[InstCombine] Simplify funnel shift with zero/undef operand to shift
The following simplifications are implemented:
* `fshl(X, 0, C) -> shl X, C%BW`
* `fshl(X, undef, C) -> shl X, C%BW` (assuming undef = 0)
* `fshl(0, X, C) -> lshr X, BW-C%BW`
* `fshl(undef, X, C) -> lshr X, BW-C%BW` (assuming undef = 0)
* `fshr(X, 0, C) -> shl X, (BW-C%BW)`
* `fshr(X, undef, C) -> shl X, BW-C%BW` (assuming undef = 0)
* `fshr(0, X, C) -> lshr X, C%BW`
* `fshr(undef, X, C) -> lshr, X, C%BW` (assuming undef = 0)
The simplification is only performed if the shift amount C is constant,
because we can explicitly compute C%BW and BW-C%BW in this case.
Differential Revision: https://reviews.llvm.org/D54778
llvm-svn: 347505
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index a99eaf0..cbaa0dd 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1992,6 +1992,29 @@
case Intrinsic::fshl:
case Intrinsic::fshr: {
+ const APInt *SA;
+ if (match(II->getArgOperand(2), m_APInt(SA))) {
+ Value *Op0 = II->getArgOperand(0), *Op1 = II->getArgOperand(1);
+ unsigned BitWidth = SA->getBitWidth();
+ uint64_t ShiftAmt = SA->urem(BitWidth);
+ assert(ShiftAmt != 0 && "SimplifyCall should have handled zero shift");
+ // Normalize to funnel shift left.
+ if (II->getIntrinsicID() == Intrinsic::fshr)
+ ShiftAmt = BitWidth - ShiftAmt;
+
+ // fshl(X, 0, C) -> shl X, C
+ // fshl(X, undef, C) -> shl X, C
+ if (match(Op1, m_Zero()) || match(Op1, m_Undef()))
+ return BinaryOperator::CreateShl(
+ Op0, ConstantInt::get(II->getType(), ShiftAmt));
+
+ // fshl(0, X, C) -> lshr X, (BW-C)
+ // fshl(undef, X, C) -> lshr X, (BW-C)
+ if (match(Op0, m_Zero()) || match(Op0, m_Undef()))
+ return BinaryOperator::CreateLShr(
+ Op1, ConstantInt::get(II->getType(), BitWidth - ShiftAmt));
+ }
+
// The shift amount (operand 2) of a funnel shift is modulo the bitwidth,
// so only the low bits of the shift amount are demanded if the bitwidth is
// a power-of-2.