2011/1/25 Andreas Raab email@example.com:
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).
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