[Bug rtl-optimization/54133] regrename introduces additional dependencies

[steven at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54133

--- Comment #4 from Steven Bosscher <steven at gcc dot gnu.org> 2012-08-01 11:58:00 UTC ---
Created attachment 27918
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27918
Hack regmove to do limited propagation of hard regs

I have a patch to make the propagation happen in regmove, resulting in the
following RTL in the IRA dump:

   78 r177:DF=r0:DF
   80 [sp:SI]=r166:DF
   81 [sp:SI+0x8]=r168:DF
   82 [sp:SI+0x10]=r170:DF
   84 r2:DF=r164:DF
   85 r0:DF=call [`bar'] argc:0x18
      REG_DEAD: r2:DF
      REG_UNUSED: r0:DF
   86 [sp:SI]=r167:DF
   87 [sp:SI+0x8]=r169:DF
   88 [sp:SI+0x10]=r171:DF
   89 r0:DF=r177:DF
      REG_DEAD: r177:DF
   90 r2:DF=r165:DF
   91 r0:DF=call [`bar'] argc:0x18

But now IRA decides to spill r177 to memory:

      Allocno a5r177 of GENERAL_REGS(9) has 4 avail. regs  4-7, node:  4-7 obj
0 (confl regs =  0-3 8-102),  obj 1 (confl regs =  0-3 8-102)

      Pushing a5(r177,l0)(potential spill: pri=469, cost=38000)
        Making a1(r159,l0) colorable
        Making a2(r172,l0) colorable
        Making a3(r163,l0) colorable
      Pushing a1(r159,l0)(cost 32000)
      Pushing a3(r163,l0)(cost 40000)
      Pushing a2(r172,l0)(cost 88000)
      Popping a2(r172,l0)  -- assign reg 4
      Popping a3(r163,l0)  -- assign reg 5
      Popping a1(r159,l0)  -- assign reg 6
      Popping a5(r177,l0)  -- (memory is more profitable 32000 vs 2147483647)
spill
      Popping a0(r175,l0)  -- assign reg 7

This makes the inner loop 2 instructions smaller, but I haven't investigated
whether that's an improvement or not.