Re: RFC: changing precision control setting in initial FPU context

["Albert D. Cahalan" <acahalan@cs.uml.edu>]

Kevin Buhr writes:

> It boils down to the fact that, under i386 Linux, the FPU control word
> has its precision control (PC) set to 3 (for 80-bit extended
> precision) while under i386 FreeBSD, NetBSD, and others, it's set to 2
> (for 64-bit double precision).  On other architectures, I assume
> there's usually no mismatch between the C "double" precision and the
> FPU's default internal precision.
...
> Initially, I was quick to dismiss the whole thing as symptomatic of a
> severe floating-point-related cluon shortage.  However, the more I
> think about it, the better the case seems for changing the Linux
> default:
> 
> 1.  First, PC=3 is a dangerous setting.  A floating point program
>     using "double"s, compiled with GCC without attention to
>     FPU-related compilation options, won't do IEEE arithmetic running
>     under this setting.  Instead, it will use a mixture of 80-bit and
>     64-bit IEEE arithmetic depending rather unpredictably on compiler
>     register allocations and optimizations.
> 
> 2.  Second, PC=3 is a mostly *useless* setting for GCC-compiled
>     programs.  There can obviously be no way to guarantee reliable
>     IEEE 80-bit arithmetic in GCC-compiled code when "double"s are
>     only 64 bits, so our only hope is to guarantee reliable IEEE
>     64-bit arithmetic.  But, then we should have set PC=2 in the first
>     place.

So you change it to 2... but what about the "float" type? It gets
a mixture of 64-bit and 32-bit IEEE arithmetic depending rather
unpredictably on compiler register allocations and optimizations???

If a "float" will have excess precision, then a "double" might
as well have it too. Usually it helps, but sometimes it hurts.
This is life with C on x86.

> So, on a related note, is it reasonable to consider resurrecting the
> "sys_setfpucw" idea at this point, to push the decision on the correct
> initial control word up to the C library level where it belongs?  (For
> those who don't remember the proposal, the idea is that the C library
> can use "sys_setfpucw" to set the desired initial control word.

Ugh, more start-up crud.