+bool MachineSinking::isWorthBreakingCriticalEdge(MachineInstr *MI,
+ MachineBasicBlock *From,
+ MachineBasicBlock *To) {
+ // FIXME: Need much better heuristics.
+
+ // If the pass has already considered breaking this edge (during this pass
+ // through the function), then let's go ahead and break it. This means
+ // sinking multiple "cheap" instructions into the same block.
+ if (!CEBCandidates.insert(std::make_pair(From, To)))
+ return true;
+
+ if (!MI->isCopy() && !MI->getDesc().isAsCheapAsAMove())
+ return true;
+
+ // MI is cheap, we probably don't want to break the critical edge for it.
+ // However, if this would allow some definitions of its source operands
+ // to be sunk then it's probably worth it.
+ for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+ const MachineOperand &MO = MI->getOperand(i);
+ if (!MO.isReg()) continue;
+ unsigned Reg = MO.getReg();
+ if (Reg == 0 || !TargetRegisterInfo::isPhysicalRegister(Reg))
+ continue;
+ if (MRI->hasOneNonDBGUse(Reg))
+ return true;
+ }
+
+ return false;
+}
+
+MachineBasicBlock *MachineSinking::SplitCriticalEdge(MachineInstr *MI,
+ MachineBasicBlock *FromBB,
+ MachineBasicBlock *ToBB,
+ bool BreakPHIEdge) {
+ if (!isWorthBreakingCriticalEdge(MI, FromBB, ToBB))
+ return 0;
+
+ // Avoid breaking back edge. From == To means backedge for single BB loop.
+ if (!SplitEdges || FromBB == ToBB)
+ return 0;
+
+ // Check for backedges of more "complex" loops.
+ if (LI->getLoopFor(FromBB) == LI->getLoopFor(ToBB) &&
+ LI->isLoopHeader(ToBB))
+ return 0;
+
+ // It's not always legal to break critical edges and sink the computation
+ // to the edge.
+ //
+ // BB#1:
+ // v1024
+ // Beq BB#3
+ // <fallthrough>
+ // BB#2:
+ // ... no uses of v1024
+ // <fallthrough>
+ // BB#3:
+ // ...
+ // = v1024
+ //
+ // If BB#1 -> BB#3 edge is broken and computation of v1024 is inserted:
+ //
+ // BB#1:
+ // ...
+ // Bne BB#2
+ // BB#4:
+ // v1024 =
+ // B BB#3
+ // BB#2:
+ // ... no uses of v1024
+ // <fallthrough>
+ // BB#3:
+ // ...
+ // = v1024
+ //
+ // This is incorrect since v1024 is not computed along the BB#1->BB#2->BB#3
+ // flow. We need to ensure the new basic block where the computation is
+ // sunk to dominates all the uses.
+ // It's only legal to break critical edge and sink the computation to the
+ // new block if all the predecessors of "To", except for "From", are
+ // not dominated by "From". Given SSA property, this means these
+ // predecessors are dominated by "To".
+ //
+ // There is no need to do this check if all the uses are PHI nodes. PHI
+ // sources are only defined on the specific predecessor edges.
+ if (!BreakPHIEdge) {
+ for (MachineBasicBlock::pred_iterator PI = ToBB->pred_begin(),
+ E = ToBB->pred_end(); PI != E; ++PI) {
+ if (*PI == FromBB)
+ continue;
+ if (!DT->dominates(ToBB, *PI))
+ return 0;
+ }
+ }
+
+ return FromBB->SplitCriticalEdge(ToBB, this);
+}
+
+static bool AvoidsSinking(MachineInstr *MI, MachineRegisterInfo *MRI) {
+ return MI->isInsertSubreg() || MI->isSubregToReg() || MI->isRegSequence();
+}
+
+/// collectDebgValues - Scan instructions following MI and collect any
+/// matching DBG_VALUEs.
+static void collectDebugValues(MachineInstr *MI,
+ SmallVector<MachineInstr *, 2> & DbgValues) {
+ DbgValues.clear();
+ if (!MI->getOperand(0).isReg())
+ return;
+
+ MachineBasicBlock::iterator DI = MI; ++DI;
+ for (MachineBasicBlock::iterator DE = MI->getParent()->end();
+ DI != DE; ++DI) {
+ if (!DI->isDebugValue())
+ return;
+ if (DI->getOperand(0).isReg() &&
+ DI->getOperand(0).getReg() == MI->getOperand(0).getReg())
+ DbgValues.push_back(DI);
+ }
+}
+