Move PHIElimination's SplitCriticalEdge for MachineBasicBlocks out

into a utility routine, teach it how to update MachineLoopInfo, and
make use of it in MachineLICM to split critical edges on demand.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106555 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineLICM.cpp

@@ -83,7 +83,6 @@ namespace {
 
     const char *getPassName() const { return "Machine Instruction LICM"; }
 
-    // FIXME: Loop preheaders?
     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.setPreservesCFG();
       AU.addRequired<MachineLoopInfo>();
@@ -182,6 +181,10 @@ namespace {
     /// current loop preheader that may become duplicates of instructions that
     /// are hoisted out of the loop.
     void InitCSEMap(MachineBasicBlock *BB);
+
+    /// getCurPreheader - Get the preheader for the current loop, splitting
+    /// a critical edge if needed.
+    MachineBasicBlock *getCurPreheader();
   };
 } // end anonymous namespace
 
@@ -193,11 +196,11 @@ FunctionPass *llvm::createMachineLICMPass(bool PreRegAlloc) {
   return new MachineLICM(PreRegAlloc);
 }
 
-/// LoopIsOuterMostWithPreheader - Test if the given loop is the outer-most
-/// loop that has a preheader.
-static bool LoopIsOuterMostWithPreheader(MachineLoop *CurLoop) {
+/// LoopIsOuterMostWithPredecessor - Test if the given loop is the outer-most
+/// loop that has a unique predecessor.
+static bool LoopIsOuterMostWithPredecessor(MachineLoop *CurLoop) {
   for (MachineLoop *L = CurLoop->getParentLoop(); L; L = L->getParentLoop())
-    if (L->getLoopPreheader())
+    if (L->getLoopPredecessor())
       return false;
   return true;
 }
@@ -223,20 +226,11 @@ bool MachineLICM::runOnMachineFunction(MachineFunction &MF) {
 
   for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I){
     CurLoop = *I;
+    CurPreheader = 0;
 
     // If this is done before regalloc, only visit outer-most preheader-sporting
     // loops.
-    if (PreRegAlloc && !LoopIsOuterMostWithPreheader(CurLoop))
-      continue;
-
-    // Determine the block to which to hoist instructions. If we can't find a
-    // suitable loop preheader, we can't do any hoisting.
-    //
-    // FIXME: We are only hoisting if the basic block coming into this loop
-    // has only one successor. This isn't the case in general because we haven't
-    // broken critical edges or added preheaders.
-    CurPreheader = CurLoop->getLoopPreheader();
-    if (!CurPreheader)
+    if (PreRegAlloc && !LoopIsOuterMostWithPredecessor(CurLoop))
       continue;
 
     if (!PreRegAlloc)
@@ -438,13 +432,16 @@ void MachineLICM::AddToLiveIns(unsigned Reg) {
 /// operands that is safe to hoist, this instruction is called to do the
 /// dirty work.
 void MachineLICM::HoistPostRA(MachineInstr *MI, unsigned Def) {
+  MachineBasicBlock *Preheader = getCurPreheader();
+  if (!Preheader) return;
+
   // Now move the instructions to the predecessor, inserting it before any
   // terminator instructions.
   DEBUG({
       dbgs() << "Hoisting " << *MI;
-      if (CurPreheader->getBasicBlock())
+      if (Preheader->getBasicBlock())
         dbgs() << " to MachineBasicBlock "
-               << CurPreheader->getName();
+               << Preheader->getName();
       if (MI->getParent()->getBasicBlock())
         dbgs() << " from MachineBasicBlock "
                << MI->getParent()->getName();
@@ -453,7 +450,7 @@ void MachineLICM::HoistPostRA(MachineInstr *MI, unsigned Def) {
 
   // Splice the instruction to the preheader.
   MachineBasicBlock *MBB = MI->getParent();
-  CurPreheader->splice(CurPreheader->getFirstTerminator(), MBB, MI);
+  Preheader->splice(Preheader->getFirstTerminator(), MBB, MI);
 
   // Add register to livein list to all the BBs in the current loop since a 
   // loop invariant must be kept live throughout the whole loop. This is
@@ -756,6 +753,9 @@ bool MachineLICM::EliminateCSE(MachineInstr *MI,
 /// that are safe to hoist, this instruction is called to do the dirty work.
 ///
 void MachineLICM::Hoist(MachineInstr *MI) {
+  MachineBasicBlock *Preheader = getCurPreheader();
+  if (!Preheader) return;
+
   // First check whether we should hoist this instruction.
   if (!IsLoopInvariantInst(*MI) || !IsProfitableToHoist(*MI)) {
     // If not, try unfolding a hoistable load.
@@ -767,9 +767,9 @@ void MachineLICM::Hoist(MachineInstr *MI) {
   // terminator instructions.
   DEBUG({
       dbgs() << "Hoisting " << *MI;
-      if (CurPreheader->getBasicBlock())
+      if (Preheader->getBasicBlock())
         dbgs() << " to MachineBasicBlock "
-               << CurPreheader->getName();
+               << Preheader->getName();
       if (MI->getParent()->getBasicBlock())
         dbgs() << " from MachineBasicBlock "
                << MI->getParent()->getName();
@@ -779,7 +779,7 @@ void MachineLICM::Hoist(MachineInstr *MI) {
   // If this is the first instruction being hoisted to the preheader,
   // initialize the CSE map with potential common expressions.
   if (FirstInLoop) {
-    InitCSEMap(CurPreheader);
+    InitCSEMap(Preheader);
     FirstInLoop = false;
   }
 
@@ -789,7 +789,7 @@ void MachineLICM::Hoist(MachineInstr *MI) {
     CI = CSEMap.find(Opcode);
   if (!EliminateCSE(MI, CI)) {
     // Otherwise, splice the instruction to the preheader.
-    CurPreheader->splice(CurPreheader->getFirstTerminator(),MI->getParent(),MI);
+    Preheader->splice(Preheader->getFirstTerminator(),MI->getParent(),MI);
 
     // Clear the kill flags of any register this instruction defines,
     // since they may need to be live throughout the entire loop
@@ -813,3 +813,30 @@ void MachineLICM::Hoist(MachineInstr *MI) {
   ++NumHoisted;
   Changed = true;
 }
+
+MachineBasicBlock *MachineLICM::getCurPreheader() {
+  // Determine the block to which to hoist instructions. If we can't find a
+  // suitable loop predecessor, we can't do any hoisting.
+
+  // If we've tried to get a preheader and failed, don't try again.
+  if (CurPreheader == reinterpret_cast<MachineBasicBlock *>(-1))
+    return 0;
+
+  if (!CurPreheader) {
+    CurPreheader = CurLoop->getLoopPreheader();
+    if (!CurPreheader) {
+      MachineBasicBlock *Pred = CurLoop->getLoopPredecessor();
+      if (!Pred) {
+        CurPreheader = reinterpret_cast<MachineBasicBlock *>(-1);
+        return 0;
+      }
+
+      CurPreheader = Pred->SplitCriticalEdge(CurLoop->getHeader(), this);
+      if (!CurPreheader) {
+        CurPreheader = reinterpret_cast<MachineBasicBlock *>(-1);
+        return 0;
+      }
+    }
+  }
+  return CurPreheader;
+}