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 netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
