On May 3, 2007, at 11:11 AM, Uros Bizjak wrote:
Could you please post a patch with suggested wording about this
option (I was trying to write something similar to the warning that
icc has in its documentation about precision settings).
How about this? It perhaps reflects my own biases, but the term
"catastrophic loss of accuracy" is sometimes used in the technical
sense that I mean here. For the performance figures, I used the
figures you gave in your e-mail but add "or more" to be on the safe
side.
Brad
[descartes:~/Desktop] lucier% rcsdiff -u invoke.texi
===================================================================
RCS file: RCS/invoke.texi,v
retrieving revision 1.1
diff -u -r1.1 invoke.texi
--- invoke.texi 2007/05/03 15:43:01 1.1
+++ invoke.texi 2007/05/03 17:15:24
@@ -10139,12 +10139,21 @@
@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
catastrophic
+loss of accuracy when this option is used to set the precision to
less than
+extended precision.
@item -mstackrealign
@opindex mstackrealign
