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

[Andreas Färber]

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. :)

Andreas