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: [email protected]
ReportedBy: [email protected]
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.
