Re: [Qemu-devel] [PATCH 4/7] tcg: Add support for "inlining" regions of code

["Lluís Vilanova" <vilanova@ac.upc.edu>]

Richard Henderson writes:

> On 09/10/2017 09:27 AM, Lluís Vilanova wrote:
>> TCG BBLs and instructions have multiple exit points from where to raise
>> tracing events, but some of the necessary information in the generic
>> disassembly infrastructure is not available until after generating these
>> exit points.
>> 
>> This patch adds support for "inline points" (where the tracing code will
>> be placed), and "inline regions" (which identify the TCG code that must
>> be inlined). The TCG compiler will basically copy each inline region to
>> any inline points that reference it.

> I am not keen on this.

> Is there a reason you can't just emit the tracing code at the appropriate place
> to begin with?  Perhaps I have to wait to see how this is used...

As I tried to briefly explain on next patch, the main problem without inlining
is that we will see guest_tb_after_trans twice on the trace for each TB in
conditional instructions on the guest, since they have two exit points (which we
capture when emitting goto_tb in TCG).

We cannot instead emit it only once by overloading the brcond opcode in TCG,
since that can be used internally in the guest instruction emulation without
necessarily ending a TB (or we could have more than one brcond for a single
instruction).

I hope it's clearer now.


Thanks,
  Lluis