C++ TargetLowering::usesGlobalOffsetTable方法代码示例

/// EmitJumpTableInfo - Print assembly representations of the jump tables used
/// by the current function to the current output stream.  
///
void AsmPrinter::EmitJumpTableInfo(MachineJumpTableInfo *MJTI,
                                   MachineFunction &MF) {
  const std::vector<MachineJumpTableEntry> &JT = MJTI->getJumpTables();
  if (JT.empty()) return;
  bool IsPic = TM.getRelocationModel() == Reloc::PIC_;
  
  // Use JumpTableDirective otherwise honor the entry size from the jump table
  // info.
  const char *JTEntryDirective = TAI->getJumpTableDirective();
  bool HadJTEntryDirective = JTEntryDirective != NULL;
  if (!HadJTEntryDirective) {
    JTEntryDirective = MJTI->getEntrySize() == 4 ?
      TAI->getData32bitsDirective() : TAI->getData64bitsDirective();
  }
  
  // Pick the directive to use to print the jump table entries, and switch to 
  // the appropriate section.
  TargetLowering *LoweringInfo = TM.getTargetLowering();

  const char* JumpTableDataSection = TAI->getJumpTableDataSection();  
  if ((IsPic && !(LoweringInfo && LoweringInfo->usesGlobalOffsetTable())) ||
     !JumpTableDataSection) {
    // In PIC mode, we need to emit the jump table to the same section as the
    // function body itself, otherwise the label differences won't make sense.
    // We should also do if the section name is NULL.
    const Function *F = MF.getFunction();
    SwitchToTextSection(getSectionForFunction(*F).c_str(), F);
  } else {
    SwitchToDataSection(JumpTableDataSection);
  }
  
  EmitAlignment(Log2_32(MJTI->getAlignment()));
  
  for (unsigned i = 0, e = JT.size(); i != e; ++i) {
    const std::vector<MachineBasicBlock*> &JTBBs = JT[i].MBBs;
    
    // If this jump table was deleted, ignore it. 
    if (JTBBs.empty()) continue;

    // For PIC codegen, if possible we want to use the SetDirective to reduce
    // the number of relocations the assembler will generate for the jump table.
    // Set directives are all printed before the jump table itself.
    std::set<MachineBasicBlock*> EmittedSets;
    if (TAI->getSetDirective() && IsPic)
      for (unsigned ii = 0, ee = JTBBs.size(); ii != ee; ++ii)
        if (EmittedSets.insert(JTBBs[ii]).second)
          printSetLabel(i, JTBBs[ii]);
    
    // On some targets (e.g. darwin) we want to emit two consequtive labels
    // before each jump table.  The first label is never referenced, but tells
    // the assembler and linker the extents of the jump table object.  The
    // second label is actually referenced by the code.
    if (const char *JTLabelPrefix = TAI->getJumpTableSpecialLabelPrefix())
      O << JTLabelPrefix << "JTI" << getFunctionNumber() << '_' << i << ":\n";
    
    O << TAI->getPrivateGlobalPrefix() << "JTI" << getFunctionNumber() 
      << '_' << i << ":\n";
    
    for (unsigned ii = 0, ee = JTBBs.size(); ii != ee; ++ii) {
      O << JTEntryDirective << ' ';
      // If we have emitted set directives for the jump table entries, print 
      // them rather than the entries themselves.  If we're emitting PIC, then
      // emit the table entries as differences between two text section labels.
      // If we're emitting non-PIC code, then emit the entries as direct
      // references to the target basic blocks.
      if (!EmittedSets.empty()) {
        O << TAI->getPrivateGlobalPrefix() << getFunctionNumber()
          << '_' << i << "_set_" << JTBBs[ii]->getNumber();
      } else if (IsPic) {
        printBasicBlockLabel(JTBBs[ii], false, false);
        // If the arch uses custom Jump Table directives, don't calc relative to
        // JT
        if (!HadJTEntryDirective) 
          O << '-' << TAI->getPrivateGlobalPrefix() << "JTI"
            << getFunctionNumber() << '_' << i;
      } else {
        printBasicBlockLabel(JTBBs[ii], false, false);
      }
      O << '\n';
    }
  }
}