Get Predecessors for BasicBlock in LLVM

Question

What is the easiest way to get the predecessors of a BasicBlock in the LLVM framework?

I have taken a look at DepthFirstIterator and idf_iterator<BasicBlock*>, but I actually need to do a breadth-first search on the control flow graph.

I feel like this should be easy, but it's not obvious from the documentation or the examples I have been exploring online.

score 22 · Accepted Answer · answered Feb 11 '14 at 17:28

22

It isn't clear from the documentation, but the basic block class has support for a pred iterator, which gives the predecessors to the basic block. In C++11 style, one could loop through the predecessors of a block as follows:

BasicBlock* B = ...
for (auto it = pred_begin(B), et = pred_end(B); it != et; ++it)
{
  BasicBlock* predecessor = *it;
  ...
}

answered Feb 11 '14 at 17:28

Brian

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8

The BasicBlock docs are not helpful here. The logic lies in `llvm/Support/CFG.h`. I found the answer here: http://llvm.org/docs/ProgrammersManual.html#iterating-over-predecessors-successors-of-blocks – adu Feb 12 '14 at 19:51

score 9 · Answer 2 · answered Mar 20 '18 at 20:16

A simpler way to iterate through the predecessors or successors is shown by using a for-each loop in the Programmer's Manual:

Iterating over the predecessors and successors of a block is quite easy with the routines defined in llvm/IR/CFG.h. Just use code like this to iterate over all predecessors of BB:
#include "llvm/IR/CFG.h"
BasicBlock *BB = ...; 
for (BasicBlock *Pred : predecessors(BB)) {
  // ...
}
Similarly, to iterate over successors use successors.

This is a lot cleaner than using explicit iteration with pred_begin and pred_end.

Get Predecessors for BasicBlock in LLVM

2 Answers2