Re: [Qemu-devel] [RFC 12/28] target-xtensa: implement shifts (ST1 and RST1 groups)

[Richard Henderson]

On 05/03/2011 05:59 PM, Max Filippov wrote:
> +                    HAS_OPTION(XTENSA_OPTION_MISC_OP);
> +                    {
> +#define gen_bit_bisect(w) do { \
> +        int label = gen_new_label(); \
> +        tcg_gen_brcondi_i32(TCG_COND_LTU, tmp, 1 << (w), label); \
> +        tcg_gen_shri_i32(tmp, tmp, (w)); \
> +        tcg_gen_subi_i32(res, res, (w)); \
> +        gen_set_label(label); \
> +    } while (0)
> +
> +                        int label = gen_new_label();
> +                        TCGv_i32 res = tcg_temp_local_new_i32();
> +
> +                        tcg_gen_movi_i32(res, 32);
> +                        tcg_gen_brcondi_i32(
> +                                TCG_COND_EQ, cpu_R[RRR_S], 0, label);
> +                        {
> +                            TCGv_i32 tmp = tcg_temp_local_new_i32();
> +                            tcg_gen_mov_i32(tmp, cpu_R[RRR_S]);
> +                            tcg_gen_movi_i32(res, 31);
> +
> +                            gen_bit_bisect(16);
> +                            gen_bit_bisect(8);
> +                            gen_bit_bisect(4);
> +                            gen_bit_bisect(2);
> +                            gen_bit_bisect(1);
> +
> +                            tcg_temp_free(tmp);
> +                        }
> +                        gen_set_label(label);
> +                        tcg_gen_mov_i32(cpu_R[RRR_T], res);
> +                        tcg_temp_free(res);
> +#undef gen_bit_bisect

This instruction is probably right at the edge of the size restrictions
on the number of ops allowed to be emitted per guest insn.  It probably
makes more sense to move this to an out-of-line helper function.

Also note that this is implementable more efficiently on hosts that have
a count-leading-zeros function:

uint32_t HELPER(nsau)(uint32_t val)
{
    return val ? clz32(val) : 32;
}

uint32_t HELPER(nsa)(int32_t val)
{
    if (val < 0) {
        val = ~val;
    }
    if (val == 0) {
        return 31;
    }
    return clz32(val) - 1;
}

> +            case 9: /*SRL*/
> +                {
> +                    TCGv_i64 v = tcg_temp_new_i64();
> +                    tcg_gen_extu_i32_i64(v, cpu_R[RRR_T]);
> +                    gen_shift(shr);
> +                }
> +                break;
> +
> +            case 10: /*SLL*/
> +                {
> +                    TCGv_i64 v = tcg_temp_new_i64();
> +                    TCGv_i32 s = tcg_const_i32(32);
> +                    tcg_gen_sub_i32(s, s, cpu_SR[SAR]);
> +                    tcg_gen_extu_i32_i64(v, cpu_R[RRR_S]);
> +                    gen_shift_reg(shl, s);
> +                    tcg_temp_free(s);
> +                }
> +                break;
> +
> +            case 11: /*SRA*/
> +                {
> +                    TCGv_i64 v = tcg_temp_new_i64();
> +                    tcg_gen_ext_i32_i64(v, cpu_R[RRR_T]);
> +                    gen_shift(sar);
> +                }

Are you implementing some of these as 64-bit shifts simply
to get a shift count of 32 correct?  While I admit that it's
probably the most efficient mechanism when the host is 64-bit,
it's somewhat less than clear.  You could stand to add some
commentary here about your choice.

As a future enhancement, it might be worthwhile to track any
known contents of SAR within the TB (see how other ports put
information about the state of the flags register in the 
DisassContext).  If you have a known value in the SAR, you
can emit the proper 32-bit shift directly.


r~