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Re: [Xen-devel] libxc: Casting of xen virtual address type xen_vaddr_t to signed int64 type: (int64_t)vaddr
- To: Viktor Mitin <viktor.mitin.19@xxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 20 May 2019 10:21:55 +0100
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, "andrii_anisov@xxxxxxxx" <andrii_anisov@xxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
- Delivery-date: Mon, 20 May 2019 09:22:06 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
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))
Have you tried compiling these two examples and seeing how they differ?
~Andrew
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