lkml.org 
[lkml]   [2007]   [Aug]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] make atomic_t volatile on all architectures
From
Date
On Sun, 2007-08-12 at 07:53 +0200, Segher Boessenkool wrote:
> > 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 ?

> > 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.

--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.


-
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 11:51    [W:0.111 / U:0.032 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site