Subject: Re: powerpc Linux scv support and scv system call ABI proposal

From: Florian Weimer <[email-masked]>
Date: Thu, 30 Jan 2020 13:03:53 +0100

* Segher Boessenkool:

>> I *assumed* that I would still get
>> the value of r0 (the register) from the associated extended asm in this
>> expression, even if it may now be a different register.  Your comment
>> made me think that this is undefined.
>
> Please show full(er) examples, I think we are talking about something
> else?

No, I think we are in agreement here how things should behave under the
new model.  But I have doubts whether that is implemented in GCC 9.

>> The GCC documentation has this warning:
>> 
>> |  _Warning:_ In the above example, be aware that a register (for
>> | example 'r0') can be call-clobbered by subsequent code, including
>> | function calls and library calls for arithmetic operators on other
>> | variables (for example the initialization of 'p2').
>
> Yes.  This does not matter for the only supported use.

I'm not so sure.  See below.

> This is why that *is* the only supported use.  The documentation could
> use a touch-up, I think.  Unless we still have problems here?

I really don't know.  GCC still has *some* support for the old behavior,
though.  For example, local register variables are treated as
initialized, and I think you can still use registers like global
variables.  GCC does not perform copy propagation here:

int f1 (int);

int
f (void)
{
  register int edi __asm__ ("edi");
  int edi_copy = edi;
  return f1 (1) + edi_copy;
}

And the case that we agreed should be defined in fact is not:

void f1 (int);

int
f (void)
{
  register int edi __asm__ ("edi");
  asm ("#" : "=r" (edi));
  f1 (1);
  return edi;
}

On x86-64, %edi is used to pass the first function parameter, so the
call clobbers %edi.  It is simply ambiguous whether edi (the variable)
should retain the value prior to the call to f1 (which I think is what
should happen under the new model, where register variables are only
affect asm operands), or if edi (the variable) should have the value of
%edi (the register) after the call (the old model).

Should we move this to the gcc list?

Thanks,
Florian