Sure, usually a tb chain is setup after a subsequent tb is found/constructed in the loop in cpu_exec when a tb returns. Taken/non-taken branch chaining is implemented by indicating the branch direction by the two least significant digits of the the 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
On Thu, Jan 12, 2012 at 7:41 PM, 陳韋任 <che...@iis.sinica.edu.tw> wrote: > On Thu, Jan 12, 2012 at 11:00:43AM -0800, James Greensky wrote: >> Hello all, I have a question about icount and tb chaining that I hope >> somebody can clear up. In cpu-exec.c, when the icount_decr.u16.low >> counter expires, it passes back the current tb as the next_tb and add >> a jump with the least significant bits = 2. This falls through to tb >> add jump, which then updates the jmp_first field of the current tb. >> why is this done? Thanks -Jim > > Could you elaborate on the whole sequence? I am not sure where should > I look into to answer your question. Thanks. > > 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
