[cfe-dev] Summer of Code idea -- detecting undefined behavior (fwd)

[Ahmed Charles]

On Tue, Mar 23, 2010 at 10:20 PM, Chris Hacking
<chacking at cs.washington.edu> wrote:
> I suspect some of these tests might incur a significant performance penalty
> if run as part of compilation. Simple and easy tests are good, but anything
> that is too expensive might make more sense to move into the Clang Static
> Analyzer instead.
>
> That said, some work from the CSA might be usable here. For example, I
> believe that the CSA already detects divide-by-zero. While a full analysis
> of the source code would be overkill for the compiler to do every time,
> adding the ability to detect divide-by-zero from literals:
>
> int a = 5; int b = 5/0;
>
> or using constant propagation:
>
> int a = 5; int b = 0; int c = a/b;
>
> would certainly be useful and probably relatively easy.
>
> Chris Hacking

Assuming the static analyzer doesn't actually produce executables (why
would it, it's static), it wouldn't be useful here, since finding
undefined behavior in general, is a runtime thing, like:

int i;
cin >> i;
int x = 5 / i;

static analysis can't detect this case of divide by zero. It could
tell that i is runtime defined and not proven to be zero, therefore it
should be tested for zero before used in the denominator of a divide.

That said, you could eliminate checks for cases where it's proven that
undefined behavior could not occur, based on static analysis, which
would result in the resulting program running faster.