Re: [Qemu-devel] [PATCH v3 2/4] target-tilegx: Add single floating point implementation

[Chen Gang <chengang@emindsoft.com.cn>]

After tried, I guess, this way below is incorrect: float64_to_float32()
assumes the input 'd' is already a standard (packed) float64 variable.
But in fact, it is not (e.g. the input from floatsisf2).

And we have to still check TILEGX_F_CALC_CVT, for they are really two
different format: TILEGX_F_CALC_CVT has no HBIT, but TILEGX_F_CALC_NCVT
has HBIT (which we need process it specially).

For me, the way like helper_fdouble_pack2 (the double implementation) is
OK to TILEGX_F_CALC_NCVT format, too.

 - Shift left to get HBIT, and change the related vexp (use vexp instead
   of exp to process overflow cases -- like double implementation does).

 - Use (u)int32_to_float32 for the mantissa.

 - Then process exp again.


Thanks.

On 12/11/15 06:14, Chen Gang wrote:
>> In particular, if gcc decided to optimize fractional fixed-point types, it
>> > would do something very similar to the current floatsisf2 code sequence, except
>> > that it wouldn't use 0x9e as the exponent; it would use something smaller, so
>> > that some number of low bits of the mantessa would be below the radix point.
>> > 
> Oh, really.
> 
>> > Therefore, I think that fsingle_pack2 should do the following: Take the
>> > (sign,exp,man) tuple and slot them into a double -- recall that a single only
>> > has 23 bits in its mantessa, and this temp format has 32 -- then convert the
>> > double to a single.  Pre-rounded single results from fsingle_* will be
>> > unchanged, while integer data that gcc has constructed will be properly rounded.
>> > 
>> > E.g.
>> > 
>> >   uint32_t sign = get_fsingle_sign(sfmt);
>> >   uint32_t exp = get_fsingle_exp(sfmt);
>> >   uint32_t man = get_fsingle_man(sfmt);
>> >   uint64_t d;
>> > 
>> >   /* Adjust the exponent for double precision, preserving Inf/NaN.  */
>> >   if (exp == 0xff) {
>> >     exp = 0x7ff;
>> >   } else {
>> >     exp += 1023 - 127;
>> >   }
>> > 
>> >   d = (uint64_t)sign << 63;
>> >   d = deposit64(d, 53, 11, exp);
>> >   d = deposit64(d, 21, 32, man);
>> >   return float64_to_float32(d, fp_status);
>> > 
>> > Note that this does require float32_to_sfmt to store the mantissa
>> > left-justified. That is, not in bits [54-32] as you're doing now, but in bits
>> > [63-41].
>> > 
> For me, it is a good idea! :-)
> 
> 

-- 
Chen Gang (陈刚)

Open, share, and attitude like air, water, and life which God blessed