[InstCombine] fold a shifted bool zext to a select

For a constant shift amount, add the following fold.
shl (zext (i1 X)), ShAmt --> select (X, 1 << ShAmt, 0)

https://rise4fun.com/Alive/IZ9

Fixes PR42257.

Based on original patch by @zvi (Zvi Rackover)

Differential Revision: https://reviews.llvm.org/D63382

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@374828 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -934,6 +934,12 @@ Instruction *InstCombiner::visitShl(BinaryOperator &I) {
     return BinaryOperator::CreateLShr(
         ConstantInt::get(Ty, APInt::getSignMask(BitWidth)), X);
 
+  // shl (zext (i1 X)), C1 --> select (X, 1 << C1, 0)
+  if (match(Op0, m_ZExt(m_Value(X))) && X->getType()->isIntOrIntVectorTy(1)) {
+    auto *NewC = ConstantExpr::getShl(ConstantInt::get(Ty, 1), C1);
+    return SelectInst::Create(X, NewC, ConstantInt::getNullValue(Ty));
+  }
+
   return nullptr;
 }
 

test/Transforms/InstCombine/and.ll

@@ -346,8 +346,7 @@ define i32 @test30(i1 %X) {
 
 define i32 @test31(i1 %X) {
 ; CHECK-LABEL: @test31(
-; CHECK-NEXT:    [[Y:%.*]] = zext i1 %X to i32
-; CHECK-NEXT:    [[Z:%.*]] = shl nuw nsw i32 [[Y]], 4
+; CHECK-NEXT:    [[Z:%.*]] = select i1 [[X:%.*]], i32 16, i32 0
 ; CHECK-NEXT:    ret i32 [[Z]]
 ;
   %Y = zext i1 %X to i32

test/Transforms/InstCombine/shift.ll

@@ -1181,8 +1181,7 @@ define <2 x i65> @test_63(<2 x i64> %t) {
 
 define i32 @test_shl_zext_bool(i1 %t) {
 ; CHECK-LABEL: @test_shl_zext_bool(
-; CHECK-NEXT:    [[EXT:%.*]] = zext i1 [[T:%.*]] to i32
-; CHECK-NEXT:    [[SHL:%.*]] = shl nuw nsw i32 [[EXT]], 2
+; CHECK-NEXT:    [[SHL:%.*]] = select i1 [[T:%.*]], i32 4, i32 0
 ; CHECK-NEXT:    ret i32 [[SHL]]
 ;
   %ext = zext i1 %t to i32
@@ -1192,8 +1191,7 @@ define i32 @test_shl_zext_bool(i1 %t) {
 
 define <2 x i32> @test_shl_zext_bool_splat(<2 x i1> %t) {
 ; CHECK-LABEL: @test_shl_zext_bool_splat(
-; CHECK-NEXT:    [[EXT:%.*]] = zext <2 x i1> [[T:%.*]] to <2 x i32>
-; CHECK-NEXT:    [[SHL:%.*]] = shl nuw nsw <2 x i32> [[EXT]], <i32 3, i32 3>
+; CHECK-NEXT:    [[SHL:%.*]] = select <2 x i1> [[T:%.*]], <2 x i32> <i32 8, i32 8>, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[SHL]]
 ;
   %ext = zext <2 x i1> %t to <2 x i32>
@@ -1203,8 +1201,7 @@ define <2 x i32> @test_shl_zext_bool_splat(<2 x i1> %t) {
 
 define <2 x i32> @test_shl_zext_bool_vec(<2 x i1> %t) {
 ; CHECK-LABEL: @test_shl_zext_bool_vec(
-; CHECK-NEXT:    [[EXT:%.*]] = zext <2 x i1> [[T:%.*]] to <2 x i32>
-; CHECK-NEXT:    [[SHL:%.*]] = shl <2 x i32> [[EXT]], <i32 2, i32 3>
+; CHECK-NEXT:    [[SHL:%.*]] = select <2 x i1> [[T:%.*]], <2 x i32> <i32 4, i32 8>, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[SHL]]
 ;
   %ext = zext <2 x i1> %t to <2 x i32>