[PATCH] D63382: [InstCombine] fold a shifted zext to a select

[Zvi Rackover via Phabricator via llvm-commits]

zvi created this revision.
zvi added reviewers: spatel, nikic.
Herald added a subscriber: hiraditya.
Herald added a project: LLVM.

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

Fixes pr42257


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D63382

Files:
  llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
  llvm/test/Transforms/InstCombine/and.ll
  llvm/test/Transforms/InstCombine/shift.ll


Index: llvm/test/Transforms/InstCombine/shift.ll
===================================================================
--- llvm/test/Transforms/InstCombine/shift.ll
+++ llvm/test/Transforms/InstCombine/shift.ll
@@ -1191,6 +1191,16 @@
   ret i64 %shl
 }
 
+define i32 @test_shl_zext_bool(i1 %t) {
+; CHECK-LABEL: @test_shl_zext_bool(
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 %t, i32 4, i32 0
+; CHECK-NEXT:    ret i32 [[SEL]]
+;
+  %ext = zext i1 %t to i32
+  %shl = shl i32 %ext, 2
+  ret i32 %shl
+}
+
 define <2 x i64> @test_64_splat_vec(<2 x i32> %t) {
 ; CHECK-LABEL: @test_64_splat_vec(
 ; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i32> %t, <i32 8, i32 8>
Index: llvm/test/Transforms/InstCombine/and.ll
===================================================================
--- llvm/test/Transforms/InstCombine/and.ll
+++ llvm/test/Transforms/InstCombine/and.ll
@@ -346,8 +346,7 @@
 
 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
Index: llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -587,14 +587,20 @@
     unsigned ShAmt = ShAmtAPInt->getZExtValue();
     unsigned BitWidth = Ty->getScalarSizeInBits();
 
-    // shl (zext X), ShAmt --> zext (shl X, ShAmt)
-    // This is only valid if X would have zeros shifted out.
     Value *X;
     if (match(Op0, m_ZExt(m_Value(X)))) {
       unsigned SrcWidth = X->getType()->getScalarSizeInBits();
+      // shl (zext X), ShAmt --> zext (shl X, ShAmt)
+      // This is only valid if X would have zeros shifted out.
       if (ShAmt < SrcWidth &&
           MaskedValueIsZero(X, APInt::getHighBitsSet(SrcWidth, ShAmt), 0, &I))
         return new ZExtInst(Builder.CreateShl(X, ShAmt), Ty);
+
+      // shl (zext (i1 X)), ShAmt --> select (X, 0, 1 << ShAmt)
+      if (X->getType()->isIntegerTy(1)) {
+        return SelectInst::Create(X, ConstantInt::get(Ty, 1 << ShAmt),
+                                  ConstantInt::get(Ty, 0));
+      }
     }
 
     // (X >> C) << C --> X & (-1 << C)


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