[cfe-dev] Question on inspecting information across program paths

[Venugopal Raghavan via cfe-dev]

Hi,

Firstly, apologies for the long email below.

I am new to the Clang Static Analyzer (CLA). My interest in CLA was more to
see if I can solve quickly path-sensitive data-flow analysis problems than
as a vehicle to build bug-finding checkers. I needed the solutions to such
data flow problems to extract some behavioral properties from some test
cases I have. For example, consider the following piece of code:

if (cond) {
   x = 4;
}
else {
   x = 5;
}

L: .... = ...x..; // Use of variable x

I want to identify program locations such as "L" in the code above where x
is not a constant if you aggregate data-flow information across paths, but,
on the other hand, is actually a constant if you have path-specific data
flow information. Since CFA does path-specific analysis, I was curious to
know if it would help me with such tasks. The "bug-report" I want at
location L is that x has a value 4, given the path constraint for the path
including the "then-path" of the if statement and that x has a value 5
along the else-path.

I started writing a checker to accomplish the above task, but was quickly
blocked by some basic doubts. My rough plan for the checker was to maintain
a map in ProgramState which maps ProgramPoints (or, maybe program symbols)
to constants when those variables indeed have constant values at the
ProgramPoint. For example, when CFA expands states along the then-path and
reaches "L", my map would say that at ProgramPoint "L", variable x has a
constant value 4. Then, when CFA expands nodes along the else-path, I guess
it would again create a state corresponding to L which now says that x has
the constant value 5. I want to be able to catch this scenario where the
same variable at the same ProgramPoint has two different constant values
depending on the path taken.

However, the issue is that, when the state graph along the else-path is
expanded, it would no longer have any of the map contents that was built
along the disjoint then-path. How then can I get access to the then-path
information when I am expanding the path along the else-path? Since the
checker is also required to be stateless, I cannot maintain auxiliary
information in the checker that is persistent across multiple calls to the
call-back function.

Can you help me with this question? Maybe, I am trying to use CFA in ways
it was not meant to be used viz. as a vehicle to solve data flow problems
which consults information cutting across paths. Maybe, the CFA is meant to
handle problems which require the examination of information restricted to
single paths. Is that the case? Or, am I missing something?

Thanks.

Regards,
Venugopal Raghavan.
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