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tests/qht-bench: Adjust threshold computation

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In 06c4cc3660, 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.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200626200950.1015121-3-richard.henderson@linaro.org>
This commit is contained in:
Richard Henderson 2020-06-26 13:09:50 -07:00 committed by Alex Bennée
parent d11f824991
commit 78441c04ca
1 changed files with 16 additions and 2 deletions
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18
tests/qht-bench.c
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@ -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;
}
}