BranchRelaxation

LLVM 后端的 BranchRelaxationPass 是一个专门用于处理跳转指令距离限制的 MachineFunctionPass。它的主要作用是确保生成的汇编代码中，所有的跳转指令（如 b、bl、beq 等）都能正确跳转到目标位置，避免因跳转距离超出指令编码范围而导致的错误。
原因：在某些目标架构（如 AArch64、RISC-V 等）中，跳转指令的目标地址是通过指令中的偏移量编码的，而这个偏移量有一定的范围限制。例如：

AArch64 的 B 指令支持 ±128MB 的跳转范围。
RISC-V 的 JAL 指令支持 ±1MB 的跳转范围。
当超出限制时，就需要进行 BranchRelaxtion 了。
其 Pass 入口在文件尾部：

PreservedAnalyses  
BranchRelaxationPass::run(MachineFunction &MF,  
                          MachineFunctionAnalysisManager &MFAM) {  
  if (!BranchRelaxation().run(MF))  
    return PreservedAnalyses::all();  
  
  return getMachineFunctionPassPreservedAnalyses();  
}  
  
bool BranchRelaxation::run(MachineFunction &mf) {  
  MF = &mf;  
  
  LLVM_DEBUG(dbgs() << "***** BranchRelaxation *****\n");  
  
  const TargetSubtargetInfo &ST = MF->getSubtarget();  
  TII = ST.getInstrInfo();  
  TM = &MF->getTarget();  
  
  TRI = ST.getRegisterInfo();  
  if (TRI->trackLivenessAfterRegAlloc(*MF))  
    RS.reset(new RegScavenger());  
  
  // Renumber all of the machine basic blocks in the function, guaranteeing that  
  // the numbers agree with the position of the block in the function.  MF->RenumberBlocks();  
  MF->RenumberBlocks();
  
  // Do the initial scan of the function, building up information about the  
  // sizes of each block.  scanFunction();  
  scanFunction();
  
  LLVM_DEBUG(dbgs() << "  Basic blocks before relaxation\n"; dumpBBs(););  
  
  bool MadeChange = false;  
  while (relaxBranchInstructions())  
    MadeChange = true;  
  
  // After a while, this might be made debug-only, but it is not expensive.  
  verify();  
  
  LLVM_DEBUG(dbgs() << "  Basic blocks after relaxation\n\n"; dumpBBs());  
  
  BlockInfo.clear();  
  RelaxedUnconditionals.clear();  
  
  return MadeChange;  
}