Hi Russel,
On Thu, 2015-10-29 at 13:31 +0000, Russell King - ARM Linux wrote:
> On Thu, Oct 29, 2015 at 01:47:35AM +0300, Alexey Brodkin wrote:
> > diff --git a/lib/div64.c b/lib/div64.c
> > index 62a698a..3055328 100644
> > --- a/lib/div64.c
> > +++ b/lib/div64.c
> > +/*
> > + * If the divisor happens to be constant, we determine the appropriate
> > + * inverse at compile time to turn the division into a few inline
> > + * multiplications instead which is much faster.
> > + */
> > uint32_t __attribute__((weak)) __div64_32(uint64_t *n, uint32_t base)
> > {
> > - uint64_t rem = *n;
> > - uint64_t b = base;
> > - uint64_t res, d = 1;
> > - uint32_t high = rem >> 32;
> > -
> > - /* Reduce the thing a bit first */
> > - res = 0;
> > - if (high >= base) {
> > - high /= base;
> > - res = (uint64_t) high << 32;
> > - rem -= (uint64_t) (high*base) << 32;
> > - }
> > + unsigned int __r, __b = base;
> >
> > - while ((int64_t)b > 0 && b < rem) {
> > - b = b+b;
> > - d = d+d;
> > - }
> > + if (!__builtin_constant_p(__b) || __b == 0) {
>
> Can you explain who __builtin_constant_p(__b) can be anything but false
> here? I can't see that this will ever be true.
>
> This is a function in its own .c file - the compiler will have no
> knowledge about the callers of this function scattered throughout the
> kernel, and it has to assume that the 'base' argument to this function
> is variable. So, __builtin_constant_p(__b) will always be false, which
> means this if () statement will always be true and the else clause will
> never be used.
Essentially constant propagation will only happen if __div64_32() is inlined.
For that we need to add "inline" specifier to __div64_32(), but that in its
turn will prevent use of arch-specific more optimal __div64_32() implementation.
And that was my main question how to implement this properly: have better
generic
do_div() or __div64_32() as its heavy lifting part and still keep an ability for
some architectures to use their own implementations.
-Alexey
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