Index: llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -72,10 +72,12 @@
 // There are many variants to this pattern:
 //   a)  (x & ((1 << MaskShAmt) - 1)) << ShiftShAmt
 //   b)  (x & (~(-1 << MaskShAmt))) << ShiftShAmt
+//   c)  (x & (-1 >> MaskShAmt)) << ShiftShAmt
 // All these patterns can be simplified to just:
 //   x << ShiftShAmt
 // iff:
 //   a,b) (MaskShAmt+ShiftShAmt) u>= bitwidth(x)
+//   c)   (ShiftShAmt-MaskShAmt) s>= 0 (i.e. ShiftShAmt u>= MaskShAmt)
 static Instruction *
 dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
                                      const SimplifyQuery &SQ) {
@@ -91,24 +93,38 @@
   auto MaskA = m_Add(m_Shl(m_One(), m_Value(MaskShAmt)), m_AllOnes());
   // (~(-1 << maskNbits))
   auto MaskB = m_Xor(m_Shl(m_AllOnes(), m_Value(MaskShAmt)), m_AllOnes());
+  // (-1 >> MaskShAmt)
+  auto MaskC = m_Shr(m_AllOnes(), m_Value(MaskShAmt));
 
   Value *X;
-  if (!match(Masked, m_c_And(m_CombineOr(MaskA, MaskB), 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.
+  if (match(Masked, m_c_And(m_CombineOr(MaskA, MaskB), m_Value(X)))) {
+    // 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.
+  } else if (match(Masked, m_c_And(MaskC, m_Value(X)))) {
+    // Can we simplify (ShiftShAmt-MaskShAmt) ?
+    Value *ShAmtsDiff =
+        SimplifySubInst(ShiftShAmt, MaskShAmt, /*IsNSW=*/false, /*IsNUW=*/false,
+                        SQ.getWithInstruction(OuterShift));
+    if (!ShAmtsDiff)
+      return nullptr; // Did not simplify.
+    // Is the difference non-negative? (is ShiftShAmt u>= MaskShAmt ?)
+    // FIXME: could also rely on ConstantRange.
+    if (!match(ShAmtsDiff, m_NonNegative()))
+      return nullptr;
+    // All good, we can do this fold.
+  } else
+    return nullptr; // Don't know anything about this pattern.
 
   // No 'NUW'/'NSW'!
   // We no longer know that we won't shift-out non-0 bits.
Index: llvm/trunk/test/Transforms/InstCombine/redundant-left-shift-input-masking-variant-c.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/redundant-left-shift-input-masking-variant-c.ll
+++ llvm/trunk/test/Transforms/InstCombine/redundant-left-shift-input-masking-variant-c.ll
@@ -21,7 +21,7 @@
 ; CHECK-NEXT:    [[T1:%.*]] = and i32 [[T0]], [[X:%.*]]
 ; CHECK-NEXT:    call void @use32(i32 [[T0]])
 ; CHECK-NEXT:    call void @use32(i32 [[T1]])
-; CHECK-NEXT:    [[T2:%.*]] = shl i32 [[T1]], [[NBITS]]
+; CHECK-NEXT:    [[T2:%.*]] = shl i32 [[X]], [[NBITS]]
 ; CHECK-NEXT:    ret i32 [[T2]]
 ;
   %t0 = lshr i32 -1, %nbits
@@ -40,7 +40,7 @@
 ; CHECK-NEXT:    call void @use32(i32 [[T0]])
 ; CHECK-NEXT:    call void @use32(i32 [[T1]])
 ; CHECK-NEXT:    call void @use32(i32 [[T2]])
-; CHECK-NEXT:    [[T3:%.*]] = shl i32 [[T1]], [[T2]]
+; CHECK-NEXT:    [[T3:%.*]] = shl i32 [[X]], [[T2]]
 ; CHECK-NEXT:    ret i32 [[T3]]
 ;
   %t0 = lshr i32 -1, %nbits
@@ -65,7 +65,7 @@
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T0]])
