Messages in this thread | | | From | Segher Boessenkool <> | Subject | Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures | Date | Sat, 18 Aug 2007 04:15:55 +0200 |
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>>>> The "asm volatile" implementation does have exactly the same >>>> reordering guarantees as the "volatile cast" thing, >>> >>> I don't think so. >> >> "asm volatile" creates a side effect. > > Yeah. > >> Side effects aren't >> allowed to be reordered wrt sequence points. > > Yeah. > >> This is exactly >> the same reason as why "volatile accesses" cannot be reordered. > > No, the code in that sub-thread I earlier pointed you at *WAS* written > such that there was a sequence point after all the uses of that > volatile > access cast macro, and _therefore_ we were safe from re-ordering > (behaviour guaranteed by the C standard).
And exactly the same is true for the "asm" version.
> Now, one cannot fantasize that "volatile asms" are also sequence > points.
Sure you can do that. I don't though.
> In fact such an argument would be sadly mistaken, because "sequence > points" are defined by the C standard and it'd be horribly wrong to > even _try_ claiming that the C standard knows about "volatile asms".
That's nonsense. GCC can extend the C standard any way they bloody well please -- witness the fact that they added an extra class of side effects...
>>> Read the relevant GCC documentation. >> >> I did, yes. > > No, you didn't read: > > http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html > > Read the bit about the need for artificial dependencies, and the > example > given there: > > asm volatile("mtfsf 255,%0" : : "f" (fpenv)); > sum = x + y; > > The docs explicitly say the addition can be moved before the "volatile > asm". Hopefully, as you know, (x + y) is an C "expression" and hence > a "sequence point" as defined by the standard.
The _end of a full expression_ is a sequence point, not every expression. And that is irrelevant here anyway.
It is perfectly fine to compute x+y any time before the assignment; the C compiler is allowed to compute it _after_ the assignment even, if it could figure out how ;-)
x+y does not contain a side effect, you know.
> I know there is also stuff written about "side-effects" there which > _could_ give the same semantic w.r.t. sequence points as the volatile > access casts,
s/could/does/
> but hey, it's GCC's own documentation, you obviously can't > find fault with _me_ if there's wrong stuff written in there. Say that > to GCC ...
There's nothing wrong there.
> See, "volatile" C keyword, for all it's ill-definition and dodgy > semantics, is still at least given somewhat of a treatment in the C > standard (whose quality is ... ummm, sadly not always good and clear, > but unsurprisingly, still about 5,482 orders-of-magnitude times > better than GCC docs).
If you find any problems/shortcomings in the GCC documentation, please file a PR, don't go whine on some unrelated mailing lists. Thank you.
> Semantics of "volatile" as applies to inline > asm, OTOH? You're completely relying on the compiler for that ...
Yes, and? GCC promises the behaviour it has documented.
>>> [ of course, if the (latest) GCC documentation is *yet again* >>> wrong, then alright, not much I can do about it, is there. ] >> >> There was (and is) nothing wrong about the "+m" documentation, if >> that is what you are talking about. It could be extended now, to >> allow "+m" -- but that takes more than just "fixing" the >> documentation. > > No, there was (and is) _everything_ wrong about the "+" documentation > as > applies to memory-constrained operands. I don't give a whit if it's > some workaround in their gimplifier, or the other, that makes it > possible > to use "+m" (like the current kernel code does). The docs suggest > otherwise, so there's obviously a clear disconnect between the docs and > actual GCC behaviour.
The documentation simply doesn't say "+m" is allowed. The code to allow it was added for the benefit of people who do not read the documentation. Documentation for "+m" might get added later if it is decided this [the code, not the documentation] is a sane thing to have (which isn't directly obvious).
> [ You seem to often take issue with _amazingly_ petty and pedantic > things, > by the way :-) ]
If you're talking details, you better get them right. Handwaving is fine with me as long as you're not purporting you're not.
And I simply cannot stand false assertions.
You can always ignore me if _you_ take issue with _that_ :-)
Segher
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