Re: [Qemu-devel] Confusion regarding temporaries with branch conditional

[Nikunj A Dadhania]

Richard Henderson <address@hidden> writes:

> On 11/29/2016 11:56 PM, Nikunj A Dadhania wrote:
>> Lets bring full example here.
>> 
>>     TCGv nb = tcg_temp_new();
>>     tcg_gen_andi_tl(nb, cpu_gpr[rB(ctx->opcode)], 0xFF);
>>     tcg_gen_brcondi_tl(TCG_COND_EQ, nb, 0, l1);
>> 
>>     /* do something */
>>     gen_set_access_type(ctx, ACCESS_INT);
>>     EA = tcg_temp_new();
>>     gen_addr_register(ctx, EA);
>>     tcg_gen_qemu_ld_i64(xtl, EA, ctx->mem_idx, MO_LEQ);
>>     tcg_gen_addi_tl(EA, EA, 8);
>>     tcg_gen_qemu_ld_i64(xth, EA, ctx->mem_idx, MO_LEQ);
>>     opc = tcg_const_i32(ctx->opcode);
>>     gen_helper_lxvl(cpu_env, opc, nb);      /* <--- That uses nb */
>>     tcg_temp_free_i32(opc);
>>     tcg_temp_free(EA);
>> 
>>     gen_set_label(l1);
>>     tcg_temp_free(nb);
>> 
>>> The plain temporary is only valid to the end of a basic
>>> block, and brcond ends a basic block. So you can free
>>> the temp after the brcond but you can't do anything
>>> else with it.
>> 
>> In the above case, assuming that nb is a plain temporary, case nb != 0
>> worked fine (by fluke?), i.e. no branch.
>> 
>> While when nb == 0, failed, i.e. branch taken to l1, and just free nb. I
>> am not using "nb" in this case.
>
> This is also a good example of why you should preferentially avoid branches
> within the tcg opcode stream.
>
> In the case of lxvl, I strongly suggest that you push *everything* into the
> helper.  In particular:
>
>  (1) Passing the full instruction opcode means you've got to re-parse.
>      Why are you not passing a pointer to the XT register like other
>      VSX helpers?

Sure, I can do that. No particular reason though.

>  (2) As I read it, this is wrong, since when NB == 0, XT is assigned 0.
>      Which you are not doing, having skipped over the helper.

I am doing that in my code. Didn't want to complicate the example code
above.

>  (3) Use cpu_ldq_data_ra within the helper to perform the memory
>  loads.

Let me try that.

Regards
Nikunj