Re: Effects of newly introduced -mpcX 80387 precision flag

[Uros Bizjak <ubizjak at gmail dot com>]

Bradley Lucier wrote:

> > > > What about "significant loss of accuracy" as these options probably 
> > > > won't cause a nuclear reactor meltdown ;)
> > >
> > > Well, I did some googling, and the technical term I was thinking of 
> > > was "catastrophic cancellation".  So how about
> > >
> > > Note that some mathematical routines in such libraries could suffer 
> > > significant loss of accuracy, typically through so-called 
> > > "catastrophic cancellation", when this option is used to set the 
> > > precision to less than extended precision.
> >
> > I think this is OK. So if nobody objects, this patch is OK for mainline.
>
> I'm sorry, but I don't have checkin privileges.

I have committed following patch attached patch in your name:

2007-05-04  Bradley Lucier  <lucier@math.purdue.edu>

       * doc/invoke.texi (i386 and x86-64 Options) [-mpc32, -mpc64, 
-mpc80]:
       Add the note about a significant loss of accuracy of some
       mathematical routines when these options are used.

Patch was tested by building the documentation.

Uros.


Index: doc/invoke.texi
===================================================================
--- doc/invoke.texi	(revision 124412)
+++ doc/invoke.texi	(working copy)
@@ -10129,13 +10129,22 @@
 @opindex mpc80
 
 Set 80387 floating-point precision to 32, 64 or 80 bits.  When @option{-mpc32}
-is specified, the significand of floating-point operations is rounded to 24
-bits (single precision), @option{-mpc64} rounds the significand of
-floating-point operations to 53 bits (double precision) and @option{-mpc80}
-rounds the significand of floating-point operations to 64 bits (extended
-double precision).  Note that a change of default precision control may
-affect the results returned by some of the mathematical functions.
+is specified, the significands of results of floating-point operations are
+rounded to 24 bits (single precision); @option{-mpc64} rounds the the
+significands of results of floating-point operations to 53 bits (double
+precision) and @option{-mpc80} rounds the significands of results of
+floating-point operations to 64 bits (extended double precision), which is
+the default.  When this option is used, floating-point operations in higher
+precisions are not available to the programmer without setting the FPU
+control word explicitly.
 
+Setting the rounding of floating-point operations to less than the default
+80 bits can speed some programs by 2% or more.  Note that some mathematical
+libraries assume that extended precision (80 bit) floating-point operations
+are enabled by default; routines in such libraries could suffer significant
+loss of accuracy, typically through so-called "catastrophic cancellation",
+when this option is used to set the precision to less than extended precision. 
+
 @item -mstackrealign
 @opindex mstackrealign
 Realign the stack at entry.  On the Intel x86, the