Messages in this thread | | | From | Segher Boessenkool <> | Subject | Re: [PATCH] make atomic_t volatile on all architectures | Date | Sun, 12 Aug 2007 07:53:58 +0200 |
| |
>> You'd have to use "+m". > > 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).
> 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.
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/
| |