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Re: [Xen-devel] libxc: Casting of xen virtual address type xen_vaddr_t to signed int64 type: (int64_t)vaddr



Hi Viktor,

Is there any specific reason I ended up to be CCed in a middle of an x86 thread?

Cheers,

On 20/05/2019 10:17, Viktor Mitin wrote:
Mean that result of "(int64_t)vaddr >> 63" can be 0 or 1.
So the next code may not work properly in case of another 'implementations'.
With another compiler (i.e. clang, etc) this code may introduce bugs
which are hard to find.

((int64_t)vaddr >> 47) == ((int64_t)vaddr >> 63)

For this reason it is better to avoid implementation-defined code.

Well, ideally we'd like to get away without using implementation
defined behavior. But I'm afraid we're quite far from that, and
we'd not always be willing to accept the worse source and/or
binary code that would be needed to avoid it.

How about using the next one-liner to avoid implementation-defined code :

//Returns true in case when the top left 17 bits are all zero or are all one
return ((!(vaddr >> 47)) || (((vaddr >> 47)&0x1FFFF) == 0x1FFFF))

Agree?

Thanks


--
Julien Grall

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