RE: Fixed-point branch?

["Fu, Chao-Ying" <fu at mips dot com>]

Bernd Schmidt wrote:

> >>> +ACCUM_MODE (HA, 2, 8, 7); /* s8.7 */
> >>> +ACCUM_MODE (SA, 4, 16, 15); /* s16.15 */
> >>> +ACCUM_MODE (DA, 8, 32, 31); /* s32.31 */
> >>> +ACCUM_MODE (TA, 16, 64, 63); /* s64.63 */
> >> Lots of predefined types and modes in this patch.  What 
> about targets
> >> with other requirements (the Blackfin has 40 bit (8 + 32) 
> >> accumulators)?
> > 
> >   In "bfin-modes.def", we can adjust the DA mode to (s7.32) by using
> > ADJUST_IBIT(DA, 7)
> > ADJUST_FBIT(DA, 32)
> > 
> >> For vectors, we let the targets define the supported 
> modes.  Why do we
> >> want something else for fractional support?
> > 
> >   I am not clear about this question.  The new modes 
> (FRACT, UFRACT, ACCUM,
> > and UACCUM) enables GCC to recognize the formats of the 
> underlying values
> > to perform constant folding (e.g., + - * /).
> >   To use the DA mode for vector, we can use:
> > VECTOR_MODE (ACCUM, DA, 2);
> 
> No, I was trying to make an analogy of how ports explicitly 
> define the 
> modes their hardware supports, e.g. for arm:
> 
> /* Vector modes.  */
> VECTOR_MODES (INT, 4);        /*            V4QI V2HI */
> VECTOR_MODES (INT, 8);        /*       V8QI V4HI V2SI */
> VECTOR_MODES (INT, 16);       /* V16QI V8HI V4SI V2DI */
> VECTOR_MODES (FLOAT, 8);      /*            V4HF V2SF */
> VECTOR_MODES (FLOAT, 16);     /*       V8HF V4SF V2DF */
> 
> I'm wondering whether it's a good idea to have a lot of pre-defined 
> fractional modes and types that may or may not match the target 
> hardware.  Not saying it's necessarily wrong; I'm just interested to 
> hear why you chose to do it this way.  (I also just noticed 
> that things 
> like SHORT_ACCUM_TYPE_SIZE are used but apparently not defined in the 
> patch - does it actually compile?)
> 

  Ok. I got it.  Maybe we treat fixed-point modes as the first class 
modes like other scalar modes (integer, floating, etc.), so we pre-define them.
We can argue that how about making machine modes (ex: floating-point, decimal floating-point)
not pre-defined, similar to vector modes.

  I think, the default fixed-point formats are the efficient ones
for 32-bit/64-bit processors (with or without hardware supports).
One of the goals for the fixed-point extension is that all targets in GCC will
enable the extension, so efficient formats may be set by default.

  We have all FRACT and ACCUM sizes in defaults.h.  Thanks!

#ifndef SHORT_FRACT_TYPE_SIZE
#define SHORT_FRACT_TYPE_SIZE BITS_PER_UNIT
#endif

#ifndef FRACT_TYPE_SIZE
#define FRACT_TYPE_SIZE (BITS_PER_UNIT * 2)
#endif

#ifndef LONG_FRACT_TYPE_SIZE
#define LONG_FRACT_TYPE_SIZE (BITS_PER_UNIT * 4)
#endif

#ifndef LONG_LONG_FRACT_TYPE_SIZE
#define LONG_LONG_FRACT_TYPE_SIZE (BITS_PER_UNIT * 8)
#endif

#ifndef SHORT_ACCUM_TYPE_SIZE
#define SHORT_ACCUM_TYPE_SIZE (SHORT_FRACT_TYPE_SIZE * 2)
#endif

#ifndef ACCUM_TYPE_SIZE
#define ACCUM_TYPE_SIZE (FRACT_TYPE_SIZE * 2)
#endif

#ifndef LONG_ACCUM_TYPE_SIZE
#define LONG_ACCUM_TYPE_SIZE (LONG_FRACT_TYPE_SIZE * 2)
#endif

#ifndef LONG_LONG_ACCUM_TYPE_SIZE
#define LONG_LONG_ACCUM_TYPE_SIZE (LONG_LONG_FRACT_TYPE_SIZE * 2)
#endif

Regards,
Chao-ying