Re: [Qemu-devel] Possible ppc comparision optimisation

[Torbjorn Granlund <tg@gmplib.org>]

Paolo Bonzini <pbonzini@redhat.com> writes:

  I think that would be faster on 32-bit hosts, truncs are cheap.
  
And slower perhaps on 64-bit hosts, at least for operations where
additional explicit trunctation will be needed (such as before
comparisions and after right shifts).

  > There could be a disadvantage of this compared to the old code, since
  > this has a chained algebraic dependency, while the old code's many
  > instructions might have been more independent.
  
  What about these alternatives:
  
  setcond LT, t0, arg0, arg1
  setcond EQ, t1, arg0, arg1
  trunc  s0, t0
  trunc  s1, t1
  shli   s0, s0, 1                ; s0 = (arg0 < arg1) ? 2 : 0
  subi   s1, s1, 2                ; s1 = (arg0 != arg1) ? -2 : -1
  sub    s0, s0, s1               ; < 4       == 1      > 2
  shli   s0, s0, 1                ; < 8       == 2      > 4
  
  =======
  
  setcond LT, t0, arg0, arg1
  setcond NE, t1, arg0, arg1
  trunc   s0, t0
  trunc   s1, t1
  add     s0, s0, s1              ; < 2       == 0      > 1
  movi    s1, 1
  add     s0, s0, s1              ; < 3       == 1      > 2
  shl     s1, s1, s0              ; < 8       == 2      > 4
  
Surely there are many alternative forms.
Is your aim to add micro-parallelism?

(Your sequences look a bit curious.  Did you use a super-optimiser?)

-- 
Torbjörn