[Bug rtl-optimization/39871] [4.3/4.4/4.5 regression] Code size increase on ARM due to inferior CSE

[steven at gcc dot gnu dot org]

------- Comment #12 from steven at gcc dot gnu dot org  2010-02-10 16:27 -------
Trying with r156650, I get this before regalloc (in the .184r.asmcons dump):

    1 NOTE_INSN_DELETED
    4 NOTE_INSN_BASIC_BLOCK
    2 r135:SI=r0:SI
      REG_DEAD: r0:SI
    3 NOTE_INSN_FUNCTION_BEG
    6 r136:SI=sfp:SI-0xc
    7 r137:SI=[r135:SI]
    8 [r136:SI+0x4]=r137:SI
      REG_DEAD: r137:SI
   10 r139:SI=[r135:SI+0x4]
      REG_DEAD: r135:SI
   11 r0:SI=r136:SI
      REG_DEAD: r136:SI
      REG_EQUAL: sfp:SI-0xc
   12 r1:SI=r139:SI
      REG_DEAD: r139:SI
   13 call [`func'] argc:0x0
      REG_DEAD: r1:SI
      REG_DEAD: r0:SI


To get the same code as gcc 4.2.1 of comment #0, r0 should be assigned to r136.
There is no reason why that should not happen, because there are no conflicts:

+++Allocating 32 bytes for conflict table (uncompressed size 32)
;; a0(r139,l0) conflicts: a1(r136,l0)
;;     total conflict hard regs: 0
;;     conflict hard regs: 0
;; a1(r136,l0) conflicts: a0(r139,l0) a2(r135,l0) a3(r137,l0)
;;     total conflict hard regs:
;;     conflict hard regs:
;; a2(r135,l0) conflicts: a1(r136,l0) a3(r137,l0)
;;     total conflict hard regs:
;;     conflict hard regs:
;; a3(r137,l0) conflicts: a1(r136,l0) a2(r135,l0)
;;     total conflict hard regs:
;;     conflict hard regs:

There are also no indications (not that I can find anyway) in the IRA dumps,
that suggests that IRA notices a tie between r0-r136 and r1-r139 may be
beneficial (thanks to insns 11 and 12 in the pre-regalloc dump).

IRA has done the following:
      Popping a0(r139,l0)  -- assign reg 1
      Popping a1(r136,l0)  -- assign reg 3
      Popping a2(r135,l0)  -- assign reg 0
      Popping a3(r137,l0)  -- assign reg 2

With this assignment, insn 2 and insn 12 become no-op moves. It looks like r139
ends up in r1 by pure luck, or there would have been another extra move.

After regalloc (in the .187r.ira dump) it looks like this:

    1 NOTE_INSN_DELETED
    4 NOTE_INSN_BASIC_BLOCK
    3 NOTE_INSN_FUNCTION_BEG
    6 r3:SI=sp:SI+0x4
      REG_EQUIV: sp:SI+0x4
    7 r2:SI=[r0:SI]
      REG_EQUIV: [r0:SI]
    8 [r3:SI+0x4]=r2:SI
   10 r1:SI=[r0:SI+0x4]
   11 r0:SI=r3:SI
      REG_EQUAL: sfp:SI-0xc
   13 call [`func'] argc:0x0
   16 NOTE_INSN_DELETED


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  GCC build triplet|x86_64-unknown-linux-gnu    |
   GCC host triplet|x86_64-unknown-linux-gnu    |
      Known to fail|                            |4.4.0 4.5.0
      Known to work|4.5.0                       |4.2.1
   Last reconfirmed|2009-05-20 14:17:16         |2010-02-10 16:27:56
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39871