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[RISCV][NFC] Match 3/5/9 * 3/5/9 * 2^N without a loop #165547

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[RISCV][NFC] Match 3/5/9 * 3/5/9 * 2^N without a loop #165547

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a3d3c29
[RISCV][NFC] Remove a duplicate shadowed variable
pfusik Oct 29, 2025
bd06efb
[RISCV][NFC] Extract the `expandMulToShlAddShlAdd` method
pfusik Oct 29, 2025
cdb6ba5
[RISCV][NFC] Match `3/5/9 * 3/5/9 * 2^N` without a loop
pfusik Oct 29, 2025
d1289ca
[RISCV][NFC] clang-format a comment into 80 columns
pfusik Oct 29, 2025
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98 changes: 42 additions & 56 deletions llvm/lib/Target/RISCV/RISCVISelLowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -16485,6 +16485,34 @@ static SDValue expandMulToAddOrSubOfShl(SDNode *N, SelectionDAG &DAG,
return DAG.getNode(Op, DL, VT, Shift1, Shift2);
}

static SDValue getShlAddShlAdd(SDNode *N, SelectionDAG &DAG, int ShX, int ShY) {
SDLoc DL(N);
EVT VT = N->getValueType(0);
SDValue X = N->getOperand(0);
SDValue Mul359 = DAG.getNode(RISCVISD::SHL_ADD, DL, VT, X,
DAG.getConstant(ShY, DL, VT), X);
return DAG.getNode(RISCVISD::SHL_ADD, DL, VT, Mul359,
DAG.getConstant(ShX, DL, VT), Mul359);
}

static SDValue expandMulToShlAddShlAdd(SDNode *N, SelectionDAG &DAG,
uint64_t MulAmt) {
switch (MulAmt) {
case 5 * 3:
return getShlAddShlAdd(N, DAG, 2, 1);
case 9 * 3:
return getShlAddShlAdd(N, DAG, 3, 1);
case 5 * 5:
return getShlAddShlAdd(N, DAG, 2, 2);
case 9 * 5:
return getShlAddShlAdd(N, DAG, 3, 2);
case 9 * 9:
return getShlAddShlAdd(N, DAG, 3, 3);
default:
return SDValue();
}
}

// Try to expand a scalar multiply to a faster sequence.
static SDValue expandMul(SDNode *N, SelectionDAG &DAG,
TargetLowering::DAGCombinerInfo &DCI,
Expand Down Expand Up @@ -16514,18 +16542,17 @@ static SDValue expandMul(SDNode *N, SelectionDAG &DAG,
if (Subtarget.hasVendorXqciac() && isInt<12>(CNode->getSExtValue()))
return SDValue();

// WARNING: The code below is knowingly incorrect with regards to undef semantics.
// We're adding additional uses of X here, and in principle, we should be freezing
// X before doing so. However, adding freeze here causes real regressions, and no
// other target properly freezes X in these cases either.
SDValue X = N->getOperand(0);

// WARNING: The code below is knowingly incorrect with regards to undef
// semantics. We're adding additional uses of X here, and in principle, we
// should be freezing X before doing so. However, adding freeze here causes
// real regressions, and no other target properly freezes X in these cases
// either.
if (Subtarget.hasShlAdd(3)) {
SDValue X = N->getOperand(0);
int Shift;
if (int ShXAmount = isShifted359(MulAmt, Shift)) {
// 3/5/9 * 2^N -> shl (shXadd X, X), N
SDLoc DL(N);
SDValue X = N->getOperand(0);
// Put the shift first if we can fold a zext into the shift forming
// a slli.uw.
if (X.getOpcode() == ISD::AND && isa<ConstantSDNode>(X.getOperand(1)) &&
Expand All @@ -16544,38 +16571,8 @@ static SDValue expandMul(SDNode *N, SelectionDAG &DAG,
}

// 3/5/9 * 3/5/9 -> shXadd (shYadd X, X), (shYadd X, X)
int ShX;
int ShY;
switch (MulAmt) {
case 3 * 5:
ShY = 1;
ShX = 2;
break;
case 3 * 9:
ShY = 1;
ShX = 3;
break;
case 5 * 5:
ShX = ShY = 2;
break;
case 5 * 9:
ShY = 2;
ShX = 3;
break;
case 9 * 9:
ShX = ShY = 3;
break;
default:
ShX = ShY = 0;
break;
}
if (ShX) {
SDLoc DL(N);
SDValue Mul359 = DAG.getNode(RISCVISD::SHL_ADD, DL, VT, X,
DAG.getConstant(ShY, DL, VT), X);
return DAG.getNode(RISCVISD::SHL_ADD, DL, VT, Mul359,
DAG.getConstant(ShX, DL, VT), Mul359);
}
if (SDValue V = expandMulToShlAddShlAdd(N, DAG, MulAmt))
return V;

// If this is a power 2 + 2/4/8, we can use a shift followed by a single
// shXadd. First check if this a sum of two power of 2s because that's
Expand Down Expand Up @@ -16638,23 +16635,12 @@ static SDValue expandMul(SDNode *N, SelectionDAG &DAG,
}
}

for (uint64_t Divisor : {3, 5, 9}) {
if (MulAmt % Divisor != 0)
continue;
uint64_t MulAmt2 = MulAmt / Divisor;
// 3/5/9 * 3/5/9 * 2^N - In particular, this covers multiples
// of 25 which happen to be quite common.
if (int ShBAmount = isShifted359(MulAmt2, Shift)) {
SDLoc DL(N);
SDValue Mul359A =
DAG.getNode(RISCVISD::SHL_ADD, DL, VT, X,
DAG.getConstant(Log2_64(Divisor - 1), DL, VT), X);
SDValue Mul359B =
DAG.getNode(RISCVISD::SHL_ADD, DL, VT, Mul359A,
DAG.getConstant(ShBAmount, DL, VT), Mul359A);
return DAG.getNode(ISD::SHL, DL, VT, Mul359B,
DAG.getConstant(Shift, DL, VT));
}
// 3/5/9 * 3/5/9 * 2^N - In particular, this covers multiples
// of 25 which happen to be quite common.
Shift = llvm::countr_zero(MulAmt);
if (SDValue V = expandMulToShlAddShlAdd(N, DAG, MulAmt >> Shift)) {
SDLoc DL(N);
return DAG.getNode(ISD::SHL, DL, VT, V, DAG.getConstant(Shift, DL, VT));
}
}

Expand Down