[6/8] Simplify make_extraction

[Richard Sandiford <rdsandiford at googlemail dot com>]

On a change of tack, this tackles some redundant code in combine.
It has code to convert a variable bit position to the mode required
by the bit position operand to insv, extv or extzv:

[A]
  else if (pos_rtx != 0
	   && GET_MODE_SIZE (pos_mode) < GET_MODE_SIZE (GET_MODE (pos_rtx)))
    pos_rtx = gen_lowpart (pos_mode, pos_rtx);

However, this is protected by an earlier:

[B]
  if (pos_rtx && GET_MODE (pos_rtx) != VOIDmode
      && GET_MODE_SIZE (pos_mode) < GET_MODE_SIZE (GET_MODE (pos_rtx)))
    pos_mode = GET_MODE (pos_rtx);

so [B] makes [A] redundant.  That leaves this block:

  /* Adjust mode of POS_RTX, if needed.  If we want a wider mode, we
     have to zero extend.  Otherwise, we can just use a SUBREG.  */
  if (pos_rtx != 0
      && GET_MODE_SIZE (pos_mode) > GET_MODE_SIZE (GET_MODE (pos_rtx)))
    {

as the only use of pos_mode, so [B] can go too.

Similarly, the memory case has:

[C]
      /* If we have to change the mode of memory and cannot, the desired mode
	 is EXTRACTION_MODE.  */
      if (inner_mode != wanted_inner_mode
	  && (mode_dependent_address_p (XEXP (inner, 0), MEM_ADDR_SPACE (inner))
	      || MEM_VOLATILE_P (inner)
	      || pos_rtx))
	wanted_inner_mode = extraction_mode;

but those same conditions are repeated in the code that actually
applies wanted_inner_mode:

  /* If INNER has a wider mode, and this is a constant extraction, try to
     make it smaller and adjust the byte to point to the byte containing
     the value.  */
  if (wanted_inner_mode != VOIDmode
      && inner_mode != wanted_inner_mode
      && ! pos_rtx
      && GET_MODE_SIZE (wanted_inner_mode) < GET_MODE_SIZE (is_mode)
      && MEM_P (inner)
      && ! mode_dependent_address_p (XEXP (inner, 0), MEM_ADDR_SPACE (inner))
      && ! MEM_VOLATILE_P (inner))
    {

so [C] too is unnecessary.

Tested as described in the covering note.  OK to install?

Richard


gcc/
	* combine.c (make_extraction): Remove dead wanted_inner_mode-
	and pos_rtx-related code.

Index: gcc/combine.c
===================================================================
--- gcc/combine.c	2012-10-27 22:22:45.000000000 +0100
+++ gcc/combine.c	2012-10-27 22:32:26.811370582 +0100
@@ -7211,10 +7211,6 @@ make_extraction (enum machine_mode mode,
       && GET_MODE_SIZE (extraction_mode) < GET_MODE_SIZE (mode))
     extraction_mode = mode;
 
-  if (pos_rtx && GET_MODE (pos_rtx) != VOIDmode
-      && GET_MODE_SIZE (pos_mode) < GET_MODE_SIZE (GET_MODE (pos_rtx)))
-    pos_mode = GET_MODE (pos_rtx);
-
   /* If this is not from memory, the desired mode is the preferred mode
      for an extraction pattern's first input operand, or word_mode if there
      is none.  */
@@ -7231,14 +7227,6 @@ make_extraction (enum machine_mode mode,
 	  wanted_inner_mode = GET_MODE_WIDER_MODE (wanted_inner_mode);
 	  gcc_assert (wanted_inner_mode != VOIDmode);
 	}
-
-      /* If we have to change the mode of memory and cannot, the desired mode
-	 is EXTRACTION_MODE.  */
-      if (inner_mode != wanted_inner_mode
-	  && (mode_dependent_address_p (XEXP (inner, 0), MEM_ADDR_SPACE (inner))
-	      || MEM_VOLATILE_P (inner)
-	      || pos_rtx))
-	wanted_inner_mode = extraction_mode;
     }
 
   orig_pos = pos;
@@ -7359,9 +7347,6 @@ make_extraction (enum machine_mode mode,
 	}
       pos_rtx = temp;
     }
-  else if (pos_rtx != 0
-	   && GET_MODE_SIZE (pos_mode) < GET_MODE_SIZE (GET_MODE (pos_rtx)))
-    pos_rtx = gen_lowpart (pos_mode, pos_rtx);
 
   /* Make POS_RTX unless we already have it and it is correct.  If we don't
      have a POS_RTX but we do have an ORIG_POS_RTX, the latter must