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.mayd...@linaro.org> > --- > block/qcow2.h | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) Reviewed-by: Eric Blake <ebl...@redhat.com> -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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