Index: llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -65,6 +65,53 @@
   return NewShift;
 }
 
+// If we have some pattern that leaves only some low bits set, and then performs
+// left-shift of those bits, if none of the bits that are left after the final
+// shift are modified by the mask, we can omit the mask.
+//
+// There are many variants to this pattern:
+//   a)  (x & ((1 << MaskShAmt) - 1)) << ShiftShAmt
+// All these patterns can be simplified to just:
+//   x << ShiftShAmt
+// iff:
+//   a) (MaskShAmt+ShiftShAmt) u>= bitwidth(x)
+static Instruction *
+dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
+                                     const SimplifyQuery &SQ) {
+  assert(OuterShift->getOpcode() == Instruction::BinaryOps::Shl &&
+         "The input must be 'shl'!");
+
+  Value *Masked = OuterShift->getOperand(0);
+  Value *ShiftShAmt = OuterShift->getOperand(1);
+
+  Value *MaskShAmt;
+
+  // ((1 << MaskShAmt) - 1)
+  auto MaskA = m_Add(m_Shl(m_One(), m_Value(MaskShAmt)), m_AllOnes());
+
+  Value *X;
+  if (!match(Masked, m_c_And(MaskA, m_Value(X))))
+    return nullptr;
+
+  // Can we simplify (MaskShAmt+ShiftShAmt) ?
+  Value *SumOfShAmts =
+      SimplifyAddInst(MaskShAmt, ShiftShAmt, /*IsNSW=*/false, /*IsNUW=*/false,
+                      SQ.getWithInstruction(OuterShift));
+  if (!SumOfShAmts)
+    return nullptr; // Did not simplify.
+  // Is the total shift amount *not* smaller than the bit width?
+  // FIXME: could also rely on ConstantRange.
+  unsigned BitWidth = X->getType()->getScalarSizeInBits();
+  if (!match(SumOfShAmts, m_SpecificInt_ICMP(ICmpInst::Predicate::ICMP_UGE,
+                                             APInt(BitWidth, BitWidth))))
+    return nullptr;
+  // All good, we can do this fold.
+
+  // No 'NUW'/'NSW'!
+  // We no longer know that we won't shift-out non-0 bits.
+  return BinaryOperator::Create(OuterShift->getOpcode(), X, ShiftShAmt);
+}
+
 Instruction *InstCombiner::commonShiftTransforms(BinaryOperator &I) {
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
   assert(Op0->getType() == Op1->getType());
@@ -629,6 +676,9 @@
   if (Instruction *V = commonShiftTransforms(I))
     return V;
 
+  if (Instruction *V = dropRedundantMaskingOfLeftShiftInput(&I, SQ))
+    return V;
+
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
   Type *Ty = I.getType();
   unsigned BitWidth = Ty->getScalarSizeInBits();
Index: llvm/trunk/test/Transforms/InstCombine/redundant-left-shift-input-masking-variant-a.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/redundant-left-shift-input-masking-variant-a.ll
+++ llvm/trunk/test/Transforms/InstCombine/redundant-left-shift-input-masking-variant-a.ll
@@ -25,7 +25,7 @@
 ; CHECK-NEXT:    call void @use32(i32 [[T1]])
 ; CHECK-NEXT:    call void @use32(i32 [[T2]])
 ; CHECK-NEXT:    call void @use32(i32 [[T3]])
-; CHECK-NEXT:    [[T4:%.*]] = shl i32 [[T2]], [[T3]]
+; CHECK-NEXT:    [[T4:%.*]] = shl i32 [[X]], [[T3]]
 ; CHECK-NEXT:    ret i32 [[T4]]
 ;
   %t0 = shl i32 1, %nbits
@@ -50,7 +50,7 @@
 ; CHECK-NEXT:    call void @use32(i32 [[T1]])
 ; CHECK-NEXT:    call void @use32(i32 [[T2]])
 ; CHECK-NEXT:    call void @use32(i32 [[T3]])
-; CHECK-NEXT:    [[T4:%.*]] = shl i32 [[T2]], [[T3]]
+; CHECK-NEXT:    [[T4:%.*]] = shl i32 [[X]], [[T3]]
