[Bug middle-end/27016] ARM optimizer produces severely suboptimal code

["steven at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #4 from steven at gcc dot gnu dot org  2009-06-30 13:21 -------
The auto-inc-dec pass fails because the store and the reg increment are not in
the same basic block.  The dump of the pass before auto-inc-dec (reginfo) looks
like this:

;; Function testme (testme)

   74 NOTE_INSN_BASIC_BLOCK
   72 NOTE_INSN_FUNCTION_BEG
   76 r205:SI=`code_in_ram'
   73 r203:SI=`_ram_erase_sector_start'
   87 r208:SI=`_ram_erase_sector_end'
L86:
   79 NOTE_INSN_BASIC_BLOCK
   80 r206:SI=[r203:SI]
   81 [r205:SI-0x4]=r206:SI
      REG_DEAD: r206:SI
   82 r203:SI=r203:SI+0x4
L83:
   84 NOTE_INSN_BASIC_BLOCK
   85 r205:SI=r205:SI+0x4
   88 cc:CC=cmp(r203:SI,r208:SI)
      REG_EQUAL: cmp(r203:SI,`_ram_erase_sector_end')
   89 pc={(ltu(cc:CC,0x0))?L86:pc}
      REG_DEAD: cc:CC
      REG_BR_PROB: 0x238c
   92 NOTE_INSN_BASIC_BLOCK


Then auto-inc-dec comes along and notices the opportunity to merge the load and
increment in insns 80 and 82:

starting bb 3
   82 r203:SI=r203:SI+0x4
   81 [r205:SI-0x4]=r206:SI
      REG_DEAD: r206:SI
   81 [r205:SI-0x4]=r206:SI
      REG_DEAD: r206:SI
found mem(81) *(r[205]+-4)
   80 r206:SI=[r203:SI]
   80 r206:SI=[r203:SI]
found mem(80) *(r[203]+0)
   82 r203:SI=r203:SI+0x4
found post inc(82) r[203]+=4
trying SIMPLE_POST_INC
rescanning insn with uid = 80.
deleting insn with uid = 80.
deleting insn with uid = 82.
****success    80 r206:SI=[r203:SI++]
      REG_INC: r203:SI


Merging the store and increment of insns 81 and 85 is never tried because the
insns are not in the same basic block.

Bernd Schmidt has patches that probably address this case of insns in different
basic blocks.  I will give his patches a try to see if it helps for ARM (the
patches were written for Blackfin).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27016