[Bug target/82158] _Noreturn functions that do return clobber caller's registers on ARM32 (but not other arches)

[peter at cordes dot ca]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82158

--- Comment #5 from Peter Cordes <peter at cordes dot ca> ---
(In reply to Ramana Radhakrishnan from comment #4)
> It's a "feature" - if the function really doesn't return, then there is no
> real requirement to save and restore all callee-saved registers. 
>
> A deliberate choice when computing the save register mask - all that we are
> providing is an ability to backtrace out of such cases

 That's what I thought; just be able to print backtraces.  Good point about
-fno-exceptions.  I forgot I was building as C in the first place. :P


> I can see that other backends have similar techniques - x86_64 appears to
> trash %esi in the experiments I did and it does look like %esi is a callee
> saved register.

No, %esi is call-clobbered in x86-64 (Windows and System V).  It's
call-preserved in x86-32.  (-m32)

I checked again with -fno-exceptions, and none of the other backends on godbolt
do this.  I constructed a better test-case (https://godbolt.org/g/pAgsnA) that
makes it easier to compare code-gen for a function that returns (without using
_Noreturn) vs. a _Noreturn function that calls done().  So even if you aren't
sure of the calling convention on a target, you can look for fewer store
instructions, or lack of them before a reg-reg move.

In all cases except ARM32, gcc stores two regs before moving arg regs into
them.

So there's also a missed-optimization for everything except ARM32, for
functions that really don't return.  I suppose that's a separate bug that I
should open.