On Tue, Jan 17, 2012 at 7:22 PM, 陳韋任 <che...@iis.sinica.edu.tw> wrote: >> previously returned tb. This is usually 0/1. When running icount, you >> can also get a 2 value in these least significant digits, indicating >> that the translation block was restarted due to the >> icount_decr.u16.low field being exhausted but having instructions left >> to execute in icount_extra. This 2 value falls through to tb_add_jump, >> which then updates the tb's jmp_first field, as both tb and next_tb >> refer to the same translation block. My question is why is this >> necessary, why not do nothing, and leave the previous chaining intact? >> I hope this is clearer and thanks for the response. -Jim > > I have a question here. Look at gen_icount_start and gen_icount_end in > gen-icount.h, I think the least significant bits are set to 2 by following > code. > > tcg_gen_exit_tb((tcg_target_long)tb + 2); > > And you said, > > "indicating that the translation block was restarted due to the > icount_decr.u16.low field being exhausted but having instructions left > to execute in icount_extra." > > From the code snipt of gen_icount_start below, I can only figure out something > (icount_decr.u32?) is exhausted so it will jump to label set by gen_icount_end > and execute "tcg_gen_exit_tb((tcg_target_long)tb + 2)". I only see > icount_extra > and icount_decr.u16.low are dealt in cpu_exec (cpu-exec.c). Do I miss > something > or misunderstand what you said? Thanks. > > --- > icount_label = gen_new_label(); > count = tcg_temp_local_new_i32(); > tcg_gen_ld_i32(count, cpu_env, offsetof(CPUState, icount_decr.u32)); > /* This is a horrid hack to allow fixing up the value later. */ > icount_arg = gen_opparam_ptr + 1; > tcg_gen_subi_i32(count, count, 0xdeadbeef); > > tcg_gen_brcondi_i32(TCG_COND_LT, count, 0, icount_label); > --- > > Regards, > chenwj > > -- > Wei-Ren Chen (陳韋任) > Computer Systems Lab, Institute of Information Science, > Academia Sinica, Taiwan (R.O.C.) > Tel:886-2-2788-3799 #1667 > Homepage: http://people.cs.nctu.edu.tw/~chenwj
What i mean here is that in gen-icount.h, the icount_decr.u32 field is exhausted and jumps out to the cpu-exec loop. This is where you would fall into tb_add_jump with the second argument being 2, and only if icount_extra was greater than zero. This is what I meant, and previously you showed me that the chaining is left intact by the check in tb_add_jump. Hope this is clearer. -Jim
