Re: [RFC] Adjusted VRP

[Richard Biener <richard dot guenther at gmail dot com>]

On Thu, Oct 30, 2014 at 2:19 PM, Marat Zakirov <m.zakirov@samsung.com> 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.
>>
>>         Jakub
>>
>
> 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.
>
> If making stuff inside VRP is a right thing why can't we do all
> VRP-dependent optimizations in the VRP transform phase?

Because for example you can't do RTL expansion from VRP ;)

> Why do we need
> range_infos if they are so imprecise?!

Because even imprecise range-info is better than no range-info.

Richard.

>