Re: [PATCH] middle-end IFN_ASSUME support [PR106654]

[Andrew MacLeod <amacleod@redhat.com>]

On 10/12/22 10:39, Jakub Jelinek wrote:
> On Wed, Oct 12, 2022 at 10:31:00AM -0400, Andrew MacLeod wrote:
>> I presume you are looking to get this working for this release, making the
>> priority high? :-)
> Yes.  So that we can claim we actually support C++23 Portable Assumptions
> and OpenMP assume directive's hold clauses for something non-trivial so
> people won't be afraid to actually use it.
> Of course, first the posted patch needs to be reviewed and only once it gets
> in, the ranger/GORI part can follow.  As the latter is only an optimization,
> it can be done incrementally.

I will start poking at something to find ranges for parameters from the 
return backwards.


>> Intersection I believe...?  I think the value from the assume's should add
>> restrictions to the range..
> Sure, sorry.
>
>> I figured as much, I was just wondering if there might be some way to
>> "simplify" certain things by processing it and turning each parameter query
>> into a smaller function returning the range we determined from the main
>> one...   but perhaps that is more complicated.
> We don't really know what the condition is, it can be pretty arbitrary
> expression (well, e.g. for C++ conditional expression, so say
> [[assume (var = foo ())]];
> is not valid but
> [[assume ((var = foo ()))]];
> is.  And with GNU statement expressions it can do a lot of stuff and until
> we e.g. inline into it and optimize it a little, we don't really know what
> it will be like.
>
> 	

No, I just meant that once we finally process the complicated function, 
and decide the final range we are storing is for x_1 is say [20,30], we 
could replace the assume call site with something like

   int assume03_x (x) { if (x>= 20 || x <= 30) return x; 
gcc_unreachable(); }

then at call sites:

    x_5 = assume03_x(x_3);

For that matter, once all the assume functions have been processed, we 
could textually replace the assume call with an expression which 
represents the determined range...  Kind of our own mini inlining?  
Maybe thats even better than adding any kind of support in 
fold_using_range..   just let things naturally fall into place?

.ASSUME_blah ( , , x_4);

where if x is determined to be [20, 30][50,60] could be textually 
"expanded" in the IL with

   if (x<20 || x>60 || (x>30 && x < 50)) gcc_unreachcable();

for each of the parameters?   If we processed this like early inlining, 
we could maybe expose the entire thing to optimization that way?

Andrew