如何从含目标基本块标签的分支指令提取MachineBasicBlock对象？

Great question! When working with LLVM's Machine IR, extracting the target MachineBasicBlock (MBB) objects from a conditional branch instruction is totally feasible—you just need to access the right operands and use the built-in methods provided by the LLVM API.

First, let's clarify the structure of a conditional branch MachineInstr:

• The first operand is usually the condition (e.g., an i1 register or immediate value).
• The subsequent operands are the target MachineBasicBlock objects (one for the true path, one for the false path).

Your example code is close, but you're accessing the wrong operand (the condition instead of the target blocks). Here's the corrected approach:

for(MachineBasicBlock &BB : MF){ 
    for(MachineInstr &MI : BB){ 
        if(MI.isConditionalBranch()){ 
            // Conditional branches have at least two target MBBs
            // Operand 0: condition, Operand 1: true target, Operand 2: false target
            if (MI.getOperand(1).isMBB()) {
                MachineBasicBlock *TrueTarget = MI.getOperand(1).getMBB();
                // Do something with TrueTarget
            }
            if (MI.getOperand(2).isMBB()) {
                MachineBasicBlock *FalseTarget = MI.getOperand(2).getMBB();
                // Do something with FalseTarget
            }
        }
    }
}

Key Points to Remember: #

• Use MachineOperand::isMBB() to verify that an operand actually references an MBB before accessing it (this is a safe practice to avoid crashes).
• MachineOperand::getMBB() returns a pointer to the target MachineBasicBlock—you can dereference it if you need a reference instead.
• For unconditional branches (check with MI.isUnconditionalBranch()), there's only one target MBB, which is usually at operand 0.

This method works across most LLVM targets since the operand structure for branch instructions is standardized in the Machine IR layer.

内容的提问来源于stack exchange，提问作者CodeHoarder