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      1 //===-- AArch64CleanupLocalDynamicTLSPass.cpp ---------------------*- C++ -*-=//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // Local-dynamic access to thread-local variables proceeds in three stages.
     11 //
     12 // 1. The offset of this Module's thread-local area from TPIDR_EL0 is calculated
     13 //    in much the same way as a general-dynamic TLS-descriptor access against
     14 //    the special symbol _TLS_MODULE_BASE.
     15 // 2. The variable's offset from _TLS_MODULE_BASE_ is calculated using
     16 //    instructions with "dtprel" modifiers.
     17 // 3. These two are added, together with TPIDR_EL0, to obtain the variable's
     18 //    true address.
     19 //
     20 // This is only better than general-dynamic access to the variable if two or
     21 // more of the first stage TLS-descriptor calculations can be combined. This
     22 // pass looks through a function and performs such combinations.
     23 //
     24 //===----------------------------------------------------------------------===//
     25 #include "AArch64.h"
     26 #include "AArch64InstrInfo.h"
     27 #include "AArch64MachineFunctionInfo.h"
     28 #include "AArch64TargetMachine.h"
     29 #include "llvm/CodeGen/MachineDominators.h"
     30 #include "llvm/CodeGen/MachineFunction.h"
     31 #include "llvm/CodeGen/MachineFunctionPass.h"
     32 #include "llvm/CodeGen/MachineInstrBuilder.h"
     33 #include "llvm/CodeGen/MachineRegisterInfo.h"
     34 using namespace llvm;
     35 
     36 namespace {
     37 struct LDTLSCleanup : public MachineFunctionPass {
     38   static char ID;
     39   LDTLSCleanup() : MachineFunctionPass(ID) {}
     40 
     41   bool runOnMachineFunction(MachineFunction &MF) override {
     42     AArch64FunctionInfo *AFI = MF.getInfo<AArch64FunctionInfo>();
     43     if (AFI->getNumLocalDynamicTLSAccesses() < 2) {
     44       // No point folding accesses if there isn't at least two.
     45       return false;
     46     }
     47 
     48     MachineDominatorTree *DT = &getAnalysis<MachineDominatorTree>();
     49     return VisitNode(DT->getRootNode(), 0);
     50   }
     51 
     52   // Visit the dominator subtree rooted at Node in pre-order.
     53   // If TLSBaseAddrReg is non-null, then use that to replace any
     54   // TLS_base_addr instructions. Otherwise, create the register
     55   // when the first such instruction is seen, and then use it
     56   // as we encounter more instructions.
     57   bool VisitNode(MachineDomTreeNode *Node, unsigned TLSBaseAddrReg) {
     58     MachineBasicBlock *BB = Node->getBlock();
     59     bool Changed = false;
     60 
     61     // Traverse the current block.
     62     for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;
     63          ++I) {
     64       switch (I->getOpcode()) {
     65       case AArch64::TLSDESC_BLR:
     66         // Make sure it's a local dynamic access.
     67         if (!I->getOperand(1).isSymbol() ||
     68             strcmp(I->getOperand(1).getSymbolName(), "_TLS_MODULE_BASE_"))
     69           break;
     70 
     71         if (TLSBaseAddrReg)
     72           I = replaceTLSBaseAddrCall(I, TLSBaseAddrReg);
     73         else
     74           I = setRegister(I, &TLSBaseAddrReg);
     75         Changed = true;
     76         break;
     77       default:
     78         break;
     79       }
     80     }
     81 
     82     // Visit the children of this block in the dominator tree.
     83     for (MachineDomTreeNode *N : *Node) {
     84       Changed |= VisitNode(N, TLSBaseAddrReg);
     85     }
     86 
     87     return Changed;
     88   }
     89 
     90   // Replace the TLS_base_addr instruction I with a copy from
     91   // TLSBaseAddrReg, returning the new instruction.
     92   MachineInstr *replaceTLSBaseAddrCall(MachineInstr *I,
     93                                        unsigned TLSBaseAddrReg) {
     94     MachineFunction *MF = I->getParent()->getParent();
     95     const AArch64TargetMachine *TM =
     96         static_cast<const AArch64TargetMachine *>(&MF->getTarget());
     97     const AArch64InstrInfo *TII = TM->getInstrInfo();
     98 
     99     // Insert a Copy from TLSBaseAddrReg to x0, which is where the rest of the
    100     // code sequence assumes the address will be.
    101     MachineInstr *Copy = BuildMI(*I->getParent(), I, I->getDebugLoc(),
    102                                  TII->get(TargetOpcode::COPY),
    103                                  AArch64::X0).addReg(TLSBaseAddrReg);
    104 
    105     // Erase the TLS_base_addr instruction.
    106     I->eraseFromParent();
    107 
    108     return Copy;
    109   }
    110 
    111   // Create a virtal register in *TLSBaseAddrReg, and populate it by
    112   // inserting a copy instruction after I. Returns the new instruction.
    113   MachineInstr *setRegister(MachineInstr *I, unsigned *TLSBaseAddrReg) {
    114     MachineFunction *MF = I->getParent()->getParent();
    115     const AArch64TargetMachine *TM =
    116         static_cast<const AArch64TargetMachine *>(&MF->getTarget());
    117     const AArch64InstrInfo *TII = TM->getInstrInfo();
    118 
    119     // Create a virtual register for the TLS base address.
    120     MachineRegisterInfo &RegInfo = MF->getRegInfo();
    121     *TLSBaseAddrReg = RegInfo.createVirtualRegister(&AArch64::GPR64RegClass);
    122 
    123     // Insert a copy from X0 to TLSBaseAddrReg for later.
    124     MachineInstr *Next = I->getNextNode();
    125     MachineInstr *Copy = BuildMI(*I->getParent(), Next, I->getDebugLoc(),
    126                                  TII->get(TargetOpcode::COPY),
    127                                  *TLSBaseAddrReg).addReg(AArch64::X0);
    128 
    129     return Copy;
    130   }
    131 
    132   const char *getPassName() const override {
    133     return "Local Dynamic TLS Access Clean-up";
    134   }
    135 
    136   void getAnalysisUsage(AnalysisUsage &AU) const override {
    137     AU.setPreservesCFG();
    138     AU.addRequired<MachineDominatorTree>();
    139     MachineFunctionPass::getAnalysisUsage(AU);
    140   }
    141 };
    142 }
    143 
    144 char LDTLSCleanup::ID = 0;
    145 FunctionPass *llvm::createAArch64CleanupLocalDynamicTLSPass() {
    146   return new LDTLSCleanup();
    147 }
    148