On Mon, Sep 16, 2013 at 01:38:35PM +0930, Rusty Russell wrote:
> Predates git, does anyone remember the rationale?
>
> ie:
> int test_bit(int nr, const volatile unsigned long *addr)
>
> I noticed because gcc failed to elimiate some code in a patch I was
> playing with.
>
> I'm nervous about subtle bugs involved in ripping it out, even if noone
> knows why. Should I add __test_bit()?
>
> Thanks,
> Rusty.
So looking at this some more, e.g. on x86 I see:
static inline int variable_test_bit(long nr, volatile const unsigned
long *addr)
{
int oldbit;
asm volatile("bt %2,%1\n\t"
"sbb %0,%0"
: "=r" (oldbit)
: "m" (*(unsigned long *)addr), "Ir" (nr));
return oldbit;
}
and I have a vague memory that (at least for some old versions) gcc
would assume (*(unsigned long *)addr) only refers to addr[0].
OTOH constant_test_bit is
static __always_inline int constant_test_bit(long nr, const volatile unsigned
long *addr)
{
return ((1UL << (nr & (BITS_PER_LONG-1))) &
(addr[nr >> _BITOPS_LONG_SHIFT])) != 0;
}
So there's a chance that we can drop volatile here.
I'll look at it some more.
--
MST
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