On Mon, Feb 17, 2014 at 04:18:52PM -0800, Linus Torvalds wrote: > On Mon, Feb 17, 2014 at 3:41 PM, Torvald Riegel <triegel@xxxxxxxxxx> wrote: > > > > There's an underlying problem here that's independent from the actual > > instance that you're worried about here: "no sense" is a ultimately a > > matter of taste/objectives/priorities as long as the respective > > specification is logically consistent. > > Yes. But I don't think it's "independent". > > Exactly *because* some people will read standards without applying > "does the resulting code generation actually make sense for the > programmer that wrote the code", the standard has to be pretty clear. > > The standard often *isn't* pretty clear. It wasn't clear enough when > it came to "volatile", and yet that was a *much* simpler concept than > atomic accesses and memory ordering. > > And most of the time it's not a big deal. But because the C standard > generally tries to be very portable, and cover different machines, > there tends to be a mindset that anything inherently unportable is > "undefined" or "implementation defined", and then the compiler writer > is basically given free reign to do anything they want (with > "implementation defined" at least requiring that it is reliably the > same thing). > > And when it comes to memory ordering, *everything* is basically > non-portable, because different CPU's very much have different rules. > I worry that that means that the standard then takes the stance that > "well, compiler re-ordering is no worse than CPU re-ordering, so we > let the compiler do anything". And then we have to either add > "volatile" to make sure the compiler doesn't do that, or use an overly > strict memory model at the compiler level that makes it all pointless. For whatever it is worth, this line of reasoning has been one reason why I have been objecting strenuously every time someone on the committee suggests eliminating "volatile" from the standard. Thanx, Paul > So I really really hope that the standard doesn't give compiler > writers free hands to do anything that they can prove is "equivalent" > in the virtual C machine model. That's not how you get reliable > results. > > Linus > -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
