Re: [RFA][PATCH][tree-optimization/78496] 01/03 Do not lose range information from earlier VRP passes

[Jakub Jelinek <jakub at redhat dot com>]

On Sun, Dec 03, 2017 at 10:55:27PM -0700, Jeff Law wrote:
> As we touched on in IRC, the EVRP analyzer was doing something stupid
> which caused it to not merge in existing range information under certain
> circumstances.
> 
> Consider this fragment:
> 
>   x_1 = foo ()
>   if (x_1 > 2)
>     __builtin_unreachable ();
>   if (x_1 < 0)
>     __builtin_unreachable ();

Note that for say:
  x_1 = foo ();
  bar (x_1);
  if (x_1 > 2)
    __builtin_unreachable ();
  if (x_1 < 0)
    __builtin_unreachable ();
  ...
  further uses of x_1
we can't do that anymore (at least, can't remember it in
SSA_NAME_RANGE_INFO), as bar could not return/could loop etc.  Ditto with
any other code in between foo and the unreachable asserts if it doesn't
guarantee that we'll always reach the comparisons after the x_1 setter.
Even
  x_1 = foo ();
  bar ();
  if (x_1 > 2)
    __builtin_unreachable ();
  if (x_1 < 0)
    __builtin_unreachable ();
looks unclean, if bar doesn't return, then we'd just need to hope we don't
add further uses of x_1 in between foo and bar.  Some optimizations do stuff
like that, consider foo being a pass-through function.

	Jakub