[Bug rtl-optimization/102178] [12 Regression] SPECFP 2006 470.lbm regressions on AMD Zen CPUs after r12-897-gde56f95afaaa22

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102178

--- Comment #17 from Richard Biener <rguenth at gcc dot gnu.org> ---
So in .reload we have (with unpatched trunk)

  401: NOTE_INSN_BASIC_BLOCK 6
  462: ax:DF=[`*.LC0']
      REG_EQUAL 9.850689999999999724167309977929107844829559326171875e-1
  407: xmm2:DF=ax:DF
  463: ax:DF=[`*.LC0']
      REG_EQUAL 9.850689999999999724167309977929107844829559326171875e-1
  408: xmm4:DF=ax:DF

why??!  We can load .LC0 into xmm4 directly.  IRA sees

  401: NOTE_INSN_BASIC_BLOCK 6
  407: r118:DF=r482:DF
  408: r119:DF=r482:DF

now I cannot really decipher IRA or LRA dumps but my guess would be that
inheritance (causing us to load from LC0) interferes badly with register
class assignment?

Changing pseudo 482 in operand 1 of insn 407 on equiv
9.850689999999999724167309977929107844829559326171875e-1
...
          alt=21,overall=9,losers=1,rld_nregs=1
         Choosing alt 21 in insn 407:  (0) v  (1) r {*movdf_internal}
      Creating newreg=525, assigning class GENERAL_REGS to r525
  407: r118:DF=r525:DF
    Inserting insn reload before:
  462: r525:DF=[`*.LC0']
      REG_EQUAL 9.850689999999999724167309977929107844829559326171875e-1

we should have preferred alt 14 I think (0) v (1) m, but that has

          alt=14,overall=13,losers=1,rld_nregs=0
            0 Spill pseudo into memory: reject+=3
            Using memory insn operand 0: reject+=3
            0 Non input pseudo reload: reject++
            1 Non-pseudo reload: reject+=2
            1 Non input pseudo reload: reject++
            alt=15,overall=28,losers=3 -- refuse
            0 Costly set: reject++
            alt=16: Bad operand -- refuse
            0 Costly set: reject++
            1 Costly loser: reject++
            1 Non-pseudo reload: reject+=2
            1 Non input pseudo reload: reject++
            alt=17,overall=17,losers=2 -- refuse
            0 Costly set: reject++
            1 Spill Non-pseudo into memory: reject+=3
            Using memory insn operand 1: reject+=3
            1 Non input pseudo reload: reject++
            alt=18,overall=14,losers=1 -- refuse
            0 Spill pseudo into memory: reject+=3
            Using memory insn operand 0: reject+=3
            0 Non input pseudo reload: reject++
            1 Costly loser: reject++
            1 Non-pseudo reload: reject+=2
            1 Non input pseudo reload: reject++
            alt=19,overall=29,losers=3 -- refuse
            0 Non-prefered reload: reject+=600
            0 Non input pseudo reload: reject++
            alt=20,overall=607,losers=1 -- refuse
            1 Non-pseudo reload: reject+=2
            1 Non input pseudo reload: reject++

I'm not sure I can decipher the reasoning but I don't understand how it
doesn't seem to anticipate the cost of reloading the GPR in the alternative
it chooses?

Vlad?