[Bug tree-optimization/100004] Dead write not removed when indirection is introduced.

[dancol at dancol dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100004

Daniel Colascione <dancol at dancol dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dancol at dancol dot org

--- Comment #2 from Daniel Colascione <dancol at dancol dot org> ---
Let's talk about this. GCC might, surprisingly, actually be correct here:
pointers are allowed to be cast from one object type to another object type and
then back while retaining their validity; in principle, w1 and w2 might be
"fake" wrapper instances created by casts from a struct Foo --- it's legal to
form these fake pointers as long as no access occurs through them. If
assign_via_pointer had reinterpret_cast<Foo*>()-ed w1 and w2 to Foo* (yielding
f1 and f2 respectively) and then accessed f1->x and f2->x, I think we'd agree 
that f1->x and f2->x could alias. 

The difference between the code in this bug and the reinterpret_cast<> scenario
is that here we're deriving f1 and f2 *not* via reinterpet_cast, but via
regular member access --- &w1->foo and &w2->foo. Should the use of a member
access instead of a cast propagate aliasing information? On one hand, it seems
logical to do so; on the other hand, the standard *does* say that access
through legally-derived same-type pointers can alias, and we *really do* have
two legally-derived same-type pointers here.

So what's the correct interpretation of the standard?