register unions

[Barnacle Wes]

In article <41964 at sun.uucp>, guy at gorodish.Sun.COM (Guy Harris) writes:
> > The official AT&T version of the K&R portable C compiler only
> > supports register allocation of int's.  All other types are
> > silently assigned to core.
> 
> I presume you mean "integral types", not "int"s.  The version of PCC2 that
> comes with S5R3.1 on 3B2s, for instance, quite cheerfully puts "short"s,
> "char"s, and "long"s into registers as well.

A lot of this type of problem is caused by the underlying machine
architecture.  On the M68000, you usually have 5 or 6 address
registers available, so most 68K C compilers let you put pointers in
registers as well as integral types.  The one I use the most (MWC for
Atari ST) does not allow floats of any flavor to be put in registers.

On Intel chips, a pointer in a register isn't really defined; there
are no real general-purpose registers, and there aren't any registers
big enough to hold a full address.  I guess you could probably put an
address is ES:DI or something like that, but I don't know of any
compilers that do.  MicroPort's pcc surely doesn't!

----
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