Teach regular and fast isel to set dead flags on unused implicit defs

on calls and similar instructions.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106353 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/FastISel.h

@@ -36,6 +36,7 @@ class TargetInstrInfo;
 class TargetLowering;
 class TargetMachine;
 class TargetRegisterClass;
+class TargetRegisterInfo;
 
 /// FastISel - This is a fast-path instruction selection class that
 /// generates poor code and doesn't support illegal types or non-trivial
@@ -60,6 +61,7 @@ protected:
   const TargetData &TD;
   const TargetInstrInfo &TII;
   const TargetLowering &TLI;
+  const TargetRegisterInfo &TRI;
   bool IsBottomUp;
 
 public:

include/llvm/CodeGen/MachineInstr.h

@@ -364,6 +364,11 @@ public:
   void addRegisterDefined(unsigned IncomingReg,
                           const TargetRegisterInfo *RegInfo = 0);
 
+  /// setPhysRegsDeadExcept - Mark every physreg used by this instruction as dead
+  /// except those in the UsedRegs list.
+  void setPhysRegsDeadExcept(const SmallVectorImpl<unsigned> &UsedRegs,
+                             const TargetRegisterInfo &TRI);
+
   /// isSafeToMove - Return true if it is safe to move this instruction. If
   /// SawStore is set to true, it means that there is a store (or call) between
   /// the instruction's location and its intended destination.

include/llvm/CodeGen/SelectionDAGNodes.h

@@ -549,6 +549,15 @@ public:
     return FoundNode;
   }
 
+  /// getFlaggedUser - If this node has a flag value with a user, return
+  /// the user (there is at most one). Otherwise return NULL.
+  SDNode *getFlaggedUser() const {
+    for (use_iterator UI = use_begin(), UE = use_end(); UI != UE; ++UI)
+      if (UI.getUse().get().getValueType() == MVT::Flag)
+        return *UI;
+    return 0;
+  }
+
   /// getNumValues - Return the number of values defined/returned by this
   /// operator.
   ///

lib/CodeGen/MachineInstr.cpp

@@ -1461,6 +1461,25 @@ void MachineInstr::addRegisterDefined(unsigned IncomingReg,
                                        true  /*IsImp*/));
 }
 
+void MachineInstr::setPhysRegsDeadExcept(const SmallVectorImpl<unsigned> &UsedRegs,
+                                         const TargetRegisterInfo &TRI) {
+  for (unsigned i = 0, e = getNumOperands(); i != e; ++i) {
+    MachineOperand &MO = getOperand(i);
+    if (!MO.isReg() || !MO.isDef()) continue;
+    unsigned Reg = MO.getReg();
+    if (Reg == 0) continue;
+    bool Dead = true;
+    for (SmallVectorImpl<unsigned>::const_iterator I = UsedRegs.begin(),
+         E = UsedRegs.end(); I != E; ++I)
+      if (TRI.regsOverlap(*I, Reg)) {
+        Dead = false;
+        break;
+      }
+    // If there are no uses, including partial uses, the def is dead.
+    if (Dead) MO.setIsDead();
+  }
+}
+
 unsigned
 MachineInstrExpressionTrait::getHashValue(const MachineInstr* const &MI) {
   unsigned Hash = MI->getOpcode() * 37;

lib/CodeGen/SelectionDAG/FastISel.cpp

@@ -849,6 +849,7 @@ FastISel::FastISel(MachineFunction &mf,
     TD(*TM.getTargetData()),
     TII(*TM.getInstrInfo()),
     TLI(*TM.getTargetLowering()),
+    TRI(*TM.getRegisterInfo()),
     IsBottomUp(false) {
 }
 

lib/CodeGen/SelectionDAG/InstrEmitter.cpp

@@ -677,6 +677,33 @@ EmitMachineNode(SDNode *Node, bool IsClone, bool IsCloned,
   // Create the new machine instruction.
   MachineInstr *MI = BuildMI(*MF, Node->getDebugLoc(), II);
 
+  // The MachineInstr constructor adds implicit-def operands. Scan through
+  // these to determine which are dead.
+  if (MI->getNumOperands() != 0 &&
+      Node->getValueType(Node->getNumValues()-1) == MVT::Flag) {
+    // First, collect all used registers.
+    SmallVector<unsigned, 8> UsedRegs;
+    for (SDNode *F = Node->getFlaggedUser(); F; F = F->getFlaggedUser())
+      if (F->getOpcode() == ISD::CopyFromReg)
+        UsedRegs.push_back(cast<RegisterSDNode>(F->getOperand(1))->getReg());
+      else {
+        // Collect declared implicit uses.
+        const TargetInstrDesc &TID = TII->get(F->getMachineOpcode());
+        UsedRegs.append(TID.getImplicitUses(),
+                        TID.getImplicitUses() + TID.getNumImplicitUses());
+        // In addition to declared implicit uses, we must also check for
+        // direct RegisterSDNode operands.
+        for (unsigned i = 0, e = F->getNumOperands(); i != e; ++i)
+          if (RegisterSDNode *R = dyn_cast<RegisterSDNode>(F->getOperand(i))) {
+            unsigned Reg = R->getReg();
+            if (Reg != 0 && TargetRegisterInfo::isPhysicalRegister(Reg))
+              UsedRegs.push_back(Reg);
+          }
+      }
+    // Then mark unused registers as dead.
+    MI->setPhysRegsDeadExcept(UsedRegs, *TRI);
+  }
+  
   // Add result register values for things that are defined by this
   // instruction.
   if (NumResults)

lib/Target/X86/X86FastISel.cpp

@@ -1555,6 +1555,7 @@ bool X86FastISel::X86SelectCall(const Instruction *I) {
   BuildMI(MBB, DL, TII.get(AdjStackUp)).addImm(NumBytes).addImm(0);
 
   // Now handle call return value (if any).
+  SmallVector<unsigned, 4> UsedRegs;
   if (RetVT.getSimpleVT().SimpleTy != MVT::isVoid) {
     SmallVector<CCValAssign, 16> RVLocs;
     CCState CCInfo(CC, false, TM, RVLocs, I->getParent()->getContext());
@@ -1582,6 +1583,8 @@ bool X86FastISel::X86SelectCall(const Instruction *I) {
                                     RVLocs[0].getLocReg(), DstRC, SrcRC, DL);
     assert(Emitted && "Failed to emit a copy instruction!"); Emitted=Emitted;
     Emitted = true;
+    UsedRegs.push_back(RVLocs[0].getLocReg());
+
     if (CopyVT != RVLocs[0].getValVT()) {
       // Round the F80 the right size, which also moves to the appropriate xmm
       // register. This is accomplished by storing the F80 value in memory and
@@ -1609,6 +1612,9 @@ bool X86FastISel::X86SelectCall(const Instruction *I) {
     UpdateValueMap(I, ResultReg);
   }
 
+  // Set all unused physreg defs as dead.
+  static_cast<MachineInstr *>(MIB)->setPhysRegsDeadExcept(UsedRegs, TRI);
+
   return true;
 }