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T1]])
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T2]])
-; CHECK-NEXT:    [[T3:%.*]] = shl <3 x i32> [[T1]], [[T2]]
+; CHECK-NEXT:    [[T3:%.*]] = shl <3 x i32> [[X]], [[T2]]
 ; CHECK-NEXT:    ret <3 x i32> [[T3]]
 ;
   %t0 = lshr <3 x i32> <i32 -1, i32 -1, i32 -1>, %nbits
@@ -86,7 +86,7 @@
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T0]])
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T1]])
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T2]])
-; CHECK-NEXT:    [[T3:%.*]] = shl <3 x i32> [[T1]], [[T2]]
+; CHECK-NEXT:    [[T3:%.*]] = shl <3 x i32> [[X]], [[T2]]
 ; CHECK-NEXT:    ret <3 x i32> [[T3]]
 ;
   %t0 = lshr <3 x i32> <i32 -1, i32 -1, i32 -1>, %nbits
@@ -107,7 +107,7 @@
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T0]])
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T1]])
 ; CHECK-NEXT:    call void @use3xi32(<3 x i32> [[T2]])
-; CHECK-NEXT:    [[T3:%.*]] = shl <3 x i32> [[T1]], [[T2]]
+; CHECK-NEXT:    [[T3:%.*]] = shl <3 x i32> [[X]], [[T2]]
 ; CHECK-NEXT:    ret <3 x i32> [[T3]]
 ;
   %t0 = lshr <3 x i32> <i32 -1, i32 undef, i32 -1>, %nbits
@@ -131,7 +131,7 @@
 ; CHECK-NEXT:    [[T1:%.*]] = and i32 [[X]], [[T0]]
 ; CHECK-NEXT:    call void @use32(i32 [[T0]])
 ; CHECK-NEXT:    call void @use32(i32 [[T1]])
-; CHECK-NEXT:    [[T2:%.*]] = shl i32 [[T1]], [[NBITS]]
+; CHECK-NEXT:    [[T2:%.*]] = shl i32 [[X]], [[NBITS]]
 ; CHECK-NEXT:    ret i32 [[T2]]
 ;
   %x = call i32 @gen32()
@@ -151,7 +151,7 @@
 ; CHECK-NEXT:    call void @use32(i32 [[T0]])
 ; CHECK-NEXT:    call void @use32(i32 [[T1]])
 ; CHECK-NEXT:    call void @use32(i32 [[T2]])
-; CHECK-NEXT:    [[T3:%.*]] = shl i32 [[T2]], [[NBITS0]]
+; CHECK-NEXT:    [[T3:%.*]] = shl i32 [[T1]], [[NBITS0]]
 ; CHECK-NEXT:    ret i32 [[T3]]
 ;
   %t0 = lshr i32 -1, %nbits0
@@ -192,7 +192,7 @@
 ; CHECK-NEXT:    [[T1:%.*]] = and i32 [[T0]], [[X:%.*]]
 ; CHECK-NEXT:    call void @use32(i32 [[T0]])
 ; CHECK-NEXT:    call void @use32(i32 [[T1]])
-; CHECK-NEXT:    [[T2:%.*]] = shl nuw i32 [[T1]], [[NBITS]]
+; CHECK-NEXT:    [[T2:%.*]] = shl i32 [[X]], [[NBITS]]
 ; CHECK-NEXT:    ret i32 [[T2]]
 ;
   %t0 = lshr i32 -1, %nbits
@@ -209,7 +209,7 @@
 ; CHECK-NEXT:    [[T1:%.*]] = and i32 [[T0]], [[X:%.*]]
 ; CHECK-NEXT:    call void @use32(i32 [[T0]])
 ; CHECK-NEXT:    call void @use32(i32 [[T1]])
-; CHECK-NEXT:    [[T2:%.*]] = shl nsw i32 [[T1]], [[NBITS]]
+; CHECK-NEXT:    [[T2:%.*]] = shl i32 [[X]], [[NBITS]]
 ; CHECK-NEXT:    ret i32 [[T2]]
 ;
   %t0 = lshr i32 -1, %nbits
@@ -226,7 +226,7 @@
 ; CHECK-NEXT:    [[T1:%.*]] = and i32 [[T0]], [[X:%.*]]
 ; CHECK-NEXT:    call void @use32(i32 [[T0]])
 ; CHECK-NEXT:    call void @use32(i32 [[T1]])
-; CHECK-NEXT:    [[T2:%.*]] = shl nuw nsw i32 [[T1]], [[NBITS]]
+; CHECK-NEXT:    [[T2:%.*]] = shl i32 [[X]], [[NBITS]]
 ; CHECK-NEXT:    ret i32 [[T2]]
 ;
   %t0 = lshr i32 -1, %nbits