round4 performance

[Igor Schein <igor@txc.com>]

Hi,

here's a polynomial on which round2 heavily outperforms round4:

x^64 + 528*x^60 + 422640*x^56 + 154189440*x^52 + 46085461920*x^48 + 86643136
30464*x^44 + 1067417121457152*x^40 + 76273480007101440*x^36 + 29307987576360
23040*x^32 + 31785192406564024320*x^28 + 729662629421496287232*x^24 - 866453
9928316858220544*x^20 + 114361270118934673858560*x^16 - 51757447983683584779
87840*x^12 + 47264303406943521096007680*x^8 - 774599638688652156020195328*x^
4 + 7585622913396815983510880256

nfdisc(,2) finished in 4min with a reasonable stack size, while
nfdisc() ran out of 500MB stack after a longer period of time.

Thanks

Igor