http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56843
Bug #: 56843 Summary: PowerPC Newton-Raphson reciprocal estimates can be improved Classification: Unclassified Product: gcc Version: 4.9.0 Status: UNCONFIRMED Keywords: missed-optimization Severity: normal Priority: P3 Component: target AssignedTo: wschm...@gcc.gnu.org ReportedBy: wschm...@gcc.gnu.org Host: powerpc64-unknown-linux-gnu Target: powerpc64-unknown-linux-gnu Build: powerpc64-unknown-linux-gnu It was recently brought to my attention that the number of Newton-Raphson iterations for floating reciprocal-estimate and floating recriprocal-sqrt-estimate can be tightened. In particular, for 32-bit floating-point values targeting processors having higher precision estimates, a single iteration should suffice to produce maximum representable precision. We currently perform two. We should verify that one is actually sufficient in practice. We should also investigate whether 3 iterations is sufficient for 64-bit floating-point values when targeting processors having lower precision estimates. The theoretical math suggests 4 may be necessary, but this could be too conservative in practice as this is derived from a general bound on the method.