field-start and varying word sizes

[Geoff Keating <geoffk at geoffk dot org>]

In field-start in ifield.scm, there is this code:

     ; Note that this is only intended for situations like the m32r.
     ; If it doesn't work elsewhere, it may be that you need to
     ; do things different (use two fields instead of one).
     (cond ((= wanted-word-len recorded-word-len)
            (bitrange-start bitrange))
           ((< wanted-word-len recorded-word-len)
            ; smaller word wanted
            (if lsb0?
                (- (bitrange-start bitrange) (- recorded-word-len
                                                wanted-word-len))
                (bitrange-start bitrange)))
           (else
            ; larger word wanted
            (if lsb0?
                (+ (bitrange-start bitrange) (- wanted-word-len
                                                recorded-word-len))
                (bitrange-start bitrange))))))

I can see why someone might try to write this, but it can't work.  It
needs to know, as well as lsb0?, the endianess of the CPU.

In my case, I have:

lsb0? #t
wanted-word-len 32
recorded-word-len 16
endian little

In this case, the right thing to do is to just return
(bitrange-start bitrange)

because on a little-endian system, you can extend words all you like
but the bit numbers don't change; it is only on a big-endian system
that and adjustment for word size is necessary.  Of course, this is
why I chose lsb0? to be true, so I can number opcode fields without
worrying about the word size.

Despite the comment, m32r has 
  (insn-lsb0? #f)
and so all this code comes down to the trivial case anyway.
In fact, I can't find any port with lsb0?, big-endian, and
variable-length instructions, which is the only time that this code
does anything other than (bitrange-start bitrange).
The code has been in since cgen was written, so I believe the code
is one of those bits of never-tested never-used-before code that makes
working with cgen so much fun.

OK to commit?

-- 
- Geoffrey Keating <geoffk@geoffk.org>

===File ~/patches/cygnus/cgen-fieldstart.patch==============
2001-07-03  Geoffrey Keating  <geoffk@redhat.com>

	* ifield.scm (<ifield> 'field-start): Don't look at word-len.

Index: cgen/ifield.scm
===================================================================
RCS file: /cvs/cvsfiles/devo/cgen/ifield.scm,v
retrieving revision 1.51
diff -p -u -p -r1.51 ifield.scm
--- ifield.scm	2000/09/15 07:54:35	1.51
+++ ifield.scm	2001/07/03 20:33:07
@@ -91,42 +91,11 @@
 (define (ifld-decode-mode f) (ifld-mode f))
 
 ; Return start of ifield.
-; WORD-LEN is the length of the word in which to compute the value or
-; #f meaning to use the default length (recorded with the bitrange).
-; WORD-LEN is present for architectures like the m32r where there are insns
-; smaller than the base insn size (LIW).
-; ??? Not sure it'll be applicable to other LIW architectures.  The m32r is
-; rather easy as the insns are 16 and 32 bits.
-; ??? Another way to do this would be to either set the base-insn-size for
-; the m32r to be 16 bits, or to add a new field to hold the insn-word-size
-; and set it to 16 for the m32r.  The problem here is that there is no
-; canonicalization that works regardless of whether a "word" is shortened
-; or lengthened.
 
 (method-make-virtual!
  <ifield> 'field-start
  (lambda (self word-len)
-   (let* ((bitrange (-ifld-bitrange self))
-	  (lsb0? (bitrange-lsb0? bitrange))
-	  (recorded-word-len (bitrange-word-length bitrange))
-	  (wanted-word-len (or word-len recorded-word-len)))
-     ; Note that this is only intended for situations like the m32r.
-     ; If it doesn't work elsewhere, it may be that you need to
-     ; do things different (use two fields instead of one).
-     (cond ((= wanted-word-len recorded-word-len)
-	    (bitrange-start bitrange))
-	   ((< wanted-word-len recorded-word-len)
-	    ; smaller word wanted
-	    (if lsb0?
-		(- (bitrange-start bitrange) (- recorded-word-len
-						wanted-word-len))
-		(bitrange-start bitrange)))
-	   (else
-	    ; larger word wanted
-	    (if lsb0?
-		(+ (bitrange-start bitrange) (- wanted-word-len
-						recorded-word-len))
-		(bitrange-start bitrange))))))
+   (bitrange-start (-ifld-bitrange self)))
 )
 
 (define (ifld-start ifld word-len)
============================================================