Fix live variables issues:

1. If part of a register is re-defined, an implicit kill and an implicit def are added to denote read / mod / write. However, this should only be necessary if the register is actually read later. This is a performance issue.
2. If a sub-register is being defined, and it doesn't have a previous use, do not add a implicit kill to the last use of a super-register:
   = EAX, AX<imp-use,kill>
...
AX =
In this case, EAX is live but AX is killed, this is wrong and will cause the coalescer to do bad things.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48521 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/LiveVariables.h

@@ -165,6 +165,12 @@ private:   // Intermediate data structures
   void HandlePhysRegUse(unsigned Reg, MachineInstr *MI);
   void HandlePhysRegDef(unsigned Reg, MachineInstr *MI);
 
+  /// hasRegisterUseBelow - Return true if the specified register is used after
+  /// the current instruction and before it's next definition.
+  bool hasRegisterUseBelow(unsigned Reg,
+                           MachineBasicBlock::iterator I,
+                           MachineBasicBlock *MBB);
+
   /// analyzePHINodes - Gather information about the PHI nodes in here. In
   /// particular, we want to map the variable information of a virtual
   /// register which is used in a PHI node. We map that to the BB the vreg

lib/CodeGen/LiveVariables.cpp

@@ -267,8 +267,9 @@ bool LiveVariables::HandlePhysRegKill(unsigned Reg, const MachineInstr *RefMI,
 
   if (*SubRegs == 0) {
     // No sub-registers, just check if reg is killed by RefMI.
-    if (PhysRegInfo[Reg] == RefMI)
+    if (PhysRegInfo[Reg] == RefMI && PhysRegInfo[Reg]->readsRegister(Reg)) {
       return true;
+    }
   } else if (SubKills.empty()) {
     // None of the sub-registers are killed elsewhere.
     return true;
@@ -297,6 +298,34 @@ bool LiveVariables::HandlePhysRegKill(unsigned Reg, MachineInstr *RefMI) {
   return false;
 }
 
+/// hasRegisterUseBelow - Return true if the specified register is used after
+/// the current instruction and before it's next definition.
+bool LiveVariables::hasRegisterUseBelow(unsigned Reg,
+                                        MachineBasicBlock::iterator I,
+                                        MachineBasicBlock *MBB) {
+  if (I == MBB->end())
+    return false;
+  ++I;
+  // FIXME: This is slow. We probably need a smarter solution. Possibilities:
+  // 1. Scan all instructions once and build def / use information of physical
+  //    registers. We also need a fast way to compare relative ordering of
+  //    instructions.
+  // 2. Cache information so this function only has to scan instructions that
+  //    read / def physical instructions.
+  for (MachineBasicBlock::iterator E = MBB->end(); I != E; ++I) {
+    MachineInstr *MI = I;
+    for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+      const MachineOperand &MO = MI->getOperand(i);
+      if (!MO.isRegister() || MO.getReg() != Reg)
+        continue;
+      if (MO.isDef())
+        return false;
+      return true;
+    }
+  }
+  return false;
+}
+
 void LiveVariables::HandlePhysRegDef(unsigned Reg, MachineInstr *MI) {
   // Does this kill a previous version of this register?
   if (MachineInstr *LastRef = PhysRegInfo[Reg]) {
@@ -338,14 +367,22 @@ void LiveVariables::HandlePhysRegDef(unsigned Reg, MachineInstr *MI) {
          unsigned SuperReg = *SuperRegs; ++SuperRegs) {
       if (PhysRegInfo[SuperReg] && PhysRegInfo[SuperReg] != MI) {
         // The larger register is previously defined. Now a smaller part is
-        // being re-defined. Treat it as read/mod/write.
+        // being re-defined. Treat it as read/mod/write if there are uses
+        // below.
         // EAX =
         // AX  =        EAX<imp-use,kill>, EAX<imp-def>
+        // ...
+        ///    =  EAX
+        if (MI && hasRegisterUseBelow(SuperReg, MI, MI->getParent())) {
           MI->addOperand(MachineOperand::CreateReg(SuperReg, false/*IsDef*/,
                                                  true/*IsImp*/,true/*IsKill*/));
           MI->addOperand(MachineOperand::CreateReg(SuperReg, true/*IsDef*/,
                                                    true/*IsImp*/));
           PhysRegInfo[SuperReg] = MI;
+        } else {
+          PhysRegInfo[SuperReg]->addRegisterKilled(SuperReg, TRI, true);
+          PhysRegInfo[SuperReg] = NULL;
+        }
         PhysRegUsed[SuperReg] = false;
         PhysRegPartUse[SuperReg] = NULL;
       } else {

test/CodeGen/PowerPC/2008-03-18-RegScavengerAssert.ll

@@ -0,0 +1,6 @@
+; RUN: llvm-as < %s | llc -march=ppc64 -enable-ppc64-regscavenger
+
+define i16 @test(i8* %d1, i16* %d2) {
+ %tmp237 = call i16 asm "lhbrx $0, $2, $1", "=r,r,bO,m"( i8* %d1, i32 0, i16* %d2 )
+ ret i16 %tmp237
+}

test/CodeGen/X86/x86-64-ret0.ll

@@ -0,0 +1,8 @@
+; RUN: llvm-as < %s | llc -march=x86-64 | grep mov | count 1
+
+define i32 @f() nounwind  {
+	tail call void @t( i32 1 ) nounwind 
+	ret i32 0
+}
+
+declare void @t(i32)