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

[David Blaikie]

On Thu, 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).)
>
> 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 the strings are deduplicated elsewhere, that should be fine. (I think I
made the change originally to pull out the names because of the duplication
I saw in many constructsn needing to reference the file/directory and they
all contained the same file/directory)

I take it you'll have other constructs (