[PATCH] AsmWriter/Bitcode: Support specialized debug nodes

[Duncan P. N. Exon Smith]

> On 2015-Feb-10, at 16:05, Adrian Prantl <aprantl at apple.com> wrote:
> 
> A few nitpicky suggestions and questions:
> 
> - !MDSubrange(count: 30, lo: 2)
>  what about spelling out “low” and “high”?

It doesn't mean "low" and "high", though, right?  It means
"count"/"size" and "index of first element".

Or have I misinterpreted it?

How about "count" and "low"?

> 
> - do we still need the uniqueID on MDLexicalBlock? Could it be a distinct node instead?

I think this should be distinct instead.  I was planning to do this
as part of the upgrade (moving them into place).

> 
> - is it possible to have it print DW_OP-* constants such as in
>  !MDExpression(DW_OP_deref, DW_OP_bit_piece, 0, 8)
>  ?

I think this is pretty difficult the way that expressions are stored
right now -- it requires knowing how many arguments each thing takes,
etc.  In particular, how would the `AsmWriter` know whether something
should have a symbolic constant (`DW_OP_deref`) vs. being the number 6?

I was thinking of trying to clean this up later, but since we're
talking about it:

I think instead of `std::vector<uint64_t>` it should be
`std::vector<ExprOperand>`, where:

    struct ExprOperand {
      LocationAtom Kind;
      unsigned NumArgs;
      uint64_t Args[2];
      ExprOperand(LocationAtom Kind);
      ExprOperand(LocationAtom Kind, uint64_t Arg);
      ExprOperand(LocationAtom Kind, uint64_t Arg1, uint64_t Arg2);

      static ExprOperand getDeref() {
        return ExprOperand(dwarf::DW_OP_deref);
      }
      static ExprOperand getPiece(uint64_t A1, uint64_t A2) {
        return ExprOperand(dwarf::DW_OP_bit_piece, A1, A2);
      }
    };

    ExprOperand Ops = {
      ExprOperand::getDeref(),
      ExprOperand::getPiece(0, 8)
    };
    auto *Expr = MDExpression::get(Ops);

Then the above could trivially be pretty-printed/parsed as:

    !MDExpression(DW_OP_deref, DW_OP_bit_piece(0, 8))

Thoughts?


> 
> Will it still print the old-style comments that testcases tend to match on, or are they obsoleted by the human-readable syntax?

They're obsoleted.  The old-style comments will get stale and just
add noise.  If we've regressed somehow let me know how.

> 
> -- adrian
> 
> 
>