[PATCH] Optimize UBSAN_NULL checks, add sanopt.c

[Yury Gribov]

On 11/05/2014 12:33 PM, Jakub Jelinek wrote:
> On Wed, Nov 05, 2014 at 12:19:22PM +0300, Yury Gribov wrote:
>> On 11/03/2014 05:27 PM, Marek Polacek wrote:
>>> Another shot at optimizing redundant UBSAN_NULL statements.
>>>
>>> This time we walk the dominator tree - that should result in
>>> more effective optimization - and keep a list of UBSAN_NULL
>>> statements that dominate the current block, see the comment
>>> before sanopt_optimize_walker.
>>
>> Marek,
>>
>> A general question - have you considered coding this as a dataflow loop
>> instead of dominator walk?  That would allow to also remove checks for
>> variables defined via PHI nodes provided that all arguments of PHI have
>> already been checked.
>
> I'd be afraid that we'd turn sanopt into another var-tracking that way,
> with possibly huge hash tables being copied on write, merging of the tables
> etc., with big memory and time requirements, having to add --param limits
> to give up if the sum size of the tables go over certain limit.

Sure, that would be slower.  I was just curious whether you considered 
alternatives (looks like you did).

> The way Marek has coded it up is pretty cheap optimization.

Right.

> BTW, as discussed privately with Marek last time, we probably want to
> optimize UBSAN_NULL (etc.) only if -fno-sanitize-recover=null (etc.)
> or if location_t is the same, otherwise such optimizations lead to only
> one problem being reported instead of all of them.

Are you going to work on ASan soon?  I could rebase my patches on top of 
Marek's infrastructure.

-Y