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[TargetLowering] Improve expansion of ROTL/ROTR

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Using a negation instead of a subtraction from a constant can save an
instruction on some targets.

Nothing much uses this until D77152 changes the translation of fshl and
fshr intrinsics.

Differential Revision: https://reviews.llvm.org/D82539
This commit is contained in:
Jay Foad 2020-04-03 14:11:56 +01:00
parent 515e16cc38
commit 5fecaffff0
1 changed files with 10 additions and 9 deletions
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19
lib/CodeGen/SelectionDAG/TargetLowering.cpp
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@ -6177,12 +6177,15 @@ bool TargetLowering::expandROT(SDNode *Node, SDValue &Result,
SDLoc DL(SDValue(Node, 0));
EVT ShVT = Op1.getValueType();
SDValue BitWidthC = DAG.getConstant(EltSizeInBits, DL, ShVT);
SDValue Zero = DAG.getConstant(0, DL, ShVT);
// If a rotate in the other direction is legal, use it.
assert(isPowerOf2_32(EltSizeInBits) && EltSizeInBits > 1 &&
"Expecting the type bitwidth to be a power of 2");
// If a rotate in the other direction is supported, use it.
unsigned RevRot = IsLeft ? ISD::ROTR : ISD::ROTL;
if (isOperationLegal(RevRot, VT)) {
SDValue Sub = DAG.getNode(ISD::SUB, DL, ShVT, BitWidthC, Op1);
if (isOperationLegalOrCustom(RevRot, VT)) {
SDValue Sub = DAG.getNode(ISD::SUB, DL, ShVT, Zero, Op1);
Result = DAG.getNode(RevRot, DL, VT, Op0, Sub);
return true;
}
@ -6195,15 +6198,13 @@ bool TargetLowering::expandROT(SDNode *Node, SDValue &Result,
return false;
// Otherwise,
// (rotl x, c) -> (or (shl x, (and c, w-1)), (srl x, (and w-c, w-1)))
// (rotr x, c) -> (or (srl x, (and c, w-1)), (shl x, (and w-c, w-1)))
// (rotl x, c) -> (or (shl x, (and c, w-1)), (srl x, (and -c, w-1)))
// (rotr x, c) -> (or (srl x, (and c, w-1)), (shl x, (and -c, w-1)))
//
assert(isPowerOf2_32(EltSizeInBits) && EltSizeInBits > 1 &&
"Expecting the type bitwidth to be a power of 2");
unsigned ShOpc = IsLeft ? ISD::SHL : ISD::SRL;
unsigned HsOpc = IsLeft ? ISD::SRL : ISD::SHL;
SDValue BitWidthMinusOneC = DAG.getConstant(EltSizeInBits - 1, DL, ShVT);
SDValue NegOp1 = DAG.getNode(ISD::SUB, DL, ShVT, BitWidthC, Op1);
SDValue NegOp1 = DAG.getNode(ISD::SUB, DL, ShVT, Zero, Op1);
SDValue And0 = DAG.getNode(ISD::AND, DL, ShVT, Op1, BitWidthMinusOneC);
SDValue And1 = DAG.getNode(ISD::AND, DL, ShVT, NegOp1, BitWidthMinusOneC);
Result = DAG.getNode(ISD::OR, DL, VT, DAG.getNode(ShOpc, DL, VT, Op0, And0),