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
