[Bug tree-optimization/111770] predicated loads inactive lane values not modelled

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111770

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
As said X + 0. -> X is an invalid transform with FP unless there are no signed
zeros (maybe also problematic with sign-dependent rounding).

I think we agree to define .MASK_LOAD to zero masked elements.  When we need
something else we need to add an explicit ELSE value.  That needs documenting
of course and also possibly testsuite coverage - I _think_ you should be able
to do a GIMPLE frontend testcase for this.

Note this behavior would extend to .MASK_GATHER_LOAD as well as
the load-lanes and -len variants.

Unfortunately we do not have _any_ internals manual documentation for
internal functions - but you can backtrack to the optabs documentation
where this would need documenting.

Now, if-conversion could indeed elide the .COND_ADD for integers.  It's
problematic there only because of signed overflow undefinedness, so
you shouldn't see it for 'unsigned' already, and adding zero is safe.
if-conversion would need to have an idea of all the ranges involved here
so it might be a bit sophisticated to get it right.