[Vm-dev] Portability problems: compiler flags

[Aleksej Saushev]

Igor Stasenko <siguctua at gmail.com> writes:

> On 25 January 2011 14:30, David T. Lewis <lewis at mail.msen.com> wrote:
>>
>> On Tue, Jan 25, 2011 at 12:05:32PM +0100, Andreas Raab wrote:
>>>
>>> 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.
>>>
>>> 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.
>>
>> In terms of requirements, is it true that bit-identical behavior
>> for FloatMathPlugin is a requirement if and only if CroquetPlugin
>> is present at runtime?
>
> well, i think it is good to have for everything , not just for Croquet

I don't like this attitude.

Is it possible to provide a switch to use better libm than fdlibm?


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