[RFC] UBSan unsafely uses VRP

[Yury Gribov]

On 11/12/2014 11:45 AM, Marek Polacek wrote:
> On Wed, Nov 12, 2014 at 11:42:39AM +0300, Yury Gribov wrote:
>> On 11/11/2014 05:15 PM, Jakub Jelinek wrote:
>>>> There are also some unsafe code in functions
>>>> ubsan_expand_si_overflow_addsub_check, ubsan_expand_si_overflow_mul_check
>>>> which uses get_range_info to reduce checks number. As seen before vrp usage
>>>> for sanitizers may decrease quality of error detection.
>>>
>>> Using VRP is completely intentional there, we don't want to generate too
>>> slow code if you decide you want to optimize your code (for -O0 VRP isn't
>>> performed of course).
>>
>> On the other hand detection quality is probably more important than
>> important regardless of optimization level. When I use a checker, I don't
>> want it to miss bugs due to overly aggressive optimization.
>
> Yes, but as said above, VRP is only run with >-O2 and -Os.

Hm, I must be missing something.  99% of users will only run their code 
under -O2 because it'll be too slow otherwise.  Why should we penalize 
them for this by lowering analysis quality?  Isn't error detection the 
main goal of sanitizers (performance being the secondary at best)?

>> I wish we had some test to check that sanitizer optimizations are indeed
>> conservative.
>
> I think most of the tests we have are tested with various optimization
> levels.

Existing tests are really a joke when we consider interblock 
optimization.  Most don't even contain any non-trivial control flow.

-Y