Re: [PATCH v3 2/2] util/cutils: Fix incorrect integer->float conversion caught by clang

[Richard Henderson <richard.henderson@linaro.org>]

On 11/22/19 9:00 AM, Markus Armbruster wrote:
> From: Fangrui Song <i@maskray.me>
> 
> Clang does not like do_strtosz()'s code to guard against overflow:
> 
>     qemu/util/cutils.c:245:23: error: implicit conversion from 'unsigned long' to 'double' changes value from 18446744073709550592 to 18446744073709551616 [-Werror,-Wimplicit-int-float-conversion]
> 
> The warning will be enabled by default in clang 10. It is not
> available for clang <= 9.
> 
> val * mul >= 0xfffffffffffffc00 is indeed wrong.  0xfffffffffffffc00
> is not representable exactly as double.  It's half-way between the
> representable values 0xfffffffffffff800 and 0x10000000000000000.
> Which one we get is implementation-defined.  Bad.
> 
> We want val * mul > (the largest uint64_t exactly representable as
> double).  That's 0xfffffffffffff800.  Write it as nextafter(0x1p64, 0)
> with a suitable comment.
> 
> Signed-off-by: Fangrui Song <i@maskray.me>
> Reviewed-by: Markus Armbruster <armbru@redhat.com>
> [Patch split, commit message improved]
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>  util/cutils.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~