Re: [Qemu-devel] [PATCH 7/7] target-mips: Add IEEE 754-2008 features support

[Leon Alrae]

On 10/02/2015 14:30, Maciej W. Rozycki wrote:
> On Tue, 10 Feb 2015, Leon Alrae wrote:
> 
>>>  These cases could be addressed by either replacing subtraction from 0.0 
>>> with multiplication by -1.0, or by tweaking the rounding mode as needed 
>>> temporarily.  Given that the computational cost of multiplication is 
>>> uncertain and likely higher or at best the same as the cost of addition or 
>>> subtraction, I'd be leaning towards the latter solution.
>>
>> My first thought was to treat zero in NEG.fmt as a special case and use
>> float32_chs() for it. But tweaking the rounding mode temporarily
>> probably is better as we will get consistent behaviour for zero as well
>> as input denormals which are squashed in float32_sub() when
>> flush_inputs_to_zero flag is set (actually I'm not sure if legacy fp
>> instructions should flush input denormals, but according to the spec
>> this is implementation dependent so I won't worry about this).
> 
>  As expected setting CP1.FCSR.FS on a randomly picked R4400 processor:
> 
> CPU0 revision is: 00000440 (R4400SC)
> FPU revision is: 00000500
> 
> does flush a NEG.fmt's input denormal to 0.  Given this program:

Good to know, thanks for checking that on the real CPU!

Leon