[patch][0/5] powerpc: Add support to fully comply with IEEE-754standard

[Kumar Gala]

On Jan 12, 2007, at 12:38 AM, Zhu Ebony-r57400 wrote:

> Hi Paul,
>
> On SPE implemented E500/E500v2 core, the embedded floating-point
> APU implements a floating-point system as defined in ANSI/IEEE
> standard754-1985 but rely on software support in order to conform  
> fully
> with the standard. Thus, whenever an input operand of a floating-point
> instruction has data values that are +infinity, –infinity, denorm,  
> or NaN,
> or when the result of an operation produces an overflow or an  
> underflow,
> an interrupt may be taken and the interrupt handler is responsible  
> for delivering
> IEEE 754-compliant behavior if desired.

In addition to some other corner cases the HW punts on.

[snip]

> The approach I've taken to solve this issue is:
> 1. Enable SPEFSCR[FINVE|FDBZE|FUNFE|FOVFE] to make sure exceptions
> can take place
> 2. Use exceptions handlers to handle the exceptions.
> 3. Restore registers and exit from exception.
>
> In arch/powerpc/math, there are some files to emulate floating  
> point instructions
> on non-FPU systems, which may come from glibc. Some macros are  
> provided to
> emulate plus, minus, multiply, divide, etc. Therefore, I re-used  
> some of the codes there
> and add some new routines to emulated SPE instruction that may  
> cause exception,
> including SPFP instructions, DPFP instructions and Vector SPFP  
> instructions.
>
> Writing some independent codes to handle the exceptions my be an  
> alternative way,
> but I think re-use the existing interfaces in kernel is the best  
> approach.

I don't believe there is any other way to solve this problem.  On  
these particular exceptions, the HW doesn't provide any real assist  
and we have to recompute the result from scratch.

Once, we agree the approach is reasonable I'll make comments on the  
actual handlers.

- k

> Ebony
>
>
>
>> -----Original Message-----
>> From: Paul Mackerras [mailto:paulus at samba.org]
>> Sent: 2007年1月12日 13:30
>> To: Zhu Ebony-r57400
>> Cc: linuxppc-dev at ozlabs.org
>> Subject: Re: [patch][0/5] powerpc: Add support to fully
>> comply with IEEE-754standard
>>
>> Zhu Ebony-r57400 writes:
>>
>>> This series of patch add support to fully comply with IEEE-754
>>> standard for E500/E500v2 core when hardware floating point
>> compiling is used.
>>
>> Your patch descriptions need to explain in detail in what way
>> the current code doesn't comply with the IEEE-754 standard,
>> and what approach you have taken to make it comply.  If there
>> are alternative approaches, explain why the approach you have
>> taken is the best.
>>
>> Thanks,
>> Paul.
>>