Index: lib/Transforms/InstCombine/InstCombineShifts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -680,6 +680,25 @@
       return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, Mask));
     }
 
+    if (match(Op0, m_SExt(m_Value(X)))) {
+      // Are we moving the sign bit to the low bit and widening with high zeros?
+      unsigned SrcTyBitWidth = X->getType()->getScalarSizeInBits();
+      if (ShAmt == BitWidth - 1 &&
+          (!Ty->isIntegerTy() || shouldChangeType(Ty, X->getType()))) {
+        // lshr (sext i1 X to iN), N-1 --> zext X to iN
+        if (SrcTyBitWidth == 1)
+          return new ZExtInst(X, Ty);
+
+        // lshr (sext iM X to iN), N-1 --> zext (lshr X, M-1) to iN
+        if (Op0->hasOneUse()) {
+          Value *NewLShr = Builder->CreateLShr(X, SrcTyBitWidth - 1);
+          return new ZExtInst(NewLShr, Ty);
+        }
+      }
+
+      // TODO: Convert to ashr+zext if the shift equals the extension amount.
+    }
+
     if (match(Op0, m_LShr(m_Value(X), m_APInt(ShOp1)))) {
       unsigned AmtSum = ShAmt + ShOp1->getZExtValue();
       // Oversized shifts are simplified to zero in InstSimplify.
Index: test/Transforms/InstCombine/lshr.ll
===================================================================
--- test/Transforms/InstCombine/lshr.ll
+++ test/Transforms/InstCombine/lshr.ll
@@ -1,6 +1,8 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -instcombine -S < %s | FileCheck %s
 
+target datalayout = "e-m:e-i64:64-n8:16:32:64"
+
 declare i32 @llvm.cttz.i32(i32, i1) nounwind readnone
 declare i32 @llvm.ctlz.i32(i32, i1) nounwind readnone
 declare i32 @llvm.ctpop.i32(i32) nounwind readnone
@@ -100,12 +102,9 @@
   ret <2 x i8> %lshr
 }
 
-; FIXME: The bool bit got smeared across a wide val, but then we zero'd out those bits. This is just a zext.
-
 define i16 @bool_zext(i1 %x) {
 ; CHECK-LABEL: @bool_zext(
-; CHECK-NEXT:    [[SEXT:%.*]] = sext i1 %x to i16
-; CHECK-NEXT:    [[HIBIT:%.*]] = lshr i16 [[SEXT]], 15
+; CHECK-NEXT:    [[HIBIT:%.*]] = zext i1 %x to i16
 ; CHECK-NEXT:    ret i16 [[HIBIT]]
 ;
   %sext = sext i1 %x to i16
@@ -115,8 +114,7 @@
 
 define <2 x i8> @bool_zext_splat(<2 x i1> %x) {
 ; CHECK-LABEL: @bool_zext_splat(
-; CHECK-NEXT:    [[SEXT:%.*]] = sext <2 x i1> %x to <2 x i8>
-; CHECK-NEXT:    [[HIBIT:%.*]] = lshr <2 x i8> [[SEXT]], <i8 7, i8 7>
+; CHECK-NEXT:    [[HIBIT:%.*]] = zext <2 x i1> %x to <2 x i8>
 ; CHECK-NEXT:    ret <2 x i8> [[HIBIT]]
 ;
   %sext = sext <2 x i1> %x to <2 x i8>
@@ -148,23 +146,34 @@
   ret <2 x i8> %hibit
 }
 
-; FIXME: All of the replicated sign bits are wiped out by the lshr. This could be lshr+zext.
-
-define i16 @fake_sext(i3 %x) {
+define i18 @fake_sext(i3 %x) {
 ; CHECK-LABEL: @fake_sext(
-; CHECK-NEXT:    [[SEXT:%.*]] = sext i3 %x to i16
-; CHECK-NEXT:    [[SH:%.*]] = lshr i16 [[SEXT]], 15
-; CHECK-NEXT:    ret i16 [[SH]]
-;
-  %sext = sext i3 %x to i16
-  %sh = lshr i16 %sext, 15
-  ret i16 %sh
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr i3 %x, 2
+; CHECK-NEXT:    [[SH:%.*]] = zext i3 [[TMP1]] to i18
+; CHECK-NEXT:    ret i18 [[SH]]
+;
+  %sext = sext i3 %x to i18
+  %sh = lshr i18 %sext, 17
+  ret i18 %sh
+}
+
+; Avoid the transform if it would change the shift from a legal to illegal type.
+
+define i32 @fake_sext_but_should_not_change_type(i3 %x) {
+; CHECK-LABEL: @fake_sext_but_should_not_change_type(
+; CHECK-NEXT:    [[SEXT:%.*]] = sext i3 %x to i32
+; CHECK-NEXT:    [[SH:%.*]] = lshr i32 [[SEXT]], 31
+; CHECK-NEXT:    ret i32 [[SH]]
+;
+  %sext = sext i3 %x to i32
+  %sh = lshr i32 %sext, 31
+  ret i32 %sh
 }
 
 define <2 x i8> @fake_sext_splat(<2 x i3> %x) {
 ; CHECK-LABEL: @fake_sext_splat(
-; CHECK-NEXT:    [[SEXT:%.*]] = sext <2 x i3> %x to <2 x i8>
-; CHECK-NEXT:    [[SH:%.*]] = lshr <2 x i8> [[SEXT]], <i8 7, i8 7>
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr <2 x i3> %x, <i3 2, i3 2>
+; CHECK-NEXT:    [[SH:%.*]] = zext <2 x i3> [[TMP1]] to <2 x i8>
 ; CHECK-NEXT:    ret <2 x i8> [[SH]]
 ;
   %sext = sext <2 x i3> %x to <2 x i8>