From: Florian Weimer <fweimer@redhat•com>
To: Segher Boessenkool <segher@kernel•crashing.org>
Cc: libc-alpha@sourceware•org,
Tulio Magno Quites Machado Filho <tuliom@linux•ibm.com>,
linuxppc-dev@lists•ozlabs.org,
Nicholas Piggin <npiggin@gmail•com>
Subject: Re: powerpc Linux scv support and scv system call ABI proposal
Date: Thu, 30 Jan 2020 13:03:53 +0100 [thread overview]
Message-ID: <87y2tp40d2.fsf@oldenburg2.str.redhat.com> (raw)
In-Reply-To: <20200130112512.GS22482@gate.crashing.org> (Segher Boessenkool's message of "Thu, 30 Jan 2020 05:25:12 -0600")
* 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
next prev parent reply other threads:[~2020-01-30 12:07 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-28 10:50 powerpc Linux scv support and scv system call ABI proposal Nicholas Piggin
2020-01-28 13:09 ` Florian Weimer
2020-01-28 14:05 ` Nicholas Piggin
2020-01-28 15:40 ` Segher Boessenkool
2020-01-28 16:04 ` Florian Weimer
2020-01-28 20:01 ` Segher Boessenkool
2020-01-29 16:19 ` Florian Weimer
2020-01-29 16:29 ` Segher Boessenkool
2020-01-29 17:02 ` Florian Weimer
2020-01-29 17:51 ` Segher Boessenkool
2020-01-30 10:42 ` Florian Weimer
2020-01-30 11:25 ` Segher Boessenkool
2020-01-30 12:03 ` Florian Weimer [this message]
2020-01-30 13:50 ` Segher Boessenkool
2020-01-30 17:04 ` Adhemerval Zanella
2020-01-30 21:41 ` Segher Boessenkool
2020-01-31 11:30 ` Adhemerval Zanella
2020-01-31 11:55 ` Segher Boessenkool
2020-01-28 15:58 ` Florian Weimer
2020-01-29 4:41 ` Nicholas Piggin
2020-01-28 17:26 ` Adhemerval Zanella
2020-01-29 4:58 ` Nicholas Piggin
2020-01-29 13:20 ` Segher Boessenkool
2020-01-29 15:51 ` Tulio Magno Quites Machado Filho
2020-02-19 11:03 ` Nicholas Piggin
2020-01-28 22:14 ` Joseph Myers
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