Re: [Qemu-devel] [PATCH] block/qcow2.h: Avoid "1LL << 63" (shifts into sign bit)

[Eric Blake <eblake@redhat.com>]

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On 08/23/2013 10:35 AM, Peter Maydell wrote:
> The expression "1LL << 63" tries to shift the 1 into the sign bit of a
> 'long long', which provokes a clang sanitizer warning:
> 
> runtime error: left shift of 1 by 63 places cannot be represented in type 'long long'

Yep, C99 6.5.7p3 states it is undefined to shift a signed number left
across the sign bit:

"The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated
bits are filled with zeros. If E1 has an unsigned type, the value of the
result is E1 × 2E2 , reduced modulo one more than the maximum value
representable in the result type. If E1 has a signed type and
nonnegative value, and E1 × 2E2 is representable in the result type,
then that is the resulting value; otherwise, the behavior is undefined."

Qemu assumes twos-complement arithmetic with sane signed left shifts,
but without a way to tell the compiler our assumptions, it's easier to
just stick with well-defined unsigned shifts.

> 
> Use "1ULL << 63" as the definition of QCOW_OFLAG_COPIED instead
> to avoid this. For consistency, we also update the other QCOW_OFLAG
> definitions to use the ULL suffix rather than LL, though only the
> shift by 63 is undefined behaviour.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>  block/qcow2.h |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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