Re: C54x port: some general technical questions

[Paul Schlie <schlie at comcast dot net>]

> James E Wilson
>> Jonathan Bastien-Filiatrault wrote:
>> * We have defined BIT_PER_WORD to 16 and UNITS_PER_WORD to 1. On this
>> DSP, there are two 40-bits accumulators. How do we make GCC take
>> advantage of this and which machine mode do we use ?
>
> GCC has little support for non-power-of-2 sized accumulators. Traditionally
> this would be done by using PSImode (because your target has a 64-bit SImode),
> which you can enable by using the undocumented PARTIAL_INT_MODE(SI) macro in
> your target modes.def file. See other targets for existing examples of this.
>
> Also, gcc has little support for targets with BITS_PER_UNIT != 8. I think the
> (ti)c4x is the only one currently supported, and it is being obsoleted due to
> lack of maintenance.

A basic thought (as no GCC language presently directly supports the full
semantics offered by the c54's ISA and various modes, including notion of
"guard bits" extending it's otherwise natural 32-bit wide accumulators);
it may be simplest to treat the accumulator as a 32 bit word composed of
a 16-bit hi and lo halves, ignoring the guard-bits, thereby more simply
modeling the c54 as a basic 16-bit machine with 16-bit char's and ints, and
32-bit long and long-longs?

Leaving the predominant remaining problem of identifying which mode maps to
which corresponding type. Personally I believe it would be most ideal if GCC
didn't presume that QImode size == BITS_PER_UNIT, but rather introduced the
definition that QImode size == BITS_PER_BYTE, where then BITS_PER_UNIT may
then more flexibly define the minimally sized addressable datum relative to
it, thereby enabling for example: (although requiring some GCC refinement)

#define BITS_PER_BYTE 8 /* or whatever */

#define QI_MODE_SIZE  1 /* BYTES, not units */
#define BITS_PER_QI_MODE (1 * BITS_PER_BYTE)
#define HI_MODE_SIZE  2 /* BYTES, not units */
#define BITS_PER_HI_MODE (2 * BITS_PER_BYTE)
#define SI_MODE_SIZE  4 /* BYTES, not units */
#define BITS_PER_SI_MODE (4 * BITS_PER_BYTE)
...

#define UNIT_SIZE HI_MODE_SIZE /* minimum naturally addressable mode size */
#define BITS_PER_UNIT (UNIT_SIZE * BITS_PER_BYTE)

#define WORD_SIZE HI_MODE_SIZE /* maximum naturally addressable mode size */
#define BITS_PER_WORD (WORD_SIZE * BITS_PER_BYTE)

#define CHAR_SIZE HI_MODE_SIZE
#define BITS_PER_CHAR (CHAR_SIZE * BITS_PER_BYTE)
#define INT_SIZE HI_MODE_SIZE
#define BITS_PER_INT (INT_SIZE * BITS_PER_BYTE)
#define LONG_SIZE SI_MODE_SIZE
#define BITS_PER_LONG (LONG_SIZE * BITS_PER_BYTE)
#define LONG_LONG_SIZE SI_MODE_SIZE
#define BITS_PER_LONG_LONG (LONG_LONG_SIZE * BITS_PER_BYTE)
...