lib/Target/AArch64/AArch64CleanupLocalDynamicTLSPass.cpp

   1 //===-- AArch64CleanupLocalDynamicTLSPass.cpp ---------------------*- C++ -*-=//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // Local-dynamic access to thread-local variables proceeds in three stages.
  10 //
  11 // 1. The offset of this Module's thread-local area from TPIDR_EL0 is calculated
  12 //    in much the same way as a general-dynamic TLS-descriptor access against
  13 //    the special symbol _TLS_MODULE_BASE.
  14 // 2. The variable's offset from _TLS_MODULE_BASE_ is calculated using
  15 //    instructions with "dtprel" modifiers.
  16 // 3. These two are added, together with TPIDR_EL0, to obtain the variable's
  17 //    true address.
  18 //
  19 // This is only better than general-dynamic access to the variable if two or
  20 // more of the first stage TLS-descriptor calculations can be combined. This
  21 // pass looks through a function and performs such combinations.
  22 //
  23 //===----------------------------------------------------------------------===//
  24 #include "AArch64.h"
  25 #include "AArch64InstrInfo.h"
  26 #include "AArch64MachineFunctionInfo.h"
  27 #include "llvm/CodeGen/MachineDominators.h"
  28 #include "llvm/CodeGen/MachineFunction.h"
  29 #include "llvm/CodeGen/MachineFunctionPass.h"
  30 #include "llvm/CodeGen/MachineInstrBuilder.h"
  31 #include "llvm/CodeGen/MachineRegisterInfo.h"
  32 using namespace llvm;
  33
  34 #define TLSCLEANUP_PASS_NAME "AArch64 Local Dynamic TLS Access Clean-up"
  35
  36 namespace {
  37 struct LDTLSCleanup : public MachineFunctionPass {
  38   static char ID;
  39   LDTLSCleanup() : MachineFunctionPass(ID) {
  40     initializeLDTLSCleanupPass(*PassRegistry::getPassRegistry());
  41   }
  42
  43   bool runOnMachineFunction(MachineFunction &MF) override {
  44     if (skipFunction(MF.getFunction()))
  45       return false;
  46
  47     AArch64FunctionInfo *AFI = MF.getInfo<AArch64FunctionInfo>();
  48     if (AFI->getNumLocalDynamicTLSAccesses() < 2) {
  49       // No point folding accesses if there isn't at least two.
  50       return false;
  51     }
  52
  53     MachineDominatorTree *DT = &getAnalysis<MachineDominatorTree>();
  54     return VisitNode(DT->getRootNode(), 0);
  55   }
  56
  57   // Visit the dominator subtree rooted at Node in pre-order.
  58   // If TLSBaseAddrReg is non-null, then use that to replace any
  59   // TLS_base_addr instructions. Otherwise, create the register
  60   // when the first such instruction is seen, and then use it
  61   // as we encounter more instructions.
  62   bool VisitNode(MachineDomTreeNode *Node, unsigned TLSBaseAddrReg) {
  63     MachineBasicBlock *BB = Node->getBlock();
  64     bool Changed = false;
  65
  66     // Traverse the current block.
  67     for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;
  68          ++I) {
  69       switch (I->getOpcode()) {
  70       case AArch64::TLSDESC_CALLSEQ:
  71         // Make sure it's a local dynamic access.
  72         if (!I->getOperand(0).isSymbol() ||
  73             strcmp(I->getOperand(0).getSymbolName(), "_TLS_MODULE_BASE_"))
  74           break;
  75
  76         if (TLSBaseAddrReg)
  77           I = replaceTLSBaseAddrCall(*I, TLSBaseAddrReg);
  78         else
  79           I = setRegister(*I, &TLSBaseAddrReg);
  80         Changed = true;
  81         break;
  82       default:
  83         break;
  84       }
  85     }
  86
  87     // Visit the children of this block in the dominator tree.
  88     for (MachineDomTreeNode *N : *Node) {
  89       Changed |= VisitNode(N, TLSBaseAddrReg);
  90     }
  91
  92     return Changed;
  93   }
  94
  95   // Replace the TLS_base_addr instruction I with a copy from
  96   // TLSBaseAddrReg, returning the new instruction.
  97   MachineInstr *replaceTLSBaseAddrCall(MachineInstr &I,
  98                                        unsigned TLSBaseAddrReg) {
  99     MachineFunction *MF = I.getParent()->getParent();
 100     const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
 101
 102     // Insert a Copy from TLSBaseAddrReg to x0, which is where the rest of the
 103     // code sequence assumes the address will be.
 104     MachineInstr *Copy = BuildMI(*I.getParent(), I, I.getDebugLoc(),
 105                                  TII->get(TargetOpcode::COPY), AArch64::X0)
 106                              .addReg(TLSBaseAddrReg);
 107
 108     // Erase the TLS_base_addr instruction.
 109     I.eraseFromParent();
 110
 111     return Copy;
 112   }
 113
 114   // Create a virtual register in *TLSBaseAddrReg, and populate it by
 115   // inserting a copy instruction after I. Returns the new instruction.
 116   MachineInstr *setRegister(MachineInstr &I, unsigned *TLSBaseAddrReg) {
 117     MachineFunction *MF = I.getParent()->getParent();
 118     const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
 119
 120     // Create a virtual register for the TLS base address.
 121     MachineRegisterInfo &RegInfo = MF->getRegInfo();
 122     *TLSBaseAddrReg = RegInfo.createVirtualRegister(&AArch64::GPR64RegClass);
 123
 124     // Insert a copy from X0 to TLSBaseAddrReg for later.
 125     MachineInstr *Copy =
 126         BuildMI(*I.getParent(), ++I.getIterator(), I.getDebugLoc(),
 127                 TII->get(TargetOpcode::COPY), *TLSBaseAddrReg)
 128             .addReg(AArch64::X0);
 129
 130     return Copy;
 131   }
 132
 133   StringRef getPassName() const override { return TLSCLEANUP_PASS_NAME; }
 134
 135   void getAnalysisUsage(AnalysisUsage &AU) const override {
 136     AU.setPreservesCFG();
 137     AU.addRequired<MachineDominatorTree>();
 138     MachineFunctionPass::getAnalysisUsage(AU);
 139   }
 140 };
 141 }
 142
 143 INITIALIZE_PASS(LDTLSCleanup, "aarch64-local-dynamic-tls-cleanup",
 144                 TLSCLEANUP_PASS_NAME, false, false)
 145
 146 char LDTLSCleanup::ID = 0;
 147 FunctionPass *llvm::createAArch64CleanupLocalDynamicTLSPass() {
 148   return new LDTLSCleanup();
 149 }