Make more use of subreg_offset_from_lsb

[Richard Sandiford]

This patch makes use of the subreg_offset_from_lsb function
added by an earlier patch in the SVE series.

Tested on aarch64-linux-gnu and x86_64-linux-gnu, and by making sure
that there were no differences in testsuite assembly output for one
target per CPU.  OK to install?

Richard


2017-08-23  Richard Sandiford  <richard.sandif...@linaro.org>
            Alan Hayward  <alan.hayw...@arm.com>
            David Sherwood  <david.sherw...@arm.com>

gcc/
        * combine.c (make_extraction): Use subreg_offset_from_lsb.

Index: gcc/combine.c
===================================================================
--- gcc/combine.c       2017-08-23 10:44:56.293484623 +0100
+++ gcc/combine.c       2017-08-23 10:45:34.493718192 +0100
@@ -7482,26 +7482,15 @@ make_extraction (machine_mode mode, rtx
                 return a new hard register.  */
              if (pos || in_dest)
                {
-                 HOST_WIDE_INT final_word = pos / BITS_PER_WORD;
-
-                 if (WORDS_BIG_ENDIAN
-                     && GET_MODE_SIZE (inner_mode) > UNITS_PER_WORD)
-                   final_word = ((GET_MODE_SIZE (inner_mode)
-                                  - GET_MODE_SIZE (tmode))
-                                 / UNITS_PER_WORD) - final_word;
-
-                 final_word *= UNITS_PER_WORD;
-                 if (BYTES_BIG_ENDIAN &&
-                     GET_MODE_SIZE (inner_mode) > GET_MODE_SIZE (tmode))
-                   final_word += (GET_MODE_SIZE (inner_mode)
-                                  - GET_MODE_SIZE (tmode)) % UNITS_PER_WORD;
+                 unsigned int offset
+                   = subreg_offset_from_lsb (tmode, inner_mode, pos);
 
                  /* Avoid creating invalid subregs, for example when
                     simplifying (x>>32)&255.  */
-                 if (!validate_subreg (tmode, inner_mode, inner, final_word))
+                 if (!validate_subreg (tmode, inner_mode, inner, offset))
                    return NULL_RTX;
 
-                 new_rtx = gen_rtx_SUBREG (tmode, inner, final_word);
+                 new_rtx = gen_rtx_SUBREG (tmode, inner, offset);
                }
              else
                new_rtx = gen_lowpart (tmode, inner);