[Bug rtl-optimization/99470] Convert fixed index addition to array address offset

["redbeard0531 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99470

Mathias Stearn <redbeard0531 at gmail dot com> changed:

           What    |Removed                     |Added
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             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |---

--- Comment #4 from Mathias Stearn <redbeard0531 at gmail dot com> ---
Yes, but I believe any case where they would access different addresses would
be UB overflow in f(), making it valid to turn f() into g(), especially if you
used an internal lowering which sign extended index to pointer width and had
defined wrapping semantics. I'll note that clang already generates identical
code for f() and g() https://gcc.godbolt.org/z/j897sh, although I think gcc has
better codegen at least for g().

Also, my example was perhaps oversimplified. My indexes were actually int8_t
(which is why I'm indexing into a 256-element array), so due to int promotion,
overflow is actually impossible. However, with int8_t arguments, gcc generates
even worse code for f(), doing the sign-extension twice for some reason (8 ->
32 -> 64): https://gcc.godbolt.org/z/5r9h89


(I hope it isn't a faux pas to reopen the ticket, but I think I've provided
enough new information that this warrants another look)