HP Fortran Programmer's Guide (B3908-90031; September 2011)

Compiling and linking
Compiling with the f90 command
Chapter 2 67
The default is +Onofenvaccess.
+O[no]fastaccess
+Ofastaccess improves execution time by speeding up access to global data items.
You can use this option at optimization level 1 or higher..
Note that the +Ofastaccess option may increase link time.
The default is +Onofastaccess at optimization levels 1, 2, and 3; and
+Ofastaccess at optimization level 4.
NOTE This option is valid only on the PA-RISC systems.
+O[no]fltacc
+Onofltacc enables optimizations that follow the rules of algebra but may change the
order of expression evaluation. For example, if a, b, and c are floating-point variables,
the expressions (a + b) + c and a + (b + c) may give slightly different results
due to roundoff.
The +Onofltacc option also enables the fusion of adjacent multiply and add
operations—resulting in Fused Multiply-Add (FMA). (At optimization level 2 or
higher, FMA occurs by default.) FMA improves performance but occasionally
produces results that may differ in accuracy from results produced by code where fusion
has not occurred. In general, the differences are slight.
+Ofltacc disables optimizations that change the order of expression evaluation and
therefore may affect the accuracy of the result. The +Ofltacc option also disables
fusing.
NOTE +Ofltacc is the same as +Ofltacc=strict. +Onofltacc is
the same as +Ofltacc=relaxed.
Table 2-9 identifies the different actions taken by the optimizer, according to whether
you specify +Ofltacc, +Onofltacc, or neither option. In all cases, the table assumes
that you are compiling at optimization level 2 (+O2) or higher.
Table 2-9 Optimizations performed by +O[no]fltacc
+O[no]fltacc Expression reordering? FMA?
No Yes
+Ofltacc No No