Re: [Qemu-devel] TCG is hard to understand!

[Jun Koi]

On Fri, Dec 11, 2009 at 7:21 AM, Andreas Färber <address@hidden> wrote:
> Hi,
>
> Am 10.12.2009 um 17:44 schrieb Jun Koi:
>
>> I am trying to understand how TCG works. For example, I look at the
>> LLDT insn on x86.
>>
>> In target-i386/translate.c, we translate LLDT to TCG code, like below:
>>
>>
>> static TCGv_i32 cpu_tmp2_i32;                                      // 1
>> ...
>> gen_ldst_modrm(s, modrm, OT_WORD, OR_TMP0, 0);   // 2
>> gen_jmp_im(pc_start - s->cs_base);                                // 3
>> tcg_gen_trunc_tl_i32(cpu_tmp2_i32, cpu_T[0]);                // 4
>> gen_helper_lldt(cpu_tmp2_i32);                                       // 5
>>
>>
>> This is quite confused. I understand that:
>
> [...]
>>
>> - Line (4) generate the code to copy cpu_T[0] to the (local) variable
>> cpu_tmp2_i32.
>> However, as tcg_gen_trunc_tl_i32() put the *value* of that variable,
>> but not its *address*, into the generated code, I dont see how next
>> line (5) can generate code that use the same variable.
>> Clearly there is no connection between cpu_tmp2_i32 on line (4) and
>> line (5), so how the generated code works here??
>
> Line 4 generates code to truncate the value from the source associated with
> cpu_T[0] and put it into the destination associated with cpu_tmp2_i32, which
> may be a general-purpose register on PowerPC for instance (i.e., not an
> address). The connection between line 4 and 5 is that cpu_tmp2_i32 has as
> its value the same identifier (sort of a handle), allowing TCG internal code
> to lookup its location.
>
> Btw if the code confuses you, cpu_T[n] is actually a leftover from the
> dyngen to TCG conversion. Feel free to provide patches replacing the
> remaining occurrences by individual local TCG variables if it helps your
> understanding. :)
>

Thanks a lot for the insight! Now it is quite clear to me.

However, I still dont understand what the line (3) does. Could you
give some hints?

>> static TCGv_i32 cpu_tmp2_i32;                                      // 1
>> ...
>> gen_ldst_modrm(s, modrm, OT_WORD, OR_TMP0, 0);   // 2
>> gen_jmp_im(pc_start - s->cs_base);                                // 3
>> tcg_gen_trunc_tl_i32(cpu_tmp2_i32, cpu_T[0]);                // 4
>> gen_helper_lldt(cpu_tmp2_i32);                                       // 5


Thanks,
Jun