[PATCH] D124183: [InstCombine] Add one use limitation for (X * C2) << C1 --> X * (C2 << C1)

[chenglin.bi via Phabricator via llvm-commits]

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Follow up D123453 <https://reviews.llvm.org/D123453>, add one-use limitation for

  (X * C2) << C1 --> X * (C2 << C1) 

to make consistent with

  lshr (mul nuw x, MulC), ShAmtC -> mul nuw x, (MulC >> ShAmtC)


https://reviews.llvm.org/D124183

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


Index: llvm/test/Transforms/InstCombine/apint-shift.ll
===================================================================
--- llvm/test/Transforms/InstCombine/apint-shift.ll
+++ llvm/test/Transforms/InstCombine/apint-shift.ll
@@ -28,7 +28,7 @@
 define i55 @test6a_negative_oneuse(i55 %A) {
 ; CHECK-LABEL: @test6a_negative_oneuse(
 ; CHECK-NEXT:    [[B:%.*]] = mul i55 [[A:%.*]], 3
-; CHECK-NEXT:    [[C:%.*]] = mul i55 [[A]], 6
+; CHECK-NEXT:    [[C:%.*]] = shl i55 [[B]], 1
 ; CHECK-NEXT:    call void @use(i55 [[B]])
 ; CHECK-NEXT:    ret i55 [[C]]
 ;
Index: llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -972,7 +972,10 @@
       return BinaryOperator::CreateShl(ConstantExpr::getShl(C2, C1), X);
 
     // (X * C2) << C1 --> X * (C2 << C1)
-    if (match(Op0, m_Mul(m_Value(X), m_Constant(C2))))
+    // The one-use check is not strictly necessary, but codegen may not be
+    // able to invert the transform and perf may suffer with an extra mul
+    // instruction.
+    if (match(Op0, m_OneUse(m_Mul(m_Value(X), m_Constant(C2)))))
       return BinaryOperator::CreateMul(X, ConstantExpr::getShl(C2, C1));
 
     // shl (zext i1 X), C1 --> select (X, 1 << C1, 0)


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