Re: [Qemu-devel] [PATCH] target-sparc: fix 32-bit truncation in fpackfix

[Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>]

On 04/11/15 11:05, Richard Henderson wrote:

> On 11/04/2015 11:45 AM, Paolo Bonzini wrote:
>>>>>>>>            int32_t src = rs2 >> (word * 32);
>>>>>>>> -        int64_t scaled = src << scale;
>>>>>>>> +        int64_t scaled = (int64_t)src << scale;
>>>>>>>>            int64_t from_fixed = scaled >> 16;
> ...
>>>
>>> I do think we'd be better served by casting to uint64_t on that line.
>>> Note that fpackfix requires the same correction.  And it wouldn't hurt
>>> to cast to uint32_t in fpack16, lest we anger the self-same shifting
>>> gods.
>>
>> Hmmm.. say src = -0x80000000, scale = 1;
>>
>> scaled     = (uint64_t)-0x8000000 << 1 = 0xffffffff00000000
>> from_fixed = 0xffffffff00000000 >> 16  = 0x0000ffffffff0000
>>
>> Now from_fixed is positive and you get 32767 instead of -32768.  In
>> other words, we would have to cast to uint64_t on the scaled assignment,
>> and back to int64_t on the from_fixed assignment.  I must be
>> misunderstanding your suggestion.
> 
>   int64_t scaled = (uint64_t)src << scale;
> 
> I.e. one explicit conversion and one implicit conversion.

I suspect Richard knows more about this part of SPARC emulation than I
do, so I'd be fine with a solution similar to the above if everyone
agress. Let me know if you need me to send a SPARC pull request,
although it will probably be quicker coming from Paolo/Richard at the
moment.


ATB,

Mark.