[cfe-dev] CFGBuilder ad OpaqueValueExprs

[Ted Kremenek]

Hi Eric,

Not all expressions are included in the CFG.  Only ones that add fixed control-flow, e.g. ternary operators, are included in the CFGBlock.  Control-flow between expressions and their subexpressions is implicitly implied, and thus not all of the control-flow needs to be linearized into the CFGBlock.

As for the blank line, that looks like a missing case in the CFG printer, but I think the CFG itself looks well-formed.

Ted

On Apr 29, 2011, at 1:09 AM, Eric Niebler wrote:

> (Apologies if this comes through twice. Domain problems.)
> 
> I have an expression with OpaqueValueExprs and subexpressions to which
> they correspond. I'm trying to figure out how to make this work with
> CFGBuilder, so I'm using CFGBuilder::VisitConditionalOperator as a
> guide. For instance, this code:
> 
> int foo(int x) {
>  x = x + 1;
>  (x > 2) ? : (x += 2);
>  return x;
> }
> 
> generates the following CFG:
> 
> [ B4 (ENTRY) ]
>    Predecessors (0):
>    Successors (1): B3
> 
> [ B1 ]
>      1: [B3.2] ?: [B2.1]
>      2: return x;
>    Predecessors (2): B2 B3
>    Successors (1): B0
> 
> [ B2 ]
>      1: (x += 2)
>    Predecessors (1): B3
>    Successors (1): B1
> 
> [ B3 ]
>      1: x = x + 1
>      2: (x > 2)
>      3:
>      T: [B3.3] ? ... : ...
>    Predecessors (1): B4
>    Successors (2): B1 B2
> 
> [ B0 (EXIT) ]
>    Predecessors (1): B1
>    Successors (0):
> 
> This seems wrong to me. First is the blank on B3.3. This is the implicit
> cast of the OpaqueValueExpr (refers to "(x > 2)") to int; that is, the
> true branch of the conditional. Second, no statement for the
> OpaqueValueExpr gets emitted at all, either in the condition or in the
> true branch. Third, we have B3.T, which indicates that the condition is
> B3.3 -- the implicit conversion expression. That's not right. It should
> be B3.2, the boolean expression. B1.1 seems to get this correct, and it
> seems that B1.1 and B3.T should agree about what the condition is.
> 
> Any explanation of this CFG is greatly appreciated. And general guidance
> about how to correctly handle OpaqueValueExprs in CFGBuilding would be
> welcome as well.
> 
> Thanks in advance,
> 
> Eric
> 
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