Re: [PATCH] make atomic_t volatile on all architectures

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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 [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[Index of Archives]     [Kernel Newbies]     [Netfilter]     [Bugtraq]     [Photo]     [Stuff]     [Gimp]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Video 4 Linux]     [Linux for the blind]     [Linux Resources]
  Powered by Linux