Proposed doc changes regarding decode-assist

["Frank Ch. Eigler" <fche at redhat dot com>]

Hi -

I'm still glowing with pride from the sim decoder heuristics rework from
a bunch of months ago.  Since I still see people by default use the
explicit decode-assist field to override the heuristics, I'd like to
apply the following patch to the documentation to give contrary advice.
Comments?

- FChE


Index: rtl.texi
===================================================================
RCS file: /cvs/src/src/cgen/doc/rtl.texi,v
retrieving revision 1.4
diff -c -r1.4 rtl.texi
*** rtl.texi	2000/12/11 21:28:22	1.4
--- rtl.texi	2001/01/04 16:11:00
***************
*** 401,412 ****
    (default-insn-word-bitsize n)
    (default-insn-bitsize n)
    (base-insn-bitsize n)
!   (decode-assist (b0 b1 b2 ...)) ; optional
    (liw-insns n)
    (parallel-insns n)
    (condition ifield-name expr)
    (setup-semantics expr)
!   (decode-splits decode-split-list)
    ; ??? missing here are fetch/execute specs
  )
  @end example
--- 401,412 ----
    (default-insn-word-bitsize n)
    (default-insn-bitsize n)
    (base-insn-bitsize n)
!   ; (decode-assist (b0 b1 b2 ...)) ; generally unnecessary
    (liw-insns n)
    (parallel-insns n)
    (condition ifield-name expr)
    (setup-semantics expr)
!   ; (decode-splits decode-split-list) ; support temporarily disabled
    ; ??? missing here are fetch/execute specs
  )
  @end example
***************
*** 455,465 ****
  @subsubsection decode-assist
  @cindex decode-assist
  
! Override CGEN's guess about which bits to initially use to decode the
! instruction.  For example on the SPARC these are bits: 31 30 24 23 22 21 20 19.
! The entire decoder can be machine generated, so this field is entirely
! optional.  The intent of @code{decode-assist} is to give the machine
! generated code a head start, to prime the built-in heuristics.
  
  ??? It might be useful to provide greater control, but this is sufficient
  for now.
--- 455,467 ----
  @subsubsection decode-assist
  @cindex decode-assist
  
! Override CGEN's heuristics about which bits to initially use to decode
! instructions in a simulator.  For example on the SPARC these are bits: 
! 31 30 24 23 22 21 20 19.  The entire decoder can be machine generated, 
! so this field is entirely optional.  Since the heuristics are quite
! good, you should only use this field if you have evidence that you
! can pick a better set, in which case the CGEN developers would like to 
! hear from you!
  
  ??? It might be useful to provide greater control, but this is sufficient
  for now.

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