"musttail" statement attribute for GCC?

[Alexander Monakov]

On Fri, 23 Apr 2021, Josh Haberman via Gcc wrote:

> On Fri, Apr 23, 2021 at 1:10 PM Iain Sandoe <idsandoe@googlemail.com> wrote:
> > I did try to use it this ^ for GCC coroutines (where such a guarantee is
> > pretty important)
> >
> > However, the issue there is that not all targets support indirect tailcalls.
> 
> I'm not familiar with this limitation. What targets do not support
> indirect tail calls?

On Power64 tailcalls to anything that is not a static function are tricky, and
current Clang ICEs for musttail indirect calls (or direct calls to a function
which is not static) on that target. GCC also doesn't do indirect tailcalls on
Power64.

Also, NVPTX (being an intermediate instruction set) does not define a "tailcall
instruction", so Clang also ICEs there. Just mentioning it for completeness.

> > What about tailcalls between DSOs?
> 
> What issues are there around DSOs? Clang/LLVM don't treat these
> specially AFAIK, and it seems that tail calls through a PLT should
> work okay?

No, some targets have additional requirements for calls that potentially go via
a (PIC) PLT. The most well-known example is probably 32-bit x86, which
requries the %ebx register to hold the address of GOT when entering a PLT
trampoline. Since %ebx is callee-saved, this makes tailcalling impossible.

LLVM solves this by transforming such calls to "no-plt" GOT-indirect calls under
'musttail', so evidently it does treat them specially.

Another example is mips64, where even non-PIC PLT is special (but looks like
LLVM does not do any tailcalls on mips64 at all).

Alexander