Re: [cc65] Floating point support (was: C++ compiler)

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From: Ullrich von Bassewitz (
Date: 2003-03-07 00:20:12

On Thu, Mar 06, 2003 at 02:58:28PM -0800, Shawn Jefferson wrote:
> Wouldn't/Couldn't it be possible to implement some standard routines for
> floating point math that the compiler knows about, but could either be
> implemented with machine specific calls (the Atari has floating point routines
> in ROM and I guess from what you guys have been saying the C64 does too.)

That would be...

  * ...ineffective. For example checking the sign of a fp variable is quite
    easy, so generating a call to a subroutine is overkill. However, the
    compiler needs to know the format used.

  * ...difficult and messy. How should the compiler generate floating point
    data if the format is not known? One solution would be to implement
    several fp formats in the compiler and switch them with the -t option,
    but this means that each target platform needs changes in the compiler.

  * ...standard violating. The standard requires that the compiler uses the
    same rules as the target platform when calculating floating point results.
    The result of

        double a = 1.0 / 3.0;   /* Evaluated at compile time */

    must be the same as

        double a = 1.0;
        double b = 3.0;
        double c = a / b;       /* Evaluated at runtime */

    So either the compiler would reimplement all ROM routines for all existing
    platforms or it would violate the standard.

> That
> would cut down on the amount of code required to support floating point, but
> those targets without ROM routines would have to have their own code.

As I see it, it would effectively require a lot more code, both in the
compiler and in the library. There are machines without floating point, so
floating point library subroutines are needed anyway. In addition, platforms
with fp support in ROM need wrappers. And, much more code in the compiler is
needed (see above).

More problems:

  * Binary data cannot be exchanged between the platforms if it contains

  * On several platforms, the ROMs containing the fp routines are banked out
    (examples are the C64 and the Plus/4). A lot of additional effort is
    needed when calling fp routines in these systems.
So you're writing much more code for this solution, and it is not even a
better one.



Ullrich von Bassewitz                        
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