| Message ID | 20200617201309.1640952-7-richard.henderson@linaro.org |
|---|---|
| State | New |
| Headers | show |
| Series | Vs clang-10 and gcc-9 warnings | expand |
On 6/17/20 1:13 PM, Richard Henderson wrote: > - *threshold = rate * UINT64_MAX; > + *threshold = (rate * 0xffff000000000000ull) > + + (rate * 0x0000ffffffffffffull); Well, while this does silence the warning, it produces the exact same results as before, since the intermediate double result still only has 53 bits of precision. We're probably be better off with the nextafter(0x1p64, 0.0) form that we've used elsewhere. r~
On Wed, 17 Jun 2020 at 21:59, Richard Henderson <richard.henderson@linaro.org> wrote: > > On 6/17/20 1:13 PM, Richard Henderson wrote: > > - *threshold = rate * UINT64_MAX; > > + *threshold = (rate * 0xffff000000000000ull) > > + + (rate * 0x0000ffffffffffffull); > > Well, while this does silence the warning, it produces the exact same results > as before, since the intermediate double result still only has 53 bits of > precision. > > We're probably be better off with the nextafter(0x1p64, 0.0) form that we've > used elsewhere. Yeah, the code does look a bit odd. I'm going to apply this one to master as well just to suppress all the patchew nagmails, but we should figure out the correct thing to do here. thanks -- PMM
diff --git a/tests/qht-bench.c b/tests/qht-bench.c index e3b512f26f..eb88a90137 100644 --- a/tests/qht-bench.c +++ b/tests/qht-bench.c @@ -284,7 +284,8 @@ static void do_threshold(double rate, uint64_t *threshold) if (rate == 1.0) { *threshold = UINT64_MAX; } else { - *threshold = rate * UINT64_MAX; + *threshold = (rate * 0xffff000000000000ull) + + (rate * 0x0000ffffffffffffull); } }
tests/qht-bench.c:287:29: error: implicit conversion from 'unsigned long' to 'double' changes value from 18446744073709551615 to 18446744073709551616 [-Werror,-Wimplicit-int-float-conversion] *threshold = rate * UINT64_MAX; ~ ^~~~~~~~~~ Fix this by splitting the 64-bit constant into two halves, each of which is individually perfectly representable, the sum of which produces the correct arithmetic result. Cc: Emilio G. Cota <cota@braap.org> Reported-by: Philippe Mathieu-Daudé <philmd@redhat.com> Signed-off-by: Richard Henderson <richard.henderson@linaro.org> --- Question: Should we really be scaling by UINT64_MAX? The comparisons against info->r mean that 1.0 is exceedingly unlikely to hit. Or if that is supposed to be the point, why is is the test r >= threshold not r > threshold where, if threshold == UINT64_MAX, there is zero chance of the test being true for 1.0. --- tests/qht-bench.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)