[PowerPC][Future] Remove "unskipableSimplifyCode()" in PPCMIPeephole.cpp

[NeHuang]

Summary

unskipableSimplifyCode() was added to handle unsafe BL8_NOTOC instruction when TOC was not completely removed. The function is not needed after confirming no TOC usage with PC Relative addressing.

[anil9]

llvm/lib/Target/PowerPC/PPCMIPeephole.cpp

128–132

nit : For both the comment and the message in the assert. Ending the sentences with just "PC Relative" seems incorrect, may be use PC Relative instructions/functionality/operations. I see many asserts ending with or without full stops and some with exclamation sign. Not sure which one is correct.

[stefanp]

Minor wording as Anil pointed out.
LGTM.

llvm/lib/Target/PowerPC/PPCMIPeephole.cpp

128–132

You could use PC Relative addressing because that's what it really is. It is a way to generate the address of something without using the TOC pointer in R2.

[nemanjai]

LGTM. Thanks for cleaning this up.

llvm/lib/Target/PowerPC/PPCMIPeephole.cpp

128–132

I don't think we should be mentioning R2 as that doesn't really indicate what the problem is:
TOC pointer use in function that uses PC-Relative addressing

[NeHuang]

Thanks all for the review comment.
Addressed review comment.

llvm/lib/Target/PowerPC/PPCMIPeephole.cpp

STATISTIC(NumFunctionsEnteredInMIPeephole,
           "Number of functions entered in PPC MI Peepholes");
 STATISTIC(NumFixedPointIterations,
           "Number of fixed-point iterations converting reg-reg instructions "
           "to reg-imm ones");
 STATISTIC(NumRotatesCollapsed,
           "Number of pairs of rotate left, clear left/right collapsed");
 STATISTIC(NumEXTSWAndSLDICombined,
           "Number of pairs of EXTSW and SLDI combined as EXTSWSLI");
-STATISTIC(NumX2FoundForPCRel, "Number of times the X2 TOC pointer has been "
-                              "found when PC relative NOTOC is being used.");
 
 static cl::opt<bool>
 FixedPointRegToImm("ppc-reg-to-imm-fixed-point", cl::Hidden, cl::init(true),
                    cl::desc("Iterate to a fixed point when attempting to "
                             "convert reg-reg instructions to reg-imm"));
 
 static cl::opt<bool>
 ConvertRegReg("ppc-convert-rr-to-ri", cl::Hidden, cl::init(true),
private:
   MachineDominatorTree *MDT;
   MachinePostDominatorTree *MPDT;
   MachineBlockFrequencyInfo *MBFI;
   uint64_t EntryFreq;
 
   // Initialize class variables.
   void initialize(MachineFunction &MFParm);
 
-  // Perform peepholes that cannot be skipped.
-  // Some peephole simplifications are required for correctness and will not
-  // be skipped even if skipFunction(MF.getFunction()) returns true.
-  void unskipableSimplifyCode(void);
-
   // Perform peepholes.
   bool simplifyCode(void);
 
   // Perform peepholes.
   bool eliminateRedundantCompare(void);
   bool eliminateRedundantTOCSaves(std::map<MachineInstr *, bool> &TOCSaves);
   bool combineSEXTAndSHL(MachineInstr &MI, MachineInstr *&ToErase);
   bool emitRLDICWhenLoweringJumpTables(MachineInstr &MI);
void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.addPreserved<MachinePostDominatorTree>();
     AU.addPreserved<MachineBlockFrequencyInfo>();
     MachineFunctionPass::getAnalysisUsage(AU);
   }
 
   // Main entry point for this pass.
   bool runOnMachineFunction(MachineFunction &MF) override {
     initialize(MF);
-    // FIXME: This introduces another complete traversal of the instructions
-    // in the function in the common case (function is not skipped). Although
-    // this is less than ideal for compile time, this code will go away once
-    // our PC-Rel implementation is complete.
-    unskipableSimplifyCode();
+    // At this point, TOC pointer should not be used in a function that uses
+    // PC-Relative addressing.
+    assert((MF.getRegInfo().use_empty(PPC::X2) ||
+            !MF.getSubtarget<PPCSubtarget>().isUsingPCRelativeCalls()) &&
+           "TOC pointer used in a function using PC-Relative addressing!");
     if (skipFunction(MF.getFunction()))
       return false;
     return simplifyCode();
   }
 };
 
 // Initialize class variables.
 void PPCMIPeephole::initialize(MachineFunction &MFParm) {
if (MDT->dominates(CurrInst, MI)) {
       Keep = false;
       break;
     }
   }
   // Add new instruction to map.
   TOCSaves[MI] = Keep;
 }
 
-void PPCMIPeephole::unskipableSimplifyCode(void) {
-  // If this function has no uses of R2 there is nothing to do here.
-  if(MF->getRegInfo().use_empty(PPC::X2))
-    return;
-
-  // This is only for PCRelative calls.
-  if (!MF->getSubtarget<PPCSubtarget>().isUsingPCRelativeCalls()) {
-    return;
-  }
-
-  // This function has R2 so we need to mark an implicit def for it.
-  PPCFunctionInfo *FuncInfo = MF->getInfo<PPCFunctionInfo>();
-  FuncInfo->setUsesTOCBasePtr();
-  for (MachineBasicBlock &MBB : *MF) {
-    for (MachineInstr &MI : MBB) {
-      if (MI.getOpcode() == PPC::BL8_NOTOC) {
-        // At this point the BL8_NOTOC instruction is not really safe because it
-        // assumes that the caller does not need the TOC. It will be safe
-        // later once the full PC relative implementation is complete but it is
-        // not now.
-        // Here we are looking for X2. Since this is Pre-RA the only uses of X2
-        // would indicate the use of the TOC. We want to detect all uses of the
-        // TOC. Once the work is done we should not see any uses of the TOC.
-        // TODO: Once the implementation is complete this should be turned into
-        // an assert
-        Register Reg = MF->getSubtarget<PPCSubtarget>().getTOCPointerRegister();
-        MachineOperand MO = MachineOperand::CreateReg(Reg, false, true);
-        MI.addOperand(*MF, MO);
-        MI.setDesc(TII->get(PPC::BL8_NOP));
-        ++NumX2FoundForPCRel;
-      }
-    }
-  }
-}
-
 // Perform peephole optimizations.
 bool PPCMIPeephole::simplifyCode(void) {
   bool Simplified = false;
   MachineInstr* ToErase = nullptr;
   std::map<MachineInstr *, bool> TOCSaves;
   const TargetRegisterInfo *TRI = &TII->getRegisterInfo();
   NumFunctionsEnteredInMIPeephole++;
   if (ConvertRegReg) {