Using and Porting GNU Fortran
14.4.10 Floating-point precision
If your program depends on exact IEEE 754 floating-point handling it may
help on some systems--specifically x86 or m68k hardware--to use
the `-ffloat-store' option or to reset the precision flag on the
floating-point unit.
See section 5.7 Options That Control Optimization.
However, it might be better simply to put the FPU into double precision
mode and not take the performance hit of `-ffloat-store'. On x86
and m68k GNU systems you can do this with a technique similar to that
for turning on floating-point exceptions
(see section 15.3.29 Floating-point Exception Handling).
The control word could be set to double precision by some code like this
one:
| | #include <fpu_control.h>
{
fpu_control_t cw = (_FPU_DEFAULT & ~_FPU_EXTENDED) | _FPU_DOUBLE;
_FPU_SETCW(cw);
}
|
(It is not clear whether this has any effect on the operation of the GNU
maths library, but we have no evidence of it causing trouble.)
Some targets (such as the Alpha) may need special options for full IEEE
conformance.
See section `Hardware Models and Configurations' in Using the GNU Compiler Collection (GCC).
