A select between a constant and zero, when fed by a bit test, can be efficiently
lowered using a series of shifts.
Fixes <rdar://problem/8285015>.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114599 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 1bd1e00..be95c08 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -6808,6 +6808,35 @@
}
}
+ // fold (select_cc seteq (and x, y), 0, 0, A) -> (and (shr (shl x)) A)
+ // where y is has a single bit set.
+ // A plaintext description would be, we can turn the SELECT_CC into an AND
+ // when the condition can be materialized as an all-ones register. Any
+ // single bit-test can be materialized as an all-ones register with
+ // shift-left and shift-right-arith.
+ if (CC == ISD::SETEQ && N0->getOpcode() == ISD::AND &&
+ N0->getValueType(0) == VT &&
+ N1C && N1C->isNullValue() &&
+ N2C && N2C->isNullValue()) {
+ SDValue AndLHS = N0->getOperand(0);
+ ConstantSDNode *ConstAndRHS = dyn_cast<ConstantSDNode>(N0->getOperand(1));
+ if (ConstAndRHS && ConstAndRHS->getAPIntValue().countPopulation() == 1) {
+ // Shift the tested bit over the sign bit.
+ APInt AndMask = ConstAndRHS->getAPIntValue();
+ SDValue ShlAmt =
+ DAG.getConstant(AndMask.countLeadingZeros(), getShiftAmountTy());
+ SDValue Shl = DAG.getNode(ISD::SHL, N0.getDebugLoc(), VT, AndLHS, ShlAmt);
+
+ // Now arithmetic right shift it all the way over, so the result is either
+ // all-ones, or zero.
+ SDValue ShrAmt =
+ DAG.getConstant(AndMask.getBitWidth()-1, getShiftAmountTy());
+ SDValue Shr = DAG.getNode(ISD::SRA, N0.getDebugLoc(), VT, Shl, ShrAmt);
+
+ return DAG.getNode(ISD::AND, DL, VT, Shr, N3);
+ }
+ }
+
// fold select C, 16, 0 -> shl C, 4
if (N2C && N3C && N3C->isNullValue() && N2C->getAPIntValue().isPowerOf2() &&
TLI.getBooleanContents() == TargetLowering::ZeroOrOneBooleanContent) {