 ; CHECK-NEXT:    ret i32 [[T4]]
 ;
   %t0 = shl i32 1, %nbits
@@ -77,7 +77,7 @@
 ; CHECK-NEXT:    call void @use32(i32 [[T2]])
 ; CHECK-NEXT:    call void @use32(i32 [[T3]])
 ; CHECK-NEXT:    call void @use32(i32 [[T4]])
-; CHECK-NEXT:    [[T5:%.*]] = shl i32 [[T3]], [[T4]]
+; CHECK-NEXT:    [[T5:%.*]] = shl i32 [[X]], [[T4]]
 ; CHECK-NEXT:    ret i32 [[T5]]
 ;
   %t0 = add i32 %nbits, 1
@@ -109,7 +109,7 @@
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T2]])
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T3]])
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T4]])
-; CHECK-NEXT:    [[T5:%.*]] = shl <3 x i32> [[T3]], [[T4]]
+; CHECK-NEXT:    [[T5:%.*]] = shl <3 x i32> [[X]], [[T4]]
 ; CHECK-NEXT:    ret <3 x i32> [[T5]]
 ;
   %t0 = add <3 x i32> %nbits, <i32 0, i32 0, i32 0>
@@ -138,7 +138,7 @@
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T2]])
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T3]])
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T4]])
-; CHECK-NEXT:    [[T5:%.*]] = shl <3 x i32> [[T3]], [[T4]]
+; CHECK-NEXT:    [[T5:%.*]] = shl <3 x i32> [[X]], [[T4]]
 ; CHECK-NEXT:    ret <3 x i32> [[T5]]
 ;
   %t0 = add <3 x i32> %nbits, <i32 -1, i32 0, i32 1>
@@ -166,7 +166,7 @@
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T2]])
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T3]])
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T4]])
-; CHECK-NEXT:    [[T5:%.*]] = shl <3 x i32> [[T3]], [[T4]]
+; CHECK-NEXT:    [[T5:%.*]] = shl <3 x i32> [[X]], [[T4]]
 ; CHECK-NEXT:    ret <3 x i32> [[T5]]
 ;
   %t0 = add <3 x i32> %nbits, <i32 0, i32 undef, i32 0>
@@ -198,7 +198,7 @@
 ; CHECK-NEXT:    call void @use32(i32 [[T1]])
 ; CHECK-NEXT:    call void @use32(i32 [[T2]])
 ; CHECK-NEXT:    call void @use32(i32 [[T3]])
-; CHECK-NEXT:    [[T4:%.*]] = shl i32 [[T2]], [[T3]]
+; CHECK-NEXT:    [[T4:%.*]] = shl i32 [[X]], [[T3]]
 ; CHECK-NEXT:    ret i32 [[T4]]
 ;
   %x = call i32 @gen32()
@@ -260,7 +260,7 @@
 ; CHECK-NEXT:    call void @use32(i32 [[T3]])
 ; CHECK-NEXT:    call void @use32(i32 [[T4]])
 ; CHECK-NEXT:    call void @use32(i32 [[T5]])
-; CHECK-NEXT:    [[T6:%.*]] = shl i32 [[T4]], [[T5]]
+; CHECK-NEXT:    [[T6:%.*]] = shl i32 [[T1]], [[T5]]
 ; CHECK-NEXT:    ret i32 [[T6]]
 ;
   %t0 = shl i32 1, %nbits0
@@ -291,7 +291,7 @@
 ; CHECK-NEXT:    call void @use32(i32 [[T1]])
 ; CHECK-NEXT:    call void @use32(i32 [[T2]])
 ; CHECK-NEXT:    call void @use32(i32 [[T3]])
-; CHECK-NEXT:    [[T4:%.*]] = shl nuw i32 [[T2]], [[T3]]
+; CHECK-NEXT:    [[T4:%.*]] = shl i32 [[X]], [[T3]]
 ; CHECK-NEXT:    ret i32 [[T4]]
 ;
   %t0 = shl i32 1, %nbits
@@ -316,7 +316,7 @@
 ; CHECK-NEXT:    call void @use32(i32 [[T1]])
 ; CHECK-NEXT:    call void @use32(i32 [[T2]])
 ; CHECK-NEXT:    call void @use32(i32 [[T3]])
-; CHECK-NEXT:    [[T4:%.*]] = shl nsw i32 [[T2]], [[T3]]
+; CHECK-NEXT:    [[T4:%.*]] = shl i32 [[X]], [[T3]]
 ; CHECK-NEXT:    ret i32 [[T4]]
 ;
   %t0 = shl i32 1, %nbits
@@ -341,7 +341,7 @@
 ; CHECK-NEXT:    call void @use32(i32 [[T1]])
 ; CHECK-NEXT:    call void @use32(i32 [[T2]])
 ; CHECK-NEXT:    call void @use32(i32 [[T3]])
-; CHECK-NEXT:    [[T4:%.*]] = shl nuw nsw i32 [[T2]], [[T3]]
+; CHECK-NEXT:    [[T4:%.*]] = shl i32 [[X]], [[T3]]
 ; CHECK-NEXT:    ret i32 [[T4]]
 ;
   %t0 = shl i32 1, %nbits