; that's POISON
+  %cmp = icmp ne i16 %sh, 1
+  ret i1 %cmp
+}
+
+define i1 @eq_shl_low_bits_set(i8 %x) {
+; CHECK-LABEL: @eq_shl_low_bits_set(
+; CHECK-NEXT:    ret i1 false
+;
+  %clear_high_bit = and i8 %x, 127
+  %set_next_high_bits = or i8 %clear_high_bit, 112 ; 0x70
+  %poison_shift = shl nsw i8 %set_next_high_bits, 3
+  %cmp = icmp eq i8 %poison_shift, 15
+  ret i1 %cmp
+}
+