DAGCombiner: Allow the DAGCombiner to combine multiple FDIVs with the same divisor info FMULs by the reciprocal.
E.g., ( a / D; b / D ) -> ( recip = 1.0 / D; a * recip; b * recip)
A hook is added to allow the target to control whether it needs to do such combine.
Reviewed in http://reviews.llvm.org/D6334
llvm-svn: 222510
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index a1291ed..860423a 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7104,6 +7104,44 @@
}
}
+ // Combine multiple FDIVs with the same divisor into multiple FMULs by the
+ // reciprocal.
+ // E.g., (a / D; b / D;) -> (recip = 1.0 / D; a * recip; b * recip)
+ // Notice that this is not always beneficial. One reason is different target
+ // may have different costs for FDIV and FMUL, so sometimes the cost of two
+ // FDIVs may be lower than the cost of one FDIV and two FMULs. Another reason
+ // is the critical path is increased from "one FDIV" to "one FDIV + one FMUL".
+ if (Options.UnsafeFPMath) {
+ // Skip if current node is a reciprocal.
+ if (N0CFP && N0CFP->isExactlyValue(1.0))
+ return SDValue();
+
+ SmallVector<SDNode *, 4> Users;
+ // Find all FDIV users of the same divisor.
+ for (SDNode::use_iterator UI = N1.getNode()->use_begin(),
+ UE = N1.getNode()->use_end();
+ UI != UE; ++UI) {
+ SDNode *User = UI.getUse().getUser();
+ if (User->getOpcode() == ISD::FDIV && User->getOperand(1) == N1)
+ Users.push_back(User);
+ }
+
+ if (TLI.combineRepeatedFPDivisors(Users.size())) {
+ SDValue FPOne = DAG.getConstantFP(1.0, VT); // floating point 1.0
+ SDValue Reciprocal = DAG.getNode(ISD::FDIV, SDLoc(N), VT, FPOne, N1);
+
+ // Dividend / Divisor -> Dividend * Reciprocal
+ for (auto I = Users.begin(), E = Users.end(); I != E; ++I) {
+ if ((*I)->getOperand(0) != FPOne) {
+ SDValue NewNode = DAG.getNode(ISD::FMUL, SDLoc(*I), VT,
+ (*I)->getOperand(0), Reciprocal);
+ DAG.ReplaceAllUsesWith(*I, NewNode.getNode());
+ }
+ }
+ return SDValue();
+ }
+ }
+
return SDValue();
}