Implement PPCInstrInfo::FoldImmediate

There are certain PPC instructions into which we can fold a zero immediate
operand. We can detect such cases by looking at the register class required
by the using operand (so long as it is not otherwise constrained).

llvm-svn: 178961

llvm/lib/Target/PowerPC/PPCInstrInfo.cpp

@@ -806,6 +806,74 @@ ReverseBranchCondition(SmallVectorImpl<MachineOperand> &Cond) const {
   return false;
 }
 
+bool PPCInstrInfo::FoldImmediate(MachineInstr *UseMI, MachineInstr *DefMI,
+                             unsigned Reg, MachineRegisterInfo *MRI) const {
+  // For some instructions, it is legal to fold ZERO into the RA register field.
+  // A zero immediate should always be loaded with a single li.
+  unsigned DefOpc = DefMI->getOpcode();
+  if (DefOpc != PPC::LI && DefOpc != PPC::LI8)
+    return false;
+  if (!DefMI->getOperand(1).isImm())
+    return false;
+  if (DefMI->getOperand(1).getImm() != 0)
+    return false;
+
+  // Note that we cannot here invert the arguments of an isel in order to fold
+  // a ZERO into what is presented as the second argument. All we have here
+  // is the condition bit, and that might come from a CR-logical bit operation.
+
+  const MCInstrDesc &UseMCID = UseMI->getDesc();
+
+  // Only fold into real machine instructions.
+  if (UseMCID.isPseudo())
+    return false;
+
+  unsigned UseIdx;
+  for (UseIdx = 0; UseIdx < UseMI->getNumOperands(); ++UseIdx)
+    if (UseMI->getOperand(UseIdx).isReg() &&
+        UseMI->getOperand(UseIdx).getReg() == Reg)
+      break;
+
+  assert(UseIdx < UseMI->getNumOperands() && "Cannot find Reg in UseMI");
+  assert(UseIdx < UseMCID.getNumOperands() && "No operand description for Reg");
+
+  const MCOperandInfo *UseInfo = &UseMCID.OpInfo[UseIdx];
+
+  // We can fold the zero if this register requires a GPRC_NOR0/G8RC_NOX0
+  // register (which might also be specified as a pointer class kind).
+  if (UseInfo->isLookupPtrRegClass()) {
+    if (UseInfo->RegClass /* Kind */ != 1)
+      return false;
+  } else {
+    if (UseInfo->RegClass != PPC::GPRC_NOR0RegClassID &&
+        UseInfo->RegClass != PPC::G8RC_NOX0RegClassID)
+      return false;
+  }
+
+  // Make sure this is not tied to an output register (or otherwise
+  // constrained). This is true for ST?UX registers, for example, which
+  // are tied to their output registers.
+  if (UseInfo->Constraints != 0)
+    return false;
+
+  unsigned ZeroReg;
+  if (UseInfo->isLookupPtrRegClass()) {
+    bool isPPC64 = TM.getSubtargetImpl()->isPPC64();
+    ZeroReg = isPPC64 ? PPC::ZERO8 : PPC::ZERO;
+  } else {
+    ZeroReg = UseInfo->RegClass == PPC::G8RC_NOX0RegClassID ?
+              PPC::ZERO8 : PPC::ZERO;
+  }
+
+  bool DeleteDef = MRI->hasOneNonDBGUse(Reg);
+  UseMI->getOperand(UseIdx).setReg(ZeroReg);
+
+  if (DeleteDef)
+    DefMI->eraseFromParent();
+
+  return true;
+}
+
 /// GetInstSize - Return the number of bytes of code the specified
 /// instruction may be.  This returns the maximum number of bytes.
 ///

llvm/lib/Target/PowerPC/PPCInstrInfo.h

@@ -157,6 +157,9 @@ public:
   virtual
   bool ReverseBranchCondition(SmallVectorImpl<MachineOperand> &Cond) const;
 
+  virtual bool FoldImmediate(MachineInstr *UseMI, MachineInstr *DefMI,
+                             unsigned Reg, MachineRegisterInfo *MRI) const;
+
   /// GetInstSize - Return the number of bytes of code the specified
   /// instruction may be.  This returns the maximum number of bytes.
   ///

llvm/lib/Target/PowerPC/PPCRegisterInfo.cpp

@@ -76,6 +76,8 @@ PPCRegisterInfo::PPCRegisterInfo(const PPCSubtarget &ST,
 const TargetRegisterClass *
 PPCRegisterInfo::getPointerRegClass(const MachineFunction &MF, unsigned Kind)
                                                                        const {
+  // Note that PPCInstrInfo::FoldImmediate also directly uses this Kind value
+  // when it checks for ZERO folding.
   if (Kind == 1) {
     if (Subtarget.isPPC64())
       return &PPC::G8RC_NOX0RegClass;

llvm/test/CodeGen/PowerPC/fold-zero.ll

@@ -0,0 +1,14 @@
+; RUN: llc < %s -mtriple=powerpc64-unknown-linux-gnu -mcpu=pwr7 | FileCheck %s
+target datalayout = "E-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-f128:128:128-v128:128:128-n32:64"
+target triple = "powerpc64-unknown-linux-gnu"
+
+define i32 @test1(i1 %a, i32 %c) nounwind  {
+  %x = select i1 %a, i32 %c, i32 0
+  ret i32 %x
+
+; CHECK: @test1
+; CHECK-NOT: li {{[0-9]+}}, 0
+; CHECK: isel 3, 0,
+; CHECK: blr
+}
+