Re: [Qemu-devel] [ARM - FCVT inst] : Difference in calculated value

[Gaurav Sharma]

Can some one confirm is this is an issue with qemu implementation ?

Thanks,

Gaurav

On Thu, Aug 14, 2014 at 1:07 PM, Gaurav Sharma <address@hidden> wrote:

I am trying to execute a 'fcvt' instruction for conversion from single precision to half precision value.

with the following values :

[Instrn]

fcvt h16 s25

[register values]

SIMD register [25] = 0x9EA82A22AB98FBA8L

FPCR = 0x40ae2f54 [with according mask removing the UFE and other unnecessary bit]

As per the softfloat algorithm, if the exp is < -10, we return a signed / unsigned zero. In this case 0x8000 is the value set in h16.

However,  when trying to match the same with arm implementation, the value for h16 comes out as 0x8001.

While trying to map the softfloat implementation with arm manual, depending upon rounding mode  it seems we can get both 0x8000 and 0x8001.

I am still in the process of checking the algorithm for conversion in softfloat, but in the arm manual, i do not see we returning a zero.

Thanks,

Gaurav