[Bug rtl-optimization/39871] [4.3/4.4/4.5 regression] Code size increase on ARM due to poor register allocation

[steven at gcc dot gnu dot org]

------- Comment #15 from steven at gcc dot gnu dot org  2010-02-10 19:24 -------
The difference between r118474 (left) and r118475 just before register
allocation (in the .life2 dumps) is this:

    2 NOTE_INSN_DELETED                     2 NOTE_INSN_DELETED
    8 NOTE_INSN_BASIC_BLOCK                 8 NOTE_INSN_BASIC_BLOCK
    6 r101:SI=r0:SI                         6 r101:SI=r0:SI
      REG_DEAD: r0:SI                         REG_DEAD: r0:SI
    7 NOTE_INSN_FUNCTION_BEG                7 NOTE_INSN_FUNCTION_BEG
   11 r102:SI=sfp:SI-0xc                   11 r102:SI=sfp:SI-0xc
   12 r103:SI=[r101:SI]                    12 r103:SI=[r101:SI]
   13 [sfp:SI-0x8]=r103:SI      |          13 [r102:SI+0x4]=r103:SI
      REG_DEAD: r103:SI                       REG_DEAD: r103:SI
   16 r105:SI=[r101:SI+0x4]                16 r105:SI=[r101:SI+0x4]
      REG_DEAD: r101:SI                       REG_DEAD: r101:SI
   17 r0:SI=r102:SI                        17 r0:SI=r102:SI
      REG_DEAD: r102:SI                       REG_DEAD: r102:SI
      REG_EQUAL: sfp:SI-0xc                   REG_EQUAL: sfp:SI-0xc
   18 r1:SI=r105:SI                        18 r1:SI=r105:SI
      REG_DEAD: r105:SI                       REG_DEAD: r105:SI
   19 call [`func'] argc:0x0               19 call [`func'] argc:0x0
      REG_DEAD: r0:SI                         REG_DEAD: r0:SI
      REG_DEAD: r1:SI                         REG_DEAD: r1:SI
      REG_UNUSED: lr:SI                       REG_UNUSED: lr:SI
   20 NOTE_INSN_FUNCTION_END               20 NOTE_INSN_FUNCTION_END


In r118474 the cse1 pass transforms the code on the left (.jump dump) to the
code on the right (.cse1 dump):

   11 r102:SI=sfp:SI-0xc                   11 r102:SI=sfp:SI-0xc
   12 r103:SI=[r101:SI]                    12 r103:SI=[r101:SI]
   13 [r102:SI+0x4]=r103:SI           |    13 [sfp:SI-0x8]=r103:SI

apparently noticing that "sfp-0xc+0x4" == "sfp-0x8". It would be interesting to
know why fwprop is not doing this transformation.


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