[Bug c++/107500] Useless atexit entry for ~constant_init in eh_globals.cc

[rdiezmail-gcc at yahoo dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107500

--- Comment #5 from R. Diez <rdiezmail-gcc at yahoo dot de> ---
I know very little about GCC, but it is a very smart compiler, so I am having a
hard time understanding how GCC could miss so many optimisations. After all,
even when compiling with little optimisation, GCC seems able to discard unused
code rather well.

In my project, I am building my own toolchain with this makefile:

https://github.com/rdiez/DebugDue/blob/master/Toolchain/Makefile

I haven't verified it this time around, but I believe that Newlib is being
built with '-Os' optimisation. Search for COMMON_CFLAGS_FOR_TARGET in that
makefile, which eventually gets passed to Newlib in CFLAGS_FOR_TARGET.

First of all, GCC seems unable to generate an empty routine or destructor, or
at least flag it as being effectively empty. The caller should then realise
that it is empty, so it is not worth generating an atexit call for it.

Secondly, I am not ARM Thumb assembly expert either, but shouldn't "add r7, sp,
#0" be optimised away? After all, nobody is really using R7 in that routine.

And finally, what is the point of generating a function prolog and epilog which
saves and restores context from the stack? If the routine is pretty much empty,
but cannot be really empty, shouldn't some kind of RET instruction suffice?