Re: [Qemu-devel] [PATCH moxie 3/5] Moxie target code

[Anthony Green]

On Thu, Feb 14, 2013 at 6:19 PM, Richard Henderson <address@hidden> wrote:
> On 02/13/2013 02:26 PM, Anthony Green wrote:
>> +        case 0x03: /* jsra */
>> +          {
>> +            /* Load the stack pointer into T0.  */
>> +            TCGv t1 = new_tmp();
>> +            TCGv t2 = new_tmp();
>> +
>> +            tcg_gen_movi_i32(t1, ctx->pc+6);
>> +
>> +            /* Make space for the static chain and return address.  */
>> +            tcg_gen_subi_i32(t2, REG(1), 8);
>> +            tcg_gen_mov_i32(REG(1), t2);
>> +            tcg_gen_qemu_st32(t1, REG(1), ctx->memidx);
>> +
>> +            /* Push the current frame pointer.  */
>> +            tcg_gen_subi_i32(t2, REG(1), 4);
>> +            tcg_gen_mov_i32(REG(1), t2);
>> +            tcg_gen_qemu_st32(REG(0), REG(1), ctx->memidx);
>
> There are two exceptions that can be taken here, for the two stores.
> Are you certain that REG(1) should be updated before both are handled?
> Should the write to REG(1) be delayed until after the second store?

You raise a good point, but that's approximately how the hardware works today.
We write to $sp midway through the multi-cycle jsr instruction (before
the second write).
https://github.com/atgreen/moxie-cores/blob/master/cores/MoxieLite/moxielite.vhd#L932
Something to put on the TODO list for both hw and qemu.

>> +        case 0x04: /* ret */
>> +          {
>> +            TCGv t1 = new_tmp();
>> +
>> +            /* The new $sp is the old $fp.  */
>> +            tcg_gen_mov_i32(REG(1), REG(0));
>> +
>> +            /* Pop the frame pointer.  */
>> +            tcg_gen_qemu_ld32u(REG(0), REG(1), ctx->memidx);
>> +            tcg_gen_addi_i32(t1, REG(1), 4);
>> +            tcg_gen_mov_i32(REG(1), t1);
>> +
>> +
>> +            /* Pop the return address and skip over the static chain
>> +               slot.  */
>> +            tcg_gen_qemu_ld32u(cpu_pc, REG(1), ctx->memidx);
>> +            tcg_gen_addi_i32(t1, REG(1), 8);
>> +            tcg_gen_mov_i32(REG(1), t1);
>
> Similarly, should any global variable be updated before the second load?
> Same comments apply to JSR and SWI.

Same answer applies.

Thanks,

AG