fix: if-statement CFG construction

[gfeng-certik]

Notes

When constructing if-statement cfgs, the true- and false-branch nodes were constructed and connected to the condition node before connecting anything to the end-if node. The issue with this is that when the true-branch is an empty block, the true-branch node is set to still be the condition node. Thus, when the false-branch node was constructed and connected, it became the first child of the condition node, leading it to be interpreted as being the true-branch.

To fix this, the connection from the true-branch node to the end-if node now happens before constructing and connecting the false-branch node. Now, in the case of an empty block true-branch, the condition node is connected to the end-if node first.

Testing

• Run the slither cfg printer with the following test file:

  pragma solidity ^0.8.0;
  
  contract IfElseBug {
      function test_used_in_one_path_buggy(uint a, uint b) public view returns(uint) {
        uint v = a + b;
        uint w;
  
        // empty true-branch; previously buggy
        if( a > b ) {}
        else {
            w = v;
        }
       // both branches empty
        if (a > b) {}
        else {}
        // empty false-branch
        if (a <= b) {
            w = v;
        }
        else {}
        // no false-branch
        if (v <= w) {
            w = v;
        }
        // both branches nonempty
        if (a == b) {
            v = w;
        } else {
            v = 2 * w;
        }
        return w;
    }
  }

• Ensure the cfg generated is correct for each type of if-statement.

Related Issue

https://github.com/CertiKProject/slither-task/issues/241

[duckki]

Nice catch! The fix looks good. I've confirmed the example code's CFG is fixed.