m68k machine flags

[Nathan Sidwell <nathan at codesourcery dot com>]

Benm
the m68k cpu selection and elf flags are somewhat of a mess. I'd like to clean 
it up in two ways.  AFAICT, the elf header flags are not being set by the m68k 
assembler, so bfd's arch info struct is always defaulting to the plain m68k one. 
 This means we can rationalize the m68k bfd_mach_ enumeration without casing 
binary incompatibilities (correct me if I'm wrong).

My motivation for this is that we'll be adding more ColdFire variants, and using 
a simple cpu enumeration is going to be awkward and unordered.  I don't mind 
leaving the 68k specific pieces as unchanged as possible -- fortunately they're 
pretty linear.

Coldfire has defined a family of architectures (v2, v3, v4, v4e), however they 
also have a set of ISAs, which don't match up with the above families.  IMHO, 
using the ISAs is the way to go to specify compatibility in object files.

The current capability bits specified in include/opcodes/m68k.h have
isa_a, isa_a+, isa_b, hwdiv, mac, emac, usp, float

Although hwdiv is separated from isa_a, I can't find a CF chip that has just isa 
without the divide instructions.  That seems an unnecessary separation.  (gas 
can assemble for just isa w/o hwdiv, but I don't see the need for that.)

If I'm right, and taking the current ARM implementation as a model to aim for, 
we should specify a set of ISAs for the bfd_mach enumeration, and then map 
specific CPUs onto that set in the assembler.  The above capabilities are not 
completely orthogonal, so we can specify some ordering for them.  The 
distinctions needed, I think are

isa_a  + hwdiv			(52xx)
isa_a  + hwdiv + mac		(5206)
isa_a  + hwdiv + emac		(5249)
isa_a+ + hwdiv			(no chip)
isa_a+ + hwdiv + mac		(5208)
isa_a+ + hwdiv + emac		(5329)
isa_a+ + hwdiv + usp		(no chip)
isa_a+ + hwdiv + usp + mac	(no chip)
isa_a+ + hwdiv + usp + emac	(521x)
isa_b  + hwdiv			(no chip)
isa_b  + hwdiv + mac		(5407)
isa_b  + hwdiv + emac		(no chip)
isa_b  + hwdiv + usp + float	(no chip)
isa_b  + hwdiv + usp + float + mac (no chip)
isa_b  + hwdiv + usp + float + emac (547x)

Even though some of these configurations have no chip, I suspect they are still 
useful categorizations, and it wouldn't surprise me if the gaps got filled at 
some point.

I'd like to add new gas options for -march=foo and -mcpu=foo, deprecating the 
existing option mechanism (i.e. leave that as it is, but not advance it for new 
chips).  I think separate switches for specifying mac/emac and for float support 
would also be good.  Thus we'd have
-march=[68kvariants]isaa,isaa+,isab
-musp  (with -mnousp?)
-mfloat (with -mnofloat?)
-mmac -mmac=mac,emac (with -mnomac?)
-mcpu=<cpuname>

Clearly some combinations of these would be invalid.  I'm not sure having a 
literal '+' in an option is a good idea, perhaps 'isaaplus'?

thoughts?

nathan

--
Nathan Sidwell    ::   http://www.codesourcery.com   ::         CodeSourcery
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk