+void StrongPHIElimination::isolatePHI(MachineInstr *PHI) {
+ assert(PHI->isPHI());
+ Node *Node = RegNodeMap[PHI->getOperand(0).getReg()];
+ Node->parent.setInt(Node->parent.getInt() | Node::kPHIIsolatedFlag);
+}
+
+/// SplitInterferencesForBasicBlock - traverses a basic block, splitting any
+/// interferences found between registers in the same congruence class. It
+/// takes two DenseMaps as arguments that it also updates:
+///
+/// 1) CurrentDominatingParent, which maps a color to the register in that
+/// congruence class whose definition was most recently seen.
+///
+/// 2) ImmediateDominatingParent, which maps a register to the register in the
+/// same congruence class that most immediately dominates it.
+///
+/// This function assumes that it is being called in a depth-first traversal
+/// of the dominator tree.
+///
+/// The algorithm used here is a generalization of the dominance-based SSA test
+/// for two variables. If there are variables a_1, ..., a_n such that
+///
+/// def(a_1) dom ... dom def(a_n),
+///
+/// then we can test for an interference between any two a_i by only using O(n)
+/// interference tests between pairs of variables. If i < j and a_i and a_j
+/// interfere, then a_i is alive at def(a_j), so it is also alive at def(a_i+1).
+/// Thus, in order to test for an interference involving a_i, we need only check
+/// for a potential interference with a_i+1.
+///
+/// This method can be generalized to arbitrary sets of variables by performing
+/// a depth-first traversal of the dominator tree. As we traverse down a branch
+/// of the dominator tree, we keep track of the current dominating variable and
+/// only perform an interference test with that variable. However, when we go to
+/// another branch of the dominator tree, the definition of the current dominating
+/// variable may no longer dominate the current block. In order to correct this,
+/// we need to use a stack of past choices of the current dominating variable
+/// and pop from this stack until we find a variable whose definition actually
+/// dominates the current block.
+///
+/// There will be one push on this stack for each variable that has become the
+/// current dominating variable, so instead of using an explicit stack we can
+/// simply associate the previous choice for a current dominating variable with
+/// the new choice. This works better in our implementation, where we test for
+/// interference in multiple distinct sets at once.
+void
+StrongPHIElimination::SplitInterferencesForBasicBlock(
+ MachineBasicBlock &MBB,
+ DenseMap<unsigned, unsigned> &CurrentDominatingParent,
+ DenseMap<unsigned, unsigned> &ImmediateDominatingParent) {
+ // Sort defs by their order in the original basic block, as the code below
+ // assumes that it is processing definitions in dominance order.
+ std::vector<MachineInstr*> &DefInstrs = PHISrcDefs[&MBB];
+ std::sort(DefInstrs.begin(), DefInstrs.end(), MIIndexCompare(LI));
+
+ for (std::vector<MachineInstr*>::const_iterator BBI = DefInstrs.begin(),
+ BBE = DefInstrs.end(); BBI != BBE; ++BBI) {
+ for (MachineInstr::const_mop_iterator I = (*BBI)->operands_begin(),
+ E = (*BBI)->operands_end(); I != E; ++I) {
+ const MachineOperand &MO = *I;
+
+ // FIXME: This would be faster if it were possible to bail out of checking
+ // an instruction's operands after the explicit defs, but this is incorrect
+ // for variadic instructions, which may appear before register allocation
+ // in the future.
+ if (!MO.isReg() || !MO.isDef())
+ continue;
+
+ unsigned DestReg = MO.getReg();
+ if (!DestReg || !TargetRegisterInfo::isVirtualRegister(DestReg))
+ continue;
+
+ // If the virtual register being defined is not used in any PHI or has
+ // already been isolated, then there are no more interferences to check.
+ unsigned DestColor = getRegColor(DestReg);
+ if (!DestColor)
+ continue;
+
+ // The input to this pass sometimes is not in SSA form in every basic
+ // block, as some virtual registers have redefinitions. We could eliminate
+ // this by fixing the passes that generate the non-SSA code, or we could
+ // handle it here by tracking defining machine instructions rather than
+ // virtual registers. For now, we just handle the situation conservatively
+ // in a way that will possibly lead to false interferences.
+ unsigned &CurrentParent = CurrentDominatingParent[DestColor];
+ unsigned NewParent = CurrentParent;
+ if (NewParent == DestReg)
+ continue;
+
+ // Pop registers from the stack represented by ImmediateDominatingParent
+ // until we find a parent that dominates the current instruction.
+ while (NewParent && (!DT->dominates(MRI->getVRegDef(NewParent), *BBI)
+ || !getRegColor(NewParent)))
+ NewParent = ImmediateDominatingParent[NewParent];
+
+ // If NewParent is nonzero, then its definition dominates the current
+ // instruction, so it is only necessary to check for the liveness of
+ // NewParent in order to check for an interference.
+ if (NewParent
+ && LI->getInterval(NewParent).liveAt(LI->getInstructionIndex(*BBI))) {
+ // If there is an interference, always isolate the new register. This
+ // could be improved by using a heuristic that decides which of the two
+ // registers to isolate.
+ isolateReg(DestReg);
+ CurrentParent = NewParent;