http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53134
Bug #: 53134
Summary: Request for option to disable excess precision on i387
Classification: Unclassified
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: bug...@aerifal.cx
Would it be possible to add to gcc an option to fully disable excess floating
point precision on i387? Basically it would work like this: any function
performing floating point arithmetic would be responsible for reading the
floating point control word, changing the precision to the right value for the
nominal type of the expression being evaluated, and restoring it afterwards
(before returning or making calls to any other function, to maintain
compatibility with code not compiled with this option).
Since even with the precision mode set correctly the exponent range of 387 fpu
registers still matches that of 80-bit long double, this mode would also imply
a store/load cycle after every single floating point operation, analogous to
-ffloat-store but on each step of calculation, not only on assignments. I
realize this would incur significant performance penalty, but for users trying
to obtain exact, reproducible, floating point results correctly rounded for the
type being used, it would be very useful. If the load/store cycle (stronger
version of -ffloat-store) were a separate option, the performance impact would
actually be extremely minimal when that part was turned off and only the
precision mode setting was turned on.
