Re: [Qemu-devel] [PATCH 14/14] i386: optimize setcc instructions

[Richard Henderson <rth@twiddle.net>]

On 10/06/2012 05:30 AM, Paolo Bonzini wrote:
> +static inline void gen_setcc1(DisasContext *s, int b, TCGv reg)
>  {
> +    int inv, jcc_op, size, cond;
> +    TCGv t0;
> +
> +    inv = b & 1;
>      jcc_op = (b >> 1) & 7;
> +
>      switch(s->cc_op) {
> +        /* we optimize relational operators for the cmp/jcc case */
>      case CC_OP_SUBB:
>      case CC_OP_SUBW:
>      case CC_OP_SUBL:
>      case CC_OP_SUBQ:
> +        size = s->cc_op - CC_OP_SUBB;
> +        switch(jcc_op) {
> +        case JCC_BE:
> +            cond = inv ? TCG_COND_GTU : TCG_COND_LEU;
> +            tcg_gen_add_tl(cpu_tmp4, cpu_cc_dst, cpu_cc_src);
> +            gen_extu(size, cpu_tmp4);
> +            t0 = gen_ext_tl(cpu_tmp0, cpu_cc_src, size, false);
> +            tcg_gen_setcond_tl(cond, reg, cpu_tmp4, t0);
> +            break;

I don't think this patch is going in the right direction.  In particular,
this is going to be largely redundant with gen_jcc1.

Instead, c.f. the DisasCompare structure now present in target-sparc/,
or a similar DisasCompare structure present in my jumbo target-s390x
patch set.  Here we use common code to generate a comparison, which
can then be fed into brcond, setcond, or movcond as desired.

I think that this Compare structure should be fed to gen_compute_eflags_*
so that a parent gen_condition routine can make use of them for simple
conditions like z/nz.

At which point gen_jcc1 and gen_setcc1 become fairly trivial routines.


r~