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[AArch64] When combining constant mul of power of 2 plus/minus 1, prefer shift 

plus add.  The shift can be folded into the add.  This only effects codegen
when the constant is 3.

llvm-svn: 210445
This commit is contained in:
Chad Rosier 2014-06-09 01:25:51 +00:00
parent 5c7b1aed5d
commit d96e9f14ee
2 changed files with 17 additions and 9 deletions
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18
llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
View File

@ -6344,15 +6344,6 @@ static SDValue performMulCombine(SDNode *N, SelectionDAG &DAG,
if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(N->getOperand(1))) { if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(N->getOperand(1))) {
APInt Value = C->getAPIntValue(); APInt Value = C->getAPIntValue();
EVT VT = N->getValueType(0); EVT VT = N->getValueType(0);
APInt VP1 = Value + 1;
if (VP1.isPowerOf2()) {
// Multiplying by one less than a power of two, replace with a shift
// and a subtract.
SDValue ShiftedVal =
DAG.getNode(ISD::SHL, SDLoc(N), VT, N->getOperand(0),
DAG.getConstant(VP1.logBase2(), MVT::i64));
return DAG.getNode(ISD::SUB, SDLoc(N), VT, ShiftedVal, N->getOperand(0));
}
APInt VM1 = Value - 1; APInt VM1 = Value - 1;
if (VM1.isPowerOf2()) { if (VM1.isPowerOf2()) {
// Multiplying by one more than a power of two, replace with a shift // Multiplying by one more than a power of two, replace with a shift
@ -6362,6 +6353,15 @@ static SDValue performMulCombine(SDNode *N, SelectionDAG &DAG,
DAG.getConstant(VM1.logBase2(), MVT::i64)); DAG.getConstant(VM1.logBase2(), MVT::i64));
return DAG.getNode(ISD::ADD, SDLoc(N), VT, ShiftedVal, N->getOperand(0)); return DAG.getNode(ISD::ADD, SDLoc(N), VT, ShiftedVal, N->getOperand(0));
} }
APInt VP1 = Value + 1;
if (VP1.isPowerOf2()) {
// Multiplying by one less than a power of two, replace with a shift
// and a subtract.
SDValue ShiftedVal =
DAG.getNode(ISD::SHL, SDLoc(N), VT, N->getOperand(0),
DAG.getConstant(VP1.logBase2(), MVT::i64));
return DAG.getNode(ISD::SUB, SDLoc(N), VT, ShiftedVal, N->getOperand(0));
}
} }
return SDValue(); return SDValue();
} }

8
llvm/test/CodeGen/AArch64/arm64-arith.ll
View File

@ -260,3 +260,11 @@ define i64 @f3(i64 %a) nounwind readnone ssp {
%res = mul nsw i64 %a, 17 %res = mul nsw i64 %a, 17
ret i64 %res ret i64 %res
} }
define i32 @f4(i32 %a) nounwind readnone ssp {
; CHECK-LABEL: f4:
; CHECK-NEXT: add w0, w0, w0, lsl #1
; CHECK-NEXT: ret
%res = mul i32 %a, 3
ret i32 %res
}