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Re: [Qemu-devel] [RFC] [tcg] Idea on refactoring target code generation

From: Lluís Vilanova
Subject: Re: [Qemu-devel] [RFC] [tcg] Idea on refactoring target code generation loop (gen_intermediate_code)
Date: Wed, 09 Mar 2016 19:16:20 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Richard Henderson writes:

> On 03/09/2016 09:38 AM, Lluís Vilanova wrote:
>> Hi,
>> NOTE: I won't be throwing patches anytime soon, I just want to know if 
>> there's
>> interest in this for the future.
>> While adding events for tracing guest instructions, I've found that the
>> per-target "gen_intermediate_code()" function is very similar but not exactly
>> the same for each of the targets. This makes architecture-agnostic features
>> harder to maintain across targets, specially when it comes to their relative
>> order.
>> So, would it be worth it if I generalized part of that code into an
>> architecture-agnostic function that calls into target-specific hooks 
>> wherever it
>> needs extending? There are many ways to do it that we can discuss later.

> It's worth talking about, since I do believe it would make long-term 
> maintenance
> across the targets easier.

> These "target-specific hooks" probably ought not be "hooks" in the
> traditional sense of attaching them to CPUState.  I'd be more comfortable 
> with a
> refactoring that used include files -- maybe .h or maybe .inc.c.  If we do the
> normal sort of hook, then we've got to either expose DisasContext in places we
> shouldn't, or dynamically allocate it.  Neither seems particularly appealing.

Great. I pondered about using QOM vs "template code", and I'm leaning towards
the latter:

* translate.c:

  struct DisasContextArch {
      DisasContext common;

  // implement "hooks"

  void gen_intermediate_code(CPUArchState *env, TranslationBlock *tb)
     DisasContextArch dc;
     gen_intermediate_code_template(get_cpu(env), &dc.common, tb);

* translate-template.h:

  struct DisasContext { /* ... */ };

  void gen_intermediate_code_template(CPUState *cpu, DisasContext *dc,
                                      TranslationBlock *tb)
      // init dc
      // arch-specific init dc hook

      // gen_icount, etc.
      while (true) {
          // generic processing code with calls to hooks

While initially simpler, the "template code" still feels a little dirty to
me. With QOM, you could implement a DisasContextClass that provides the
per-target hook pointers, which can be globally allocated once per target and
pointed to by the DisasContext allocated in stack.

I'm not sure about what you mean by exposing DisasContext in places it shouldn't
be, though.


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