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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>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 20 May 2019 10:43:18 +0100
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, "andrii_anisov@xxxxxxxx" <andrii_anisov@xxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
- Delivery-date: Mon, 20 May 2019 09:43:31 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 20/05/2019 10:37, Viktor Mitin wrote:
> On Mon, May 20, 2019 at 12:22 PM Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx> wrote:
>> 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?
>>
> Yes, I've tried it with the next draft test code.
> The result looks ok, however it takes time to check all 64bits values
> one by one...
> Do you see any issues with new one-liner?
Jan already identified the issue in the other thread, and my suggestion
here was to inspect the generated assembly, which you'll observe is
substantially more complicated.
However, this thread isn't being productive. You're clearly wanting to
change this expression, and the maintainers see no reason to.
So, what problem are you actually trying to solve?
~Andrew
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