Re: [Qemu-devel] Questions/comments on TCG

[Stuart Brady <sdbrady@ntlworld.com>]

On Fri, Mar 07, 2008 at 06:07:32PM +0200, Blue Swirl wrote:
> On 3/7/08, Stuart Brady <sdbrady@ntlworld.com> wrote:
> >  I do understand that the current SPARC TCG code is preliminary work.
> >  However, in some ways, I feel it still serves as a better reference than
> >  i386 and x86_64
> 
> Well, I'd still recommend using x86 as a reference until Sparc works
> or you may copy a faulty design.

Don't worry -- I still checked with the x86 targets.  I only really
needed a quick idea of what was required for a new target.

> >  Which registers should go in tcg_target_reg_alloc_order[]?  I notice
> >  that i386 includes ESP, which tcg_target_init() marks as reserved, and
> >  x86_64 includes RBX and RBP, which are again marked as reserved.
> 
> I put there only the registers that should be safe to use, the G
> registers may have issues or they are already used as global
> registers. Also we should not need frame pointer.

Sounds reasonable.  I think I really meant to ask what _shouldn't_ go in
tcg_target_reg_alloc_order[].  I was mainly confused by the inclusion of
registers which are marked as reserved on the x86 targets.

> >  Furthermore, x86_64's tcg_target_reg_alloc_order[] contains 16 elements
> >  (TCG_TARGET_NB_REGS), but only 15 are specified -- the last element is
> >  left as 0, which is TCG_REG_RAX.  SPARC also does this, but with
> >  TARGET_REG_G0 (which is marked as reserved, as it's hardwired to zero).
> 
> Maybe I missed something, but g0 isn't in the reg_alloc_order?

tcg_target_reg_alloc_order[] has 32 elements, but only 14 are used.
The rest hold 0, specifying TCG_REG_G0.

> >  On SPARC, I notice that goto_tb is handled using CALL and JMPL, placing
> >  the return address in o7... but we're returning from a TB, or jumping to
> >  another one, so surely we shouldn't link here?  Also, TCG_TYPE_TL is
> >  used for exit_tb's return value, I think this should be the host's long
> >  (using TCG_TYPE_PTR) instead.
> 
> These are bugs, thanks for spotting. I was using o7 if a register is
> needed, it will be clobbered anyway.

I don't understand -- o7 is required when returning in exit_tb, so if it
is used, it must be saved and restored.

> >  Also on SPARC, could the indentation of the OP_32_64s be improved?
> >  Yeah, it's not a serious problem, but I feel it would make the code
> >  slightly easier to read.
> 
> It's not my fault, Emacs wants to do it this way. I'm open to your
> suggestions.

Oh dear, I'm such a vim user, I don't even have Emacs installed. :)

How about something like this?

#if defined(__sparc_v9__) && !defined(__sparc_v8plus__)
#define OP_32_64(x)                             \
        glue(glue(case INDEX_op_, x), _i32:)    \
        glue(glue(case INDEX_op_, x), _i64)
#else
#define OP_32_64(x)                             \
        glue(glue(case INDEX_op_, x), _i32)
#endif
...
    OP_32_64(ld8u):
        tcg_out_ldst(s, args[0], args[1], args[2], LDUB);
        break;
...

The macro might be a bit sick, but hopefully it would make Emacs happy,
and I feel ':' does make a certain amount of sense, here.

It probably wouldn't help with indentation, but you could always do
something like this:

#if defined(__sparc_v9__) && !defined(__sparc_v8plus__)
#define v9(x) x
#else
#define v9(x)
#endif
...
    case INDEX_op_ld8u_i32:
v9( case INDEX_op_ld8u_i64: )
        tcg_out_ldst(s, args[0], args[1], args[2], LDUB);
        break;
...

I'll admit, that looks unusual, but it would avoid breaking searches for
ld8u_i32 or op_ld8u.

Cheers,
-- 
Stuart Brady