[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

["i at maskray dot me" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

Fangrui Song <i at maskray dot me> changed:

           What    |Removed                     |Added
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                 CC|                            |i at maskray dot me

--- Comment #17 from Fangrui Song <i at maskray dot me> ---
(In reply to Andrew Pinski from comment #16)
> So what is interesting is the way LLVM implements this as an IPO pass as how
> I described in comment #8, 6 years after I wrote that. Well they used
> "empty" rather than const . Note using it as an IPO pass I think is not
> wrong just having a specialized pass here where this is the same as DCE so
> just have DCE handle it instead seems better; plus with the availability of
> having pure/const flags with LTO and not having to load in the function
> seems much better really.

Yes, LLVM recognizes __cxa_atexit as a library function and removes it in
GlobalOpt (part of IPO) since 2011
(https://github.com/llvm/llvm-project/commit/ee6bc70d2f1c2434ca9ca8092216bdeab322c7e5),
likely because
GlobalOpt is already doing global variable optimizations (e.g. removal if dead,
constant folding). Technically __cxa_atexit removal can be moved elsewhere.

There are two GlobalOpt passes in the optimization pipeline (see -mllvm
-print-changed=cdiff output), one before the inliner and one after.

  SROA
  ...
  GlobalOpt
  ...
  buildInlinerPipeline
  ...
  GlobalOpt

For empty functions like `~constant_init() {}`, SROA deletes unneeded IR
instructions (spill of the "this" pointer) and actually makes the IR function
empty. Then GlobalOpt removes __cxa_atexit call sites.

For `static void empty() {} ~constant_init() { empty(); }`, the inliner removes
the `empty()` and makes the IR function empty. Then GlobalOpt removes
__cxa_atexit call sites.