Let each target determines whether a machine instruction is dead. If true, that allows late codeine passes to delete it.

This is considered a workaround. The problem is some targets are not modeling side effects correctly. PPC is apparently one of those. This patch allows ppc llvm-gcc to bootstrap on Darwin. Once we find out which instruction definitions are wrong, we can remove the PPCInstrInfo workaround.

llvm-svn: 76703

llvm/include/llvm/Target/TargetInstrInfo.h

@@ -470,6 +470,10 @@ public:
     return true;
   }
 
+  /// isDeadInstruction - Return true if the instruction is considered dead.
+  /// This allows some late codegen passes to delete them.
+  virtual bool isDeadInstruction(const MachineInstr *MI) const = 0;
+
   /// GetInstSize - Returns the size of the specified Instruction.
   /// 
   virtual unsigned GetInstSizeInBytes(const MachineInstr *MI) const {
@@ -501,6 +505,8 @@ public:
                              MachineBasicBlock::iterator MI,
                              unsigned DestReg, unsigned SubReg,
                              const MachineInstr *Orig) const;
+  virtual bool isDeadInstruction(const MachineInstr *MI) const;
+
   virtual unsigned GetFunctionSizeInBytes(const MachineFunction &MF) const;
 };
 

llvm/lib/CodeGen/TargetInstrInfoImpl.cpp

@@ -139,6 +139,27 @@ void TargetInstrInfoImpl::reMaterialize(MachineBasicBlock &MBB,
   MBB.insert(I, MI);
 }
 
+bool TargetInstrInfoImpl::isDeadInstruction(const MachineInstr *MI) const {
+  const TargetInstrDesc &TID = MI->getDesc();
+  if (TID.mayLoad() || TID.mayStore() || TID.isCall() || TID.isTerminator() ||
+      TID.isCall() || TID.isBarrier() || TID.isReturn() ||
+      TID.hasUnmodeledSideEffects())
+    return false;
+  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+    const MachineOperand &MO = MI->getOperand(i);
+    if (!MO.isReg() || !MO.getReg())
+      continue;
+    if (MO.isDef() && !MO.isDead())
+      return false;
+    if (MO.isUse() && MO.isKill())
+      // FIXME: We can't remove kill markers or else the scavenger will assert.
+      // An alternative is to add a ADD pseudo instruction to replace kill
+      // markers.
+      return false;
+  }
+  return true;
+}
+
 unsigned
 TargetInstrInfoImpl::GetFunctionSizeInBytes(const MachineFunction &MF) const {
   unsigned FnSize = 0;

llvm/lib/CodeGen/VirtRegRewriter.cpp

@@ -1349,29 +1349,6 @@ private:
     ++NumStores;
   }
 
-  /// isSafeToDelete - Return true if this instruction doesn't produce any side
-  /// effect and all of its defs are dead.
-  static bool isSafeToDelete(MachineInstr &MI) {
-    const TargetInstrDesc &TID = MI.getDesc();
-    if (TID.mayLoad() || TID.mayStore() || TID.isCall() || TID.isTerminator() ||
-        TID.isCall() || TID.isBarrier() || TID.isReturn() ||
-        TID.hasUnmodeledSideEffects())
-      return false;
-    for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
-      MachineOperand &MO = MI.getOperand(i);
-      if (!MO.isReg() || !MO.getReg())
-        continue;
-      if (MO.isDef() && !MO.isDead())
-        return false;
-      if (MO.isUse() && MO.isKill())
-        // FIXME: We can't remove kill markers or else the scavenger will assert.
-        // An alternative is to add a ADD pseudo instruction to replace kill
-        // markers.
-        return false;
-    }
-    return true;
-  }
-
   /// TransferDeadness - A identity copy definition is dead and it's being
   /// removed. Find the last def or use and mark it as dead / kill.
   void TransferDeadness(MachineBasicBlock *MBB, unsigned CurDist,
@@ -1413,7 +1390,7 @@ private:
       if (LastUD->isDef()) {
         // If the instruction has no side effect, delete it and propagate
         // backward further. Otherwise, mark is dead and we are done.
-        if (!isSafeToDelete(*LastUDMI)) {
+        if (!TII->isDeadInstruction(LastUDMI)) {
           LastUD->setIsDead();
           break;
         }
@@ -2198,7 +2175,7 @@ private:
       }
     ProcessNextInst:
       // Delete dead instructions without side effects.
-      if (!Erased && !BackTracked && isSafeToDelete(MI)) {
+      if (!Erased && !BackTracked && TII->isDeadInstruction(&MI)) {
         InvalidateKills(MI, TRI, RegKills, KillOps);
         VRM.RemoveMachineInstrFromMaps(&MI);
         MBB.erase(&MI);

llvm/lib/Target/PowerPC/PPCInstrInfo.h

@@ -156,6 +156,13 @@ public:
   virtual bool BlockHasNoFallThrough(const MachineBasicBlock &MBB) const;
   virtual
   bool ReverseBranchCondition(SmallVectorImpl<MachineOperand> &Cond) const;
+
+  virtual bool isDeadInstruction(const MachineInstr *MI) const {
+    // FIXME: Without this, ppc llvm-gcc doesn't bootstrap. That means some
+    // instruction definitions are not modeling side effects correctly.
+    // This is a workaround until we know the exact cause.
+    return false;
+  }
   
   /// GetInstSize - Return the number of bytes of code the specified
   /// instruction may be.  This returns the maximum number of bytes.