Al Viro <[email protected]> writes:
> On Fri, Jan 06, 2006 at 12:33:56PM -0700, Zan Lynx wrote:
> > On Fri, 2006-01-06 at 19:48 +0100, Jens Axboe wrote:
> > > On Fri, Jan 06 2006, Jesper Juhl wrote:
> > > > gcc is right to warn in the sense that it doesn't know if
> > > > bvec_alloc_bs() will read or write into idx when its address is passed
> > >
> > > The function is right there, on top of bio_alloc_bioset(). It's even
> > > inlined. gcc has absolutely no reason to complain.
> And yes, if you inline it manually gcc _will_ see that everything's OK.
> Path that confuses it is
> default in switch -> exit from bio_alloc_bs() -> l1 -> use of idx
> and
> return value will be NULL => we will go to l2
> is what it doesn't notice when it inlines itself.
With my compiler (gcc 4.0.2 from FC4), it's the "unlikely" construct
that confuses the gcc warning logic, not the inlining. I get the
warning if I compile the following code with "gcc -Wall -O2 -S
test.c":
int f(int x)
{
int a;
int b = 0;
if (x) {
a = 1;
b = 1;
}
if (__builtin_expect(!b, 0))
return 0;
return a;
}
However, removing the __builtin_expect makes the warning go away.
Interestingly, changing the source so that a is initialized to some
value will make the warning go away without changing the compiled
code. Apparently, the compiler eventually realizes that the initial
value of a is never used, but it realizes it after deciding if the
warning should be generated.
--
Peter Osterlund - [email protected]
http://web.telia.com/~u89404340
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