On 6/26/20 10:09 PM, Richard Henderson wrote:
> In 06c4cc3660b3, we split the multiplication in two parts to avoid
> a clang warning. But because double still rounds to 53 bits, this
> does not provide additional precision beyond multiplication by
> nextafter(0x1p64, 0), the largest representable value smaller
> than 2**64.
>
> However, since we have eliminated 1.0, mutiplying by 2**64 produces
> a better distribution of input values to the output values.
>
> Signed-off-by: Richard Henderson <richard.hender...@linaro.org>
> ---
> tests/qht-bench.c | 18 ++++++++++++++++--
> 1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/tests/qht-bench.c b/tests/qht-bench.c
> index ad885d89d0..362f03cb03 100644
> --- a/tests/qht-bench.c
> +++ b/tests/qht-bench.c
> @@ -289,11 +289,25 @@ static void pr_params(void)
>
> static void do_threshold(double rate, uint64_t *threshold)
> {
> + /*
> + * For 0 <= rate <= 1, scale to fit in a uint64_t.
> + *
> + * Scale by 2**64, with a special case for 1.0.
> + * The remainder of the possible values are scattered between 0
> + * and 0xfffffffffffff800 (nextafter(0x1p64, 0)).
> + *
> + * Note that we cannot simply scale by UINT64_MAX, because that
> + * value is not representable as an IEEE double value.
> + *
> + * If we scale by the next largest value, nextafter(0x1p64, 0),
> + * then the remainder of the possible values are scattered between
> + * 0 and 0xfffffffffffff000. Which leaves us with a gap between
> + * the final two inputs that is twice as large as any other.
> + */
> if (rate == 1.0) {
> *threshold = UINT64_MAX;
> } else {
> - *threshold = (rate * 0xffff000000000000ull)
> - + (rate * 0x0000ffffffffffffull);
> + *threshold = rate * 0x1p64;
> }
> }
>
>
Reviewed-by: Philippe Mathieu-Daudé <phi...@redhat.com>
