From: Richard Henderson <richard.hender...@linaro.org> 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> Reviewed-by: Philippe Mathieu-Daudé <phi...@redhat.com> Signed-off-by: Alex Bennée <alex.ben...@linaro.org> Message-Id: <20200626200950.1015121-3-richard.hender...@linaro.org> diff --git a/tests/qht-bench.c b/tests/qht-bench.c index ad885d89d054..362f03cb0370 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; } } -- 2.20.1