[Vm-dev] Portability problems: compiler flags

[Nicolas Cellier]

2011/1/25 Andreas Raab <andreas.raab at gmx.de>:
>
> On 1/25/2011 11:38 AM, Igor Stasenko wrote:
>>>
>>> Squeak uses highly unportable flag "-mno-fused-madd".
>>> It isn't present in many systems, even those using GCC,
>>> since many systems use older compilers.
>>>
>>> Also, why do you build fdlibm? It is old stuff that works not so well,
>>> we have better libm. Is there a way to use our libm?
>>>
>> I asked  same question few weeks ago.
>> Check mailing list archive for discussion.
>> In short: differrent libm implementations work differently and some
>> have bad support of IEEE standard.
>
> Actually, that's not quite the point. The issue is that Croquet requires
> bit-identical computations including floating point. For FPU computations,
> the use of -mno-fused-madd avoids the use of the fused multiply-add
> operation by compilers which support it which would generate different
> results from compilers not using fused madd.
>
> The usage of fdlibm is similar. As Nicolas has pointed out elsewhere, fdlibm
> is in some cases actually inferior of the platform libms (one might say
> outright broken) but the requirement for the usage in Croquet isn't really
> whether it's "correct" or "good". The requirement is bit-identical results
> across all platforms. The results can be wrong as long as they are
> consistently wrong. But they mustn't be different.
>

Yes, I was disappointed by exp(1), but fdlibm sin(pi+epsilon) is far
far superior to i86 hardwired answer (i86 approximation of pi is well
known to be poor).

Nicolas

> Having said that, for your regular Squeak VM (i.e., not requiring
> bit-identical floating point results) there really isn't a requirement to
> use either -mno-fused-madd or fdlibm.
>
> Cheers,
>  - Andreas
>