Re: [Qemu-devel] Possible ppc comparision optimisation

[Paolo Bonzini <pbonzini@redhat.com>]

Il 08/05/2013 17:44, Torbjorn Granlund ha scritto:
> 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?

Yes, I think in this respect I think the first one is better.  The
second could be three instructions on machines that have a set-nth-bit
instruction _and_ a zero register, but I'm not sure they exist...

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

No, but I am attracted to these curious sequences from my previous life
working on compilers. :)  I know your superoptimizer and, in fact, we
both worked on some parts of GCC (optimization of conditional
branches/stores), just 20 years apart.

The second is actually not too curious after you look at it for a while,
it is a variant of the usual (x > y) + (x >= y) trick used to generate a
0/1/2 result.  The first I found by trial and error based on yours; it
is basically (x < y) * 2 - (x == y) + 2, with some reordering to get
parallelism and avoid the need for subfi-like instructions.

Paolo