[Bug rtl-optimization/114664] -fno-omit-frame-pointer causes an ICE during the build of the greenlet package

["bergner at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114664

--- Comment #11 from Peter Bergner <bergner at gcc dot gnu.org> ---
(In reply to Richard Sandiford from comment #10)
> Yeah, I agree it's an error.  The PR says “ICE”, but is there an internal
> error?  The “cannot be used in ‘asm’ here” is a normal user-facing error,
> albeit with bad error recovery, leading us to report the same thing multiple
> times.

My bad for calling it an ICE.  Clearly it's not an ICE but a normal error as
you say.



> > but how are users supposed to know whether
> > -fno-omit-frame-pointer is in effect or not?  I've looked and there is no
> > pre-defined macro a user could check.
> That might be a useful thing to have, but if the programmer has no control
> over the build flags (i.e. cannot require/force -fomit-frame-pointer) then I
> think the asm has to take care to save and restore the frame pointer itself.
> 
> Dropping "31" from the asm means that the asm must preserve the register. 
> Things will go badly if the asm doesn't do that.

So r31 which we use as our frame-pointer reg is a non-volatile/callee saved
register, so it must be saved, but I guess they (greenlet) cannot use the
method of mentioning it in the asm clobber list to perform that.