lkml.org 
[lkml]   [2007]   [Aug]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH] make atomic_t volatile on all architectures
Date
>>> Yes, though I would use "=m" on the output list and "m" on the input
>>> list. The reason is that I've seen gcc fall on its face with an ICE
>>> on
>>> s390 due to "+m". The explanation I've got from our compiler people
>>> was
>>> quite esoteric, as far as I remember gcc splits "+m" to an input
>>> operand
>>> and an output operand. Now it can happen that the compiler chooses
>>> two
>>> different registers to access the same memory location. "+m" requires
>>> that the two memory references are identical which causes the ICE if
>>> they are not.
>>
>> The problem is very nicely described here, last paragraph:
>> <http://gcc.gnu.org/ml/gcc-patches/2007-07/msg01816.html>
>>
>> It's not a problem anymore in (very) recent GCC, although
>> that of course won't help you in the kernel (yet).
>
> So you are saying that gcc 3.x still has this problem ?

Yes. A warning ("read-write constraint does not allow a register")
was added for GCC-3.4, but the fix/workaround is more recent
(4.2 I believe, perhaps it was backported to the 4.1 branch).

>>> I do not know if the current compilers still do this. Has
>>> anyone else seen this happen ?
>>
>> In recent GCC, it's actually documented:
>>
>> The ordinary output operands must be write-only; GCC will assume
>> that
>> the values in these operands before the instruction are dead and need
>> not be generated. Extended asm supports input-output or read-write
>> operands. Use the constraint character `+' to indicate such an
>> operand
>> and list it with the output operands. You should only use read-write
>> operands when the constraints for the operand (or the operand in
>> which
>> only some of the bits are to be changed) allow a register.
>>
>> Note that last line.
>
> I see, thanks for the info.

My pleasure.


Segher

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-08-12 12:39    [W:2.463 / U:0.900 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site