Re: [Qemu-devel] [PATCH 0/7] tcg: conditional set and move opcodes

[Richard Henderson]

On 12/17/2009 07:32 AM, malc wrote:
> > These new opcodes are considered "required" by the backend,
> > because expanding them at the tcg level breaks the basic block.
> > There might be some way to emulate within tcg internals, but
> > that doesn't seem worthwhile, as essentially all hosts have
> > some form of support for these.
...
>   c. Historically things like that were made conditional with
>      a generic fallback (bswap, neg, not, rot, etc)

I answered this one above.  A generic fallback would break the
basic block, which would break TCGs simple register allocation.

>   b. Documentation for movcond has a typo, t0 is assigned not t1

Oops.  Will fix.

>   d. Documentation for setcond2 is missing

Ah, I see that brcond2 is missing as well; I'll fix that too.

> It would also be interesting to learn what impact adding those two
> has on performance, any results?

Hmph, not as much as I would have liked.  I suppose Intel is getting 
pretty darned good with its branch prediction.  It shaved about 3 
minutes off 183.equake from what I posted earlier this week; that's 
something around a 7% improvement, assuming it's not just all noise (I 
havn't run that test enough times to see what the variation is).

> +    case TCG_COND_NE:
> +        if (const_arg2) {
> +            if ((uint16_t) arg2 == arg2) {
> +                tcg_out32 (s, XORI | RS (arg1) | RA (0) | arg2);
> +            }
> +            else {
> +                tcg_out_movi (s, TCG_TYPE_I32, 0, arg2);
> +                tcg_out32 (s, XOR | SAB (arg1, 0, 0));
> +            }
> +        }
> +        else {
> +            tcg_out32 (s, XOR | SAB (arg1, 0, arg2));
> +        }
> +
> +        tcg_out32 (s, ADDIC | RT (arg0) | RA (0) | 0xffff);
> +        tcg_out32 (s, SUBFE | TAB (arg0, arg0, 0));
> +        return;

Heh, you know a trick that gcc doesn't for powerpc.  It just adds an xor 
at the end of the EQ sequence.


r~