On Fri, Dec 11, 2009 at 7:21 AM, Andreas Färber <andreas.faer...@web.de> 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