xtensa: optimize trampolines relaxation

xtensa: optimize trampolines relaxation

Currently every fixup in the current segment is checked when relaxing
trampoline frag. This is very expensive. Make a searchable array of
fixups pointing at potentially oversized jumps at the beginning of every
relaxation pass and only check subset of this cache in the reach of
single jump from the trampoline frag currently being relaxed.

Original profile:

% time    self  children    called     name
-----------------------------------------
        370.16  593.38 12283048/12283048     relax_segment
  98.4  370.16  593.38 12283048         xtensa_relax_frag
         58.91  269.26 2691463834/2699602236     xtensa_insnbuf_from_chars
         68.35   68.17 811266668/813338977     S_GET_VALUE
         36.85   29.51 2684369246/2685538060     xtensa_opcode_decode
         28.34    8.84 2684369246/2685538060     xtensa_format_get_slot
         12.39    5.94 2691463834/2699775044     xtensa_format_decode
          0.03    4.60 4101109/4101109     relax_frag_for_align
          0.18    1.76  994617/994617      relax_frag_immed
          0.07    0.09 24556277/24851220     new_logical_line
          0.06    0.00 12283048/14067410     as_where
          0.04    0.00 7094588/15460506     xtensa_format_num_slots
          0.00    0.00       1/712477      xtensa_insnbuf_alloc
-----------------------------------------

Same data, after optimization:

% time    self  children    called     name
-----------------------------------------
          0.51    7.47 12283048/12283048     relax_segment
  58.0    0.51    7.47 12283048         xtensa_relax_frag
          0.02    4.08 4101109/4101109     relax_frag_for_align
          0.18    1.39  994617/994617      relax_frag_immed
          0.01    0.98     555/555         xtensa_cache_relaxable_fixups
          0.21    0.25 7094588/16693271     xtensa_insnbuf_from_chars
          0.06    0.12 24556277/24851220     new_logical_line
          0.06    0.00 7094588/15460506     xtensa_format_num_slots
          0.02    0.04 7094588/16866079     xtensa_format_decode
          0.05    0.00 12283048/14067410     as_where
          0.00    0.00       1/712477      xtensa_insnbuf_alloc
          0.00    0.00   93808/93808       xtensa_find_first_cached_fixup
-----------------------------------------

2015-05-02  Max Filippov  <jcmvbkbc@gmail.com>
gas/
	* config/tc-xtensa.c (cached_fixupS, fixup_cacheS): New typedefs.
	(struct cached_fixup, struct fixup_cache): New structures.
	(fixup_order, xtensa_make_cached_fixup),
	(xtensa_realloc_fixup_cache, xtensa_cache_relaxable_fixups),
	(xtensa_find_first_cached_fixup, xtensa_delete_cached_fixup),
	(xtensa_add_cached_fixup): New functions.
	(xtensa_relax_frag): Cache fixups pointing at potentially
	oversized jumps at the beginning of every relaxation pass. Only
	check subset of this cache in the reach of single jump from the
	trampoline frag currently being relaxed.