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"
34using namespace llvm;
35
36namespace {
37struct LDTLSCleanup : public MachineFunctionPass {
38  static char ID;
39  LDTLSCleanup() : MachineFunctionPass(ID) {}
40
41  bool runOnMachineFunction(MachineFunction &MF) override {
42    if (skipFunction(*MF.getFunction()))
43      return false;
44
45    AArch64FunctionInfo *AFI = MF.getInfo<AArch64FunctionInfo>();
46    if (AFI->getNumLocalDynamicTLSAccesses() < 2) {
47      // No point folding accesses if there isn't at least two.
48      return false;
49    }
50
51    MachineDominatorTree *DT = &getAnalysis<MachineDominatorTree>();
52    return VisitNode(DT->getRootNode(), 0);
53  }
54
55  // Visit the dominator subtree rooted at Node in pre-order.
56  // If TLSBaseAddrReg is non-null, then use that to replace any
57  // TLS_base_addr instructions. Otherwise, create the register
58  // when the first such instruction is seen, and then use it
59  // as we encounter more instructions.
60  bool VisitNode(MachineDomTreeNode *Node, unsigned TLSBaseAddrReg) {
61    MachineBasicBlock *BB = Node->getBlock();
62    bool Changed = false;
63
64    // Traverse the current block.
65    for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;
66         ++I) {
67      switch (I->getOpcode()) {
68      case AArch64::TLSDESC_CALLSEQ:
69        // Make sure it's a local dynamic access.
70        if (!I->getOperand(0).isSymbol() ||
71            strcmp(I->getOperand(0).getSymbolName(), "_TLS_MODULE_BASE_"))
72          break;
73
74        if (TLSBaseAddrReg)
75          I = replaceTLSBaseAddrCall(*I, TLSBaseAddrReg);
76        else
77          I = setRegister(*I, &TLSBaseAddrReg);
78        Changed = true;
79        break;
80      default:
81        break;
82      }
83    }
84
85    // Visit the children of this block in the dominator tree.
86    for (MachineDomTreeNode *N : *Node) {
87      Changed |= VisitNode(N, TLSBaseAddrReg);
88    }
89
90    return Changed;
91  }
92
93  // Replace the TLS_base_addr instruction I with a copy from
94  // TLSBaseAddrReg, returning the new instruction.
95  MachineInstr *replaceTLSBaseAddrCall(MachineInstr &I,
96                                       unsigned TLSBaseAddrReg) {
97    MachineFunction *MF = I.getParent()->getParent();
98    const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
99
100    // Insert a Copy from TLSBaseAddrReg to x0, which is where the rest of the
101    // code sequence assumes the address will be.
102    MachineInstr *Copy = BuildMI(*I.getParent(), I, I.getDebugLoc(),
103                                 TII->get(TargetOpcode::COPY), AArch64::X0)
104                             .addReg(TLSBaseAddrReg);
105
106    // Erase the TLS_base_addr instruction.
107    I.eraseFromParent();
108
109    return Copy;
110  }
111
112  // Create a virtal register in *TLSBaseAddrReg, and populate it by
113  // inserting a copy instruction after I. Returns the new instruction.
114  MachineInstr *setRegister(MachineInstr &I, unsigned *TLSBaseAddrReg) {
115    MachineFunction *MF = I.getParent()->getParent();
116    const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
117
118    // Create a virtual register for the TLS base address.
119    MachineRegisterInfo &RegInfo = MF->getRegInfo();
120    *TLSBaseAddrReg = RegInfo.createVirtualRegister(&AArch64::GPR64RegClass);
121
122    // Insert a copy from X0 to TLSBaseAddrReg for later.
123    MachineInstr *Copy =
124        BuildMI(*I.getParent(), ++I.getIterator(), I.getDebugLoc(),
125                TII->get(TargetOpcode::COPY), *TLSBaseAddrReg)
126            .addReg(AArch64::X0);
127
128    return Copy;
129  }
130
131  const char *getPassName() const override {
132    return "Local Dynamic TLS Access Clean-up";
133  }
134
135  void getAnalysisUsage(AnalysisUsage &AU) const override {
136    AU.setPreservesCFG();
137    AU.addRequired<MachineDominatorTree>();
138    MachineFunctionPass::getAnalysisUsage(AU);
139  }
140};
141}
142
143char LDTLSCleanup::ID = 0;
144FunctionPass *llvm::createAArch64CleanupLocalDynamicTLSPass() {
145  return new LDTLSCleanup();
146}
147