Re: On-Demand range technology [3/5] - The Prototype

[Eric Botcazou <ebotcazou@adacore.com>]

> While I agree that symbolic ranges are a complication and that most
> cases it currently handles are not "value-range" things I do not agree
> with the idea that we can simply remove handling them and deal
> with the fallout in some later point in the future.  Eric may also be
> able to show you "real" symbolic value-range magic.

There are a couple of testcases in the testsuite that, I believe, require a 
minimal form of support for symbolic ranges: gcc.dg/tree-ssa/vrp94.c and 
gnat.dg/opt40.adb.  They deal with the following pattern in C:

  if (x >= y)
    return 1;

  z = y - x;
  if (z <= 0)
    abort ();

  return z;

where we want to eliminate the abort.  Of course the C version doesn't really 
make sense on its own, but it's the translation of the Ada version where the

  if (z <= 0)
    abort ();

is generated by the compiler (it's a range check in Ada parlance).

I also agree that symbolic ranges are a complication but I don't understand 
why they would conceptually be treated differently from literal ranges.  Of 
course things may been different practically speaking because I also agree 
that they are of lesser importance than literal ranges in most cases.

> Note that symbolic ranges are already restricted to PLUS_EXPR
> and MINUS_EXPR (and NEGATE_EXPR I think).  There are
> also "symbolic" (non-integer constant) ranges like [&a, &a].

Yes, the current implementation is restricted to additive operations.

-- 
Eric Botcazou