https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661
Fangrui Song <i at maskray dot me> changed:
What |Removed |Added
----------------------------------------------------------------------------
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.
