On Sun, Jun 24, 2007 at 11:04:57AM -0700, Linus Torvalds wrote:
>
>
> On Sun, 24 Jun 2007, Al Viro wrote:
> >
> > Heh... The first catches are lovely:
> > struct fxsrAlignAssert {
> > int _:!(offsetof(struct task_struct,
> > thread.i387.fxsave) & 15);
>
> Ok, that's a bit odd.
>
> > as an idiotic way to do BUILD_BUG() and
> > #define _IOC_TYPECHECK(t) \
> > ((sizeof(t) == sizeof(t[1]) && \
> > sizeof(t) < (1 << _IOC_SIZEBITS)) ? \
> > sizeof(t) : __invalid_size_argument_for_IOC)
> > poisoning _IOW() et.al., so those who do something like
> >
> > static const char *v4l1_ioctls[] = {
> > [_IOC_NR(VIDIOCGCAP)] = "VIDIOCGCAP",
>
> On the other hand, this one really does seem to be "nice".
Why? I'd say it's not better than BUILD_BUG_ON_ZERO() use
instead of that ?:. This code would remain unchanged; the
entire reason why we run into problems is that a way to
force an error at build time had produced something that
was not a constant expression. It's not a matter of having
to change something in the users of that sucker (or _IOC_NR(), or
_IOR(), or _IO()). The code in question is there for one thing -
to give (constant) sizeof(t) or an error if t doesn't look good for
us. That's all. And we have a perfectly good way to do that...
> I don't think it's a misfeature to be able to do "obvious compile-time
> constant optimizations" on initializer indexes. The bitfield size thing in
> some ways does do the same thing - it's clearly _odd_, but if I had my
> choice, I'd prefer a language that allows it over one that doesn't.
Er... That's nice, assuming you don't suddenly run into "code with
convoluted bunch of macros breaks on a different version of compiler
and/or different CFLAGS".
IOW, having the optimizer strength define the boundary between the programs
that compile and ones that give an error is not a good idea, IMO.
Where do you place that boundary? Is 0 && n good? How about 0 & n?
Or 0 * n? Or n - n? Or (n+1)-(n+1) from macro expansion? Note that
gcc does _not_ take the last one as integer constant expression, but
happens to deal with the earlier ones. OTOH, it does deal with n * m - n * m.
And I would not bet a dime on gcc versions being consistent with each other
in that area.
Now, there is one possible answer that makes some sense - allow removal
of unevaluated parts in &&, || and ?:. I can do that, but I'm not
sure if it's actually worth doing. The only case in the kernel tree
become more readable with use of BUILD_BUG_ON_ZERO() anyway and I would
be very surprised to find any real code where something of that kind
would be a problem.
> > Objections? The only reason that doesn't break gcc to hell and back is
> > that gcc has unfixed bugs in that area. It certainly is not a valid C
> > or even a remotely sane one.
>
> I agree that it's obviously not "valid C", but I don't agree that it's not
> remotely sane. Why not allow that extension?
Because it's not well-defined and because having "let me check if this
version of compiler will eat that" as the only way to tell if construct
would be OK is not a good thing...
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