[LLVMdev] Questions before moving the new debug info hierarchy into place

[Adrian Prantl]

> On Feb 19, 2015, at 7:49 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> I'm getting close to executing the transition to the new debug info
> hierarchy.  For reference, I've attached two WIP patches (which would be
> squashed before commit) and the WIP upgrade script I'm using.
> 
>    - transition-code.patch: Change the `DIDescriptor` hierarchy to
>      lightweight wrappers around the subclasses of `DebugNode` (instead
>      of rather heavier wrappers around `MDTuple`), and update
>      `DIBuilder` to generate the right things.
>    - transition-testcases.patch: Upgrade of testcases, entirely
>      generated from the upgrade script (so far).
>    - upgrade-specialized-nodes.sh: Script to upgrade LLVM assembly.
> 
> (Feel free to have a look, but I haven't updated LangRef, I don't quite
> have `make check` passing, and I haven't even started on `make
> check-clang` (I imagine that'll all be done by hand).)
> 
> There are two outstanding issues I'd like some feedback on.
> 
> Pretty-printing the flags
> =========================
> 
> I've noticed the `flags:` field is *harder* to read in the new assembly:
> 
>    !MDDerivedType(flags: 16384, ...)
> 
> than the pretty-printed comments in the old:
> 
>    !{!"...\\0016384", ...} ; ... [public] [rvalue reference]
> 
> I don't want to regress here.
> 
> In `DIDescriptor`, the flags are described in an enum bitfield:
> 
>    FlagAccessibility     = 1 << 0 | 1 << 1,
>    FlagPrivate           = 1,
>    FlagProtected         = 2,
>    FlagPublic            = 3,
>    FlagFwdDecl           = 1 << 2,
>    FlagAppleBlock        = 1 << 3,
>    FlagBlockByrefStruct  = 1 << 4,
>    FlagVirtual           = 1 << 5,
>    FlagArtificial        = 1 << 6,
>    FlagExplicit          = 1 << 7,
>    FlagPrototyped        = 1 << 8,
>    FlagObjcClassComplete = 1 << 9,
>    FlagObjectPointer     = 1 << 10,
>    FlagVector            = 1 << 11,
>    FlagStaticMember      = 1 << 12,
>    FlagLValueReference   = 1 << 13,
>    FlagRValueReference   = 1 << 14
> 
> I think the right short-term solution is to use these names directly in
> assembly, giving us:
> 
>    !MDDerivedType(flags: FlagPublic | FlagRValueReference, ...)
> 
> This is easy to implement and easy to CHECK against.
> 
> Sound good?
> 
> (Eventually, I'd like to use the `DW_AT` symbols that each of these
> corresponds to, but `FlagStaticMember` doesn't seem to line up with any
> such `DW_AT` so that will take some refactoring (and I don't think it
> makes sense for that to block moving the hierarchy into place).)

I support this proposal under the condition that it is only an intermediate step.

What I would like to evolve this to is a more generic infrastructure were we have a list of “trivial" attributes (= attributes that don’t need any special backend support) so we could write things like
   MDCompositeType(attributes: [DW_AT_artificial, DW_AT_rvalue_reference, DW_AT_public], ...)

perhaps even
   MDCompositeType(attributes: [DW_AT_vtable_elem_location(DIExpression(DW_OP_constu(3)))], ...)
?

[ Looks nice, but it isn’t perfect, of course. For example: DW_AT_rvalue_reference is technically only available in DWARF5. Should the frontend then have to know/care about these details, or should the backend perform a lowering from a DWARF5-based IR to whatever the module flags requested? ]

-- adrian


> 
> Merging the two types of files
> ==============================
> 
> In the old format, we have two types of files -- an untagged file node,
> and a DIFile/DW_TAG_file_type/0x29 which references the untagged node.
> 
>    !0 = !{!"path/to/file", !"/path/to/dir"}
>    !1 = !{!"0x29", !0}
> 
> In the actual metadata graph, most file references use !0, but in
> DIBuilder !1 is always passed in and the !0 is extracted from it.
> 
> I've been planning to merge these into:
> 
>    !1 = !MDFile(filename: "path/to/file", directory: "/path/to/dir")
> 
> Anyone see a problem with that?  Why?
> 
> If not, I'd like to roll that change into the same patch in order to
> reduce testcase churn.  (I've discovered that it won't complicate the
> code patch at all.)
> 
> <transition-code.patch><transition-testcases.patch><upgrade-specialized-nodes.sh>
>