[SDAG] Now that we have a way to communicate the exact bit on sdiv use it to simplify sdiv by a constant.
We had a hack in SDAGBuilder in place to work around this but now we
can avoid that. Call BuildExactSDIV from BuildSDIV so DAGCombiner can
perform this trick automatically.
The added check in DAGCombiner is necessary to prevent exact sdiv by pow2
from regressing as the target-specific pow2 lowering is not aware of
exact bits yet.
This is mostly covered by existing tests. One side effect is that we
get the better lowering for exact vector sdivs now too :)
llvm-svn: 240891
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 2c4ab8b..29f9df6 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2178,7 +2178,11 @@
}
// fold (sdiv X, pow2) -> simple ops after legalize
+ // FIXME: We check for the exact bit here because the generic lowering gives
+ // better results in that case. The target-specific lowering should learn how
+ // to handle exact sdivs efficiently.
if (N1C && !N1C->isNullValue() && !N1C->isOpaque() &&
+ !cast<BinaryWithFlagsSDNode>(N)->Flags.hasExact() &&
(N1C->getAPIntValue().isPowerOf2() ||
(-N1C->getAPIntValue()).isPowerOf2())) {
// If dividing by powers of two is cheap, then don't perform the following
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 329c170..85b2d5f 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -2238,17 +2238,11 @@
SDValue Op1 = getValue(I.getOperand(0));
SDValue Op2 = getValue(I.getOperand(1));
- // Turn exact SDivs into multiplications.
- // FIXME: This should be in DAGCombiner, but it doesn't have access to the
- // exact bit.
- if (isa<BinaryOperator>(&I) && cast<BinaryOperator>(&I)->isExact() &&
- !isa<ConstantSDNode>(Op1) &&
- isa<ConstantSDNode>(Op2) && !cast<ConstantSDNode>(Op2)->isNullValue())
- setValue(&I, DAG.getTargetLoweringInfo()
- .BuildExactSDIV(Op1, Op2, getCurSDLoc(), DAG));
- else
- setValue(&I, DAG.getNode(ISD::SDIV, getCurSDLoc(), Op1.getValueType(),
- Op1, Op2));
+ SDNodeFlags Flags;
+ Flags.setExact(isa<PossiblyExactOperator>(&I) &&
+ cast<PossiblyExactOperator>(&I)->isExact());
+ setValue(&I, DAG.getNode(ISD::SDIV, getCurSDLoc(), Op1.getValueType(), Op1,
+ Op2, &Flags));
}
void SelectionDAGBuilder::visitICmp(const User &I) {
diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index fc74658..e7722b3 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -2674,10 +2674,9 @@
/// \brief Given an exact SDIV by a constant, create a multiplication
/// with the multiplicative inverse of the constant.
-SDValue TargetLowering::BuildExactSDIV(SDValue Op1, SDValue Op2, SDLoc dl,
- SelectionDAG &DAG) const {
- ConstantSDNode *C = cast<ConstantSDNode>(Op2);
- APInt d = C->getAPIntValue();
+static SDValue BuildExactSDIV(const TargetLowering &TLI, SDValue Op1, APInt d,
+ SDLoc dl, SelectionDAG &DAG,
+ std::vector<SDNode *> &Created) {
assert(d != 0 && "Division by zero!");
// Shift the value upfront if it is even, so the LSB is one.
@@ -2685,10 +2684,11 @@
if (ShAmt) {
// TODO: For UDIV use SRL instead of SRA.
SDValue Amt =
- DAG.getConstant(ShAmt, dl, getShiftAmountTy(Op1.getValueType()));
+ DAG.getConstant(ShAmt, dl, TLI.getShiftAmountTy(Op1.getValueType()));
SDNodeFlags Flags;
Flags.setExact(true);
Op1 = DAG.getNode(ISD::SRA, dl, Op1.getValueType(), Op1, Amt, &Flags);
+ Created.push_back(Op1.getNode());
d = d.ashr(ShAmt);
}
@@ -2697,8 +2697,10 @@
while ((t = d*xn) != 1)
xn *= APInt(d.getBitWidth(), 2) - t;
- Op2 = DAG.getConstant(xn, dl, Op1.getValueType());
- return DAG.getNode(ISD::MUL, dl, Op1.getValueType(), Op1, Op2);
+ SDValue Op2 = DAG.getConstant(xn, dl, Op1.getValueType());
+ SDValue Mul = DAG.getNode(ISD::MUL, dl, Op1.getValueType(), Op1, Op2);
+ Created.push_back(Mul.getNode());
+ return Mul;
}
/// \brief Given an ISD::SDIV node expressing a divide by constant,
@@ -2718,6 +2720,10 @@
if (!isTypeLegal(VT))
return SDValue();
+ // If the sdiv has an 'exact' bit we can use a simpler lowering.
+ if (cast<BinaryWithFlagsSDNode>(N)->Flags.hasExact())
+ return BuildExactSDIV(*this, N->getOperand(0), Divisor, dl, DAG, *Created);
+
APInt::ms magics = Divisor.magic();
// Multiply the numerator (operand 0) by the magic value