While looking through LAPACK I notices a few patterns. For example, in drotg,
"r = dsign(1.0d0,x)*y". This is really "r = y ^ (x & sign-bit)". Compare this
with "r = (1.0 | (x & sign-bit)) * y" when expanding copysign on most systems.
Note that this can get kinda complex. See slasv2.f,
IF( PMAX.EQ.1 )
$ TSIGN = SIGN( ONE, CSR )*SIGN( ONE, CSL )*SIGN( ONE, F )
IF( PMAX.EQ.2 )
$ TSIGN = SIGN( ONE, SNR )*SIGN( ONE, CSL )*SIGN( ONE, G )
IF( PMAX.EQ.3 )
$ TSIGN = SIGN( ONE, SNR )*SIGN( ONE, SNL )*SIGN( ONE, H )
SSMAX = SIGN( SSMAX, TSIGN )
SSMIN = SIGN( SSMIN, TSIGN*SIGN( ONE, F )*SIGN( ONE, H ) )
Note that this is, depending on how you count, either 3 or 11 sequential
copysign-of-one-and-mult operations.
--
Summary: Recognize common Fortran usages of copysign.
Product: gcc
Version: 4.0.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P2
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: rth at gcc dot gnu dot org
CC: gcc-bugs at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19706
