Re: [range-ops] patch 01/04: types for VR_UNDEFINED and VR_VARYING

[Richard Biener <richard.guenther@gmail.com>]

On July 24, 2019 8:18:57 PM GMT+02:00, Jeff Law <law@redhat.com> wrote:
>On 7/24/19 11:00 AM, Richard Biener wrote:
>[ Big snip, ignore missing reply attributions... ]
>
>>> it. But I'd claim that if callers are required not to change these
>>> ranges, then the callers are fundamentally broken.  I'm not sure
>>> what the "sanitization" is really buying you here.  Can you point
>>> to something specific?
>>> 
>>>> 
>>>> But you lose the sanitizing that nobody can change it and the 
>>>> changed info leaks to other SSA vars.
>>>> 
>>>> As said, fix all callers to deal with NULL.
>>>> 
>>>> But I argue the current code is exactly optimal and safe.
>>> ANd I'd argue that it's just plain broken and that the
>>> sanitization you're referring to points to something broken
>>> elsewhere,  higher up in the callers.
>> 
>> Another option is to make get_value_range return by value and the
>> only way to change the lattice to call an appropriate set function. I
>> think we already do the latter in all cases (but we use
>> get_value_range in the setter) and returning by reference is just
>> eliding the copy.
>OK, so what I think you're getting at (and please correct me if I'm
>wrong) is that once the lattice values are set, you don't want
>something
>changing the recorded ranges underneath?
>
>ISTM the way to enforce that is to embed the concept in the class and
>enforce it by not allowing direct manipulation of range by the clients.
> So a client that wants this behavior somehow tells the class that
>ranges are "set in stone" and from that point the setters don't allow
>changing the underlying ranges.

Yes. You'll see that nearly all callers do

Value_range vr = *get_value_range (name);

Modify 

Update_value_range (name, &vr) ;

And returning by reference was mostly an optimization. We _did_ have callers Changing the range in place and the const varying catched those. 

When returning by value we can return individual VARYINGs not in the lattice if we decide that's what we want. 

>I just want to make sure we're on the same page WRT why you think the
>constant varying range object is useful.

As said it's an optimization. We do not want to reallocate the lattice. And we want lattice updating to happen in a controlled manner, so returning a pointer into the lattice is bad design at this point. 

Richard. 

>jeff