From: Mathieu Desnoyers <mathieu.desnoyers@efficios•com>
To: Christophe Leroy <christophe.leroy@csgroup•eu>
Cc: Boqun Feng <boqun.feng@gmail•com>,
linuxppc-dev <linuxppc-dev@lists•ozlabs.org>,
Michael Jeanson <mjeanson@efficios•com>
Subject: Re: Failure to build librseq on ppc
Date: Wed, 8 Jul 2020 10:32:20 -0400 (EDT) [thread overview]
Message-ID: <1137155888.2676.1594218740683.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <96994487-ae4a-3bfb-b0f1-34228e51bea2@csgroup.eu>
----- On Jul 8, 2020, at 10:21 AM, Christophe Leroy christophe.leroy@csgroup•eu wrote:
> Le 08/07/2020 à 16:00, Mathieu Desnoyers a écrit :
>> ----- On Jul 8, 2020, at 8:33 AM, Mathieu Desnoyers
>> mathieu.desnoyers@efficios•com wrote:
>>
>>> ----- On Jul 7, 2020, at 8:59 PM, Segher Boessenkool segher@kernel•crashing.org
>>> wrote:
>> [...]
>>>>
>>>> So perhaps you have code like
>>>>
>>>> int *p;
>>>> int x;
>>>> ...
>>>> asm ("lwz %0,%1" : "=r"(x) : "m"(*p));
>>>
>>> We indeed have explicit "lwz" and "stw" instructions in there.
>>>
>>>>
>>>> where that last line should actually read
>>>>
>>>> asm ("lwz%X1 %0,%1" : "=r"(x) : "m"(*p));
>>>
>>> Indeed, turning those into "lwzx" and "stwx" seems to fix the issue.
>>>
>>> There has been some level of extra CPP macro coating around those instructions
>>> to
>>> support both ppc32 and ppc64 with the same assembly. So adding %X[arg] is not
>>> trivial.
>>> Let me see what can be done here.
>>
>> I did the following changes which appear to generate valid asm.
>> See attached corresponding .S output.
>>
>> I grepped for uses of "m" asm operand in Linux powerpc code and noticed it's
>> pretty much
>> always used with e.g. "lwz%U1%X1". I could find one blog post discussing that %U
>> is about
>> update flag, and nothing about %X. Are those documented ?
>
> As far as I can see, %U is mentioned in
> https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html in the
> powerpc subpart, at the "m" constraint.
Yep, I did notice it, but mistakenly thought it was only needed for "m<>" operand,
not "m".
Thanks,
Mathieu
>
> For the %X I don't know.
>
> Christophe
>
>>
>> Although it appears to generate valid asm, I have the feeling I'm relying on
>> undocumented
>> features here. :-/
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2020-07-08 14:34 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-07 19:17 Failure to build librseq on ppc Mathieu Desnoyers
2020-07-08 0:59 ` Segher Boessenkool
2020-07-08 12:27 ` Michael Ellerman
2020-07-08 23:53 ` Segher Boessenkool
2020-07-09 0:01 ` Mathieu Desnoyers
2020-07-09 0:18 ` Segher Boessenkool
2020-07-09 13:43 ` Mathieu Desnoyers
2020-07-09 17:37 ` Segher Boessenkool
2020-07-09 17:42 ` Mathieu Desnoyers
2020-07-09 17:56 ` Mathieu Desnoyers
2020-07-09 20:46 ` Segher Boessenkool
2020-07-09 20:57 ` Mathieu Desnoyers
2020-07-09 20:31 ` Segher Boessenkool
2020-07-08 12:33 ` Mathieu Desnoyers
2020-07-08 14:00 ` Mathieu Desnoyers
2020-07-08 14:21 ` Christophe Leroy
2020-07-08 14:32 ` Mathieu Desnoyers [this message]
2020-07-08 16:11 ` Christophe Leroy
2020-07-09 0:15 ` Segher Boessenkool
2020-07-09 0:10 ` Segher Boessenkool
2020-07-09 13:33 ` Mathieu Desnoyers
2020-07-09 17:31 ` Segher Boessenkool
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