[cfe-dev] [analyzer] How to analyzer the code after an indefinite loop?

[Denis Petrov via cfe-dev]

Gabor, thank you.


Well, as I see there's no way to do this without changes/improvements in the core for now.


________________________________
Denys Petrov
Senior С++ Developer | Kharkiv, Ukraine

________________________________
От: Gábor Horváth <xazax.hun at gmail.com>
Отправлено: 30 июля 2020 г. 15:48
Кому: Denis Petrov
Копия: Artem Dergachev; cfe-dev
Тема: Re: [cfe-dev] [analyzer] How to analyzer the code after an indefinite loop?

Hi!

There are two tools in the analyzer to help these problems but both are off by default as they would need some additional improvements and testing.

One is loop unrolling which (when turned on) will detect specific loop patterns and unroll them completely (instead of stopping after 4 iterations).
The other is loop widening which involves restarting the analysis after the loop. While this increases the coverage of the analysis it can also trigger additional false positives as a large portion of the state needs to be invalidated to restart the analysis. Making this invalidation less severe is one way to improve the situation.

Cheers,
Gabor

On Thu, 30 Jul 2020 at 14:32, Denis Petrov via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:

In addition to my previous letter. I'm very concerned that we may have no chance to analyze a big chunk of code in this case:

void clang_analyzer_warnIfReached();
int f()
{
  int i = 0;
  for(int i = 0; i < 100; i++){
    if(i > 10){
      clang_analyzer_warnIfReached();
      // a lot of code will never be analyzed
    }
  }
}

Why don't we analyze loop bodies as functions, just substitute a var `i` with symbols(or constraint ranges) after reaching the limits, not generating a sinks instead?

Or there are somewhere already disscussed plans for improvement?


________________________________
Denys Petrov
Senior С++ Developer | Kharkiv, Ukraine

________________________________
От: Artem Dergachev <noqnoqneo at gmail.com<mailto:noqnoqneo at gmail.com>>
Отправлено: 28 июля 2020 г. 22:34
Кому: Denis Petrov; cfe-dev
Тема: Re: [cfe-dev] [analyzer] How to analyzer the code after an indefinite loop?

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Here's how you can find this out with the help of ExprInspection:


$ cat test.c

int f(int x, int flag)
{
  int i = 0;
  while(i < x)
    i++;

  clang_analyzer_warnIfReached();
}


$ clang --analyze -Xclang -analyzer-checker=debug.ExprInspection test.c

test.c:7:3: warning: REACHABLE [debug.ExprInspection]
  clang_analyzer_warnIfReached();
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.


Here's a slightly more interesting experiment:


$ cat test.c

int f(int x, int flag)
{
  int i = 0;
  while(i < x)
    i++;

  if (flag) {
    clang_analyzer_warnIfReached();
    clang_analyzer_numTimesReached();
    return i;
  } else {
    clang_analyzer_warnIfReached();
    clang_analyzer_numTimesReached();
    return 0;
  }
}


$ clang --analyze -Xclang -analyzer-checker=debug.ExprInspection test.c

test.c:8:5: warning: REACHABLE [debug.ExprInspection]
    clang_analyzer_warnIfReached();
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.c:9:5: warning: 4 [debug.ExprInspection]
    clang_analyzer_numTimesReached();
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.c:12:5: warning: REACHABLE [debug.ExprInspection]
    clang_analyzer_warnIfReached();
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.c:13:5: warning: 1 [debug.ExprInspection]
    clang_analyzer_numTimesReached();
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4 warnings generated.


Will you be able to figure out why is one branch reached 4 times while the other branch is reached only once? You can find all your answers on the exploded graph dump.


On 7/28/20 5:59 AM, Denis Petrov via cfe-dev wrote:

Hi, community!


A quick question.


Is CSA Core able to analyze the code after some indefinite loop?

E.g.

void f(int x)
{
  int i = 0;
  while(i < x)
    i++;
  // Interested in some code here!!
}


I found that Exploded graph grows going through the loop 4 times and then stops to analyze the code further.

P.S. I know about -analyzer-max-loop(4)?.

________________________________
Denys Petrov
Senior С++ Developer | Kharkiv, Ukraine




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