[Bug c++/116416] Missing optimization: compile time evaluation of prvalue

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116416

--- Comment #11 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 59834
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=59834&action=edit
gcc15-pr116416-opt.patch

I've tried to implement a simple optimization (on top of the PR118002 fix),
where we clear just a subobject that needs padding or other uninitialized bits
cleared rather than the whole object.
On the pr78687.C testcase, this results (in gimple dump) in:
-      D.10177 = {};
+      D.10177._storage.D.9582.D.9163._tail.D.9221._tail.D.9280._head = {};
because as I wrote, the single byte option_2() is the only part of the object
that actually needs to have padding bits cleared, rather than the rather large
whole object.
Though, SRA still punts even on that.