Re: [gcc] Enable DW_OP_VAL_EXPRESSION support in dwarf module

[Jiong Wang <jiong dot wang at foss dot arm dot com>]

On 01/11/16 15:24, Jason Merrill wrote:
> On Tue, Nov 1, 2016 at 11:12 AM, Jiong Wang <jiong.wang@foss.arm.com> wrote:
> > On 31/10/16 19:50, Jason Merrill wrote:
> > > On 10/21/2016 04:30 AM, Jiong Wang wrote:
> > > > All DW_OP_* of the expression are grouped together inside the PARALLEL,
> > > > and those operations which don't have RTL mapping are wrapped by
> > > > UNSPEC.  The parsing algorithm is simply something like:
> > > >
> > > >    foreach elem inside PARALLEL
> > > >      if (UNSPEC)
> > > >        {
> > > >          dw_op_code = INTVAL (XVECEXP (elem, 0, 0));
> > > >          oprnd1 = INTVAL (XVECEXP (elem, 0, 1));
> > > >          oprnd2 = INTVAL (XVECEXP (elem, 0, 2));
> > > >        }
> > > >      else
> > > >        call standard RTL parser.
> > > >
> > > > Any comments on the approach?
> > >
> > > If you're going to use UNSPEC, why not put the DWARF operator in the
> > > second operand?
> >    Thanks for the review, but I still don't understand your meaning.
> >
> >    Do you mean I should simply put the DWARF operator at XVECEXP (UNSPEC_RTX,
> > 0, 2) instead of at XVECEXP (UNSPEC_RTX, 0, 0)
> No, at XINT (UNSPEC_RTX, 1).  The documentation of UNSPEC says,
>
> /* A machine-specific operation.
>     1st operand is a vector of operands being used by the operation so
> that
>       any needed reloads can be done.
>     2nd operand is a unique value saying which of a number of
> machine-specific
>       operations is to be performed.

Aha, understood now, thanks for the clarification.

You mean we simply reuse the UNSPEC number field, so the RTX will be

      (UNSPEC
         [((reg) (reg)]
       DW_OP_XXX)

Yeah, I do have tried to do that, but later give up, one reason I 
remember is suppose we
want to push two value on the stack, the second value is an address, 
which we want a
follow up DW_OP_deref to operate on that. then the value expression will be

   (set (reg A)
    (parallel
      [(reg A)

        (UNSPEC
          [DW_OP_deref, const0_rtx, const0_rtx]
        UNSPEC_PRIVATE_DW);

       (UNSPEC
         [DW_OP_XXX (const0_rtx) (const0_rtx)]
        UNSPEC_PRIVATE_DW))

And there might be some other expressions we need some complex RAW 
encoding, so it seems to
me if we want to offer user the most general way to do this, then it's 
better to encode the DWARF
operation inside UNSPEC as reuse the UNSPEC number then you need to make 
sure there is no
overlap with other backend UNSPEC enumeration number.

Does this explanation make sense to you?

Thanks.