[RFC] combine: Improve change_zero_ext, call simplify_set afterwards.

[Dominik Vogt <vogt at linux dot vnet dot ibm dot com>]

There are several situations that combine.c:change_zero_ext does not
handle well yet.  One of them is

  (and:SI (subreg:SI (zero_extract:DI (reg:DI) ...) ...)
(with const_int operands to "and" and "zero_extract")

=>

  (and:SI (subreg:SI (and:DI (lshiftrt:DI ...)))

with two nested "and"s.  Another one is

  (zero_extract:DI (foo:SI) ...)

which is ignored by change_zero_ext.  Attached are two
experimental patches:

0001-*

  Deal with mode expanding zero_extracts in change_zero_ext.  The
  patch looks good to me, but not sure whether endianness is
  handled properly.  Is the second argument of gen_rtx_SUBREG
  correct?

0002-*

  This is a work in progress with the goal of fixing the first
  problem and similar ones by calling simplify_set after
  change_zero_ext to get rid of the overly complex code.  That
  works fine in principle, but replaces back the (and (lshiftrt
  ...) ...) that change_zero_ext generates back into zero_extract
  form.  Fiddling with simplify_set and make_compound_operation* a
  bit, trying to suppress undoing the transformations that
  change_zero_ext has just done, resulted in the (unfinished)
  patch.

As it's not clear to me whether this is a valid approach I'd
appreciate any advice on the patch or alternative ways of doing
that.

Ciao

Dominik ^_^  ^_^

-- 

Dominik Vogt
IBM Germany

Attachment: 0001-combine-Handle-mode-expanding-zero_extracts-in-chang.patch
Description: Text document

Attachment: 0002-combine-Call-simplify_set-after-change_zero_ext.patch
Description: Text document