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

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Oct 13, 2022 at 08:11:53AM +0000, Richard Biener wrote:
> On Wed, 12 Oct 2022, Andrew MacLeod wrote:
> 
> > 
> > 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.
> 
> If the return were
> 
>   if (return_val)
>     return return_val;
> 
> you could use path-ranger with the parameter SSA default defs as
> "interesting".  So you "only" need to somehow interpret the return
> statement as such and do path rangers compute_ranges () 

If it was easier for handling, another possible representation of the
assume_function could be not that it returns a bool where [1,1] returned
means defined behavior, otherwise UB, but that the function returns void
and the assumption is that it returns, the other paths would be
__builtin_unreachable ().  But still in both cases it needs a specialized
backwards walk from the assumption that either it returns [1,1] or that it
returns through GIMPLE_RETURN to be defined behavior.  In either case,
external exceptions, or infinite loops or other reasons why the function
might not return normally (exit/abort/longjmp/non-local goto etc.) are still
UB for assumptions.
Say normally, if we have:
extern void foo (int);

bool
assume1 (int x)
{
  foo (x);
  if (x != 42)
    __builtin_unreachable ();
  return true;
}
we can't through backwards ranger walk determine that x_1(D) at the start of
the function has [42,42] range, we can just say it is true at the end of the
function, because foo could do if (x != 42) exit (0); or if (x != 42) throw
1; or if (x != 42) longjmp (buf, 1); or while (x != 42) ; or if (x != 42)
abort ();
But with assumption functions we actually can and stick [42, 42] on the
parameters even when we know nothing about foo function.

Of course, perhaps initially, we can choose to ignore those extra
guarantees.

	Jakub