[RFC] Adjusted VRP

[Yury Gribov]

On 10/30/2014 04:19 PM, Marat Zakirov wrote:
> On 10/30/2014 02:32 PM, Jakub Jelinek wrote:
>> On Thu, Oct 30, 2014 at 02:16:04PM +0300, Yury Gribov wrote:
>>> On 10/30/2014 01:27 PM, Richard Biener wrote:
>>>> Well, VRP is not path-insensitive - it is the value-ranges we are able
>>>> to retain after removing the ASSERT_EXPRs VRP inserts.
>>>>
>>>> Why can't you do the ASAN optimizations in the VRP transform phase?
>>> I think this is not Asan-specific: Marat's point was that allowing
>>> basic-block-precise ranges would generally allow middle-end to produce
>>> better code.
>> The reason for get_range_info in the current form is that it is cheap,
>> and
>> unless we want to make some SSA_NAMEs non-propagatable [*], IMHO it
>> should
>> stay that way.  Now that we have ASAN_ internal calls, if you want to
>> optimize away ASAN_CHECK calls where VRP suggests that e.g. array
>> index will be within the right bounds and you'd optimize away
>> ASAN_CHECK to
>> a VAR_DECL access if the index was constant (say minimum or maximum of
>> the
>> range), you can do so in VRP and it is the right thing to do it there.
>>
>> [*] - that is something I've been talking about for
>> __builtin_unreachable ()
>> etc., whether it would be worth it if range_info of certain SSA_NAME that
>> would VRP want to remove again was significantly better than range
>> info of
>> the base SSA_NAME, to keep that SSA_NAME around and e.g. block
>> forwprop etc.
>> from propagating the SSA_NAME copy, unless something other than
>> SSA_NAME has
>> been propagated into it.  Richard was against that though.
>
> We didn't find reasonable performance gains to use VRP in asan. But even
> if we found we couldn't use it because it is not safe for asan. It make
> some optimistic conclusions invalid for asan.
>
> Adjusted VRP memory upper bound is #{trees that are compared} x nblocks
> which could be reduced by some threshold.

Do you have some concrete numbers at hand?

-Y