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

[Nikunj A Dadhania]

Richard Henderson <address@hidden> writes:

> 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).

Thanks for the hints, that got reduce to following:

#define haszero(v) (((v) - 0x0101010101010101UL) & ~(v) & 0x8080808080808080UL)
#define hasvalue(x,n)  (haszero((x) ^ (~0UL/255 * (n))))

uint32_t helper_cmpeqb(target_ulong ra, target_ulong rb)
{
    return !!hasvalue(rb, ra);
}

Regards
Nikunj