Re: A patch for libm-ieee754

[Geoffrey KEATING <geoffk@discus.anu.edu.au>]

> From: hjl@lucon.org (H.J. Lu)
> Date: Sat, 21 Feb 1998 18:55:18 -0800 (PST)

> It turns out those libm-ieee754 bugs are not in egcs. Here is the patch
> for glibc 2.1 to fix a few libm-ieee754 bugs. Ulrich, could you please
> take a look?
> 
> When you use fpu/cpu to do rounding, you have to mark the variable
> volatile. Otherwise, the compiler may do some thing you don't want.

libm-ieee754 expects ieee754 behaviour, and in particular that when a
value is stored to a 'double' variable or returned from a function of
type 'double', then that value is representable as a 'double' (and
similarly for 'float').  In fact, some functions basically do nothing
but invoke the FPU's rounder (like s_rint.c).

To get this behaviour with gcc, you may need to use the -ffloat-store
flag:


For most programs, the excess precision does only good, but a few
programs rely on the precise definition of IEEE floating point.  Use
`-ffloat-store' for such programs.


Do _not_ mark the variables 'volatile'.  This causes a performance
reduction on sparc and powerpc, and probably on alpha and x86 too,
because of the memory traffic it generates.

--
Geoff Keating <Geoff.Keating@anu.edu.au>