[LLVMdev] updated code size comparison

[Török Edwin]

On 01/20/2010 10:10 PM, John Regehr wrote:
> Hi Torok-
>
>> Could you also add a main() for each of these files, and do
>> a very simple test that the optimized functions actually work?
>
> Unfortunately, testing isolated C functions is much harder than just
> passing them random data!
>
> Consider this function:
>
>   int foo (int x, int y) { return x+y; }
>
> The behavior of foo() is undefined when x+y overflows.  If course it
> is trivial to come up with similar examples based on shifts,
> multiplies and divides, etc.


Indeed, but can't an analysis find at least one value for each variable
where the behavior is not undefined?
Such a value must exist, or the entire function is useless if it always
has undefined behavior.

Sure, testing on 1 such value (or a random) value won't prove that the
result is correct, but may help finding trivial
miscompilations like the neon_helper case.

Alternatively a testcase could be manually constructed for the top 10
functions in the size comparison charts,
and see whether they are miscompiled. Repeat until top 10 has no
miscompilations.

>
> A potential solution is "under-constrained execution":
>
>   http://www.stanford.edu/~engler/issta07v-engler.pdf
>
> I will bug Dawson and Daniel and see if I can get ahold of some code
> for this.

Although EXE isn't, KLEE is publicly available.

Best regards,
--Edwin