bit-field pointers / arrays

[Stuart D. Gathman]

In article <534 at cartan.Berkeley.EDU>, ballou at brahms (Kenneth R. Ballou) writes:

> In article <311 at bms-at.UUCP> stuart at bms-at.UUCP (Stuart D. Gathman) writes:

	[ bitfield array proposal deleted, see parent article ]

> 	Oh, please, here we go again with someone else who wants to redefine
> C because he wants to take advantage of very specific features of his
> machine/environment!  Big deal, so you can do this in a straightforward manner
> on your 68020.  I'm just thrilled to pieces for you!  And what happens when
> trying to implement C on a machine which doesn't have your spiffy handy-dandy
> bitfield instructions?  Two possibilities come to mind:

	[ etc, etc ]

As a matter of fact, we do not even have a 68020 machine.  I  was
merely  giving  an  example  for  what  I  assumed  was a popular
processor to illustrate the feasability.  The NEC V20/V30 and the
VAX  are  other processors which I know for a fact have bit field
instructions.  Processors that don't have such nifty instructions
generate  lots  of  code; but ONLY when bitfields are used.  This
happens already now when  using  bitfield  in  structures!   Even
then,  the  compiler probably generates less code than if you did
it yourself using shifts and 'and's.  Furthermore,  the  bitfield
code  is  a  lot more portable than lots of shifts and logic with
the accompanying assumptions regarding byte size, etc.   The  EGA
application  was  just an EXAMPLE (and was labeled as such).  The
primary portable application would be bit arrays.

The operators  in  the  'C'  language  were  designed  to  relect
functions held in common across a wide range of processors.  Many
hardware extensions can be  handled  conveniently  via  functions
(e.g.  vector  processors).   Bit  manipulation is something that
most  modern  processors  can  do  efficiently  with   built   in
instructions,  and  that  any  machine can do just as efficiently
with compiler generated code as with programmer specified  shifts
and  masks.   It  cannot  however be handled well with functions.
Current 'C' *already* has bitfield support.  I am merely throwing
out  the  idea  of rounding  out that support to allow arrays and
pointers as well.
-- 
Stuart D. Gathman	<..!seismo!dgis!bms-at!stuart>