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combiner and paradoxical subreg question....

I'm working on a new target port currently based on 3.0.4. I'm looking
into a problem in which a sign extension is inappropriately optimised
away.I suspect the problem originates in the combiner, can anyone shed
some light on whats going on here??? This is what I have so far:

The combiner eventually gets around to expanding {sign,zero}_extends in:

combine.c:expand_compound_operation ()

In this scenario it performs the following rewrite:

(sign_extend:SI (mem:QI (reg/f:SI 168) 0)) => (subreg:SI (mem:QI
(reg/f:SI 168) 0) 0)

The relevant code fragment is something like:

combine.c:9662ish:gen_lowpart_for_combine ()

      /* If we want to refer to something bigger than the original
      generate a perverse subreg instead.  That will force a reload
      of the original memref X.  */
      if (GET_MODE_SIZE (GET_MODE (x)) < GET_MODE_SIZE (mode))
     return gen_rtx_SUBREG (mode, x, 0);

I'm slightly confused by this, my understanding of paradoxical subreg's
is that they are used to refer to something in  a wider mode where we
don;t care about the value of the additional bits. Are there situations
where a paradoxical subreg implies zero or sign extension rather than

Subsequently when we get around to 'splitting' the above fragment, back
combine.c:2277ish:do_try_combine ()

   /* If *SPLIT is a paradoxical SUBREG, when we split it, it should
      be written as a ZERO_EXTEND.  */
   if (split_code == SUBREG && GET_CODE (SUBREG_REG (*split)) == MEM)
     SUBST (*split, gen_rtx_combine (ZERO_EXTEND, split_mode,
         XEXP (*split, 0)));

This performs the rewrite:
(subreg:SI (mem:QI (reg/f:SI 168) 0) 0) => (zero_extend:SI (mem:QI
(reg/f:SI 168) 0))

This code fragment switches the paradoxical subreg for a zero_extend, so
the combiner has now managed the overall rewrite:

(sign_extend:SI (mem:QI (reg/f:SI 168) 0))
(subreg:SI (mem:QI (reg/f:SI 168) 0) 0)
(zero_extend:SI (mem:QI (reg/f:SI 168) 0))

So can someone explain:
* What does a paradoxial subreg really represent.
* Is it ever safe to rewrite a sign_extend to a paradoxical subreg.


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