Re: [Qemu-ppc] [Qemu-devel] [RFC v1 09/13] target-ppc: add cmpeqb instruction

[Richard Henderson]

On 07/18/2016 10:35 PM, Nikunj A Dadhania wrote:
> +    tcg_gen_andi_tl(src1, cpu_gpr[rA(ctx->opcode)], 0xFF);
> +    for (i = 0; i < 64; i += 8) {
> +        tcg_gen_shri_tl(t0, arg1, i);
> +        tcg_gen_andi_tl(t0, t0, 0xFF);
> +        tcg_gen_brcond_tl(TCG_COND_EQ, src1, t0, l1);
> +    }
> +    tcg_gen_movi_i32(cpu_crf[crfD(ctx->opcode)], 0);
> +    tcg_gen_br(l2);
> +    gen_set_label(l1);
> +    /* Set match bit, i.e. CRF_GT */
> +    tcg_gen_movi_i32(cpu_crf[crfD(ctx->opcode)], 1 << CRF_GT);

Ew.  This can be done much better as

  http://graphics.stanford.edu/~seander/bithacks.html#ZeroInWord

Which still might be best done in a helper, because of the constants involved 
(tcg is not nearly so good as gcc in building full 64-bit constants).

C.f. target-alpha/int_helper.c, helper_cmpbe0 (which computes different 
information than cmpeqb, but is still helpful as an example).


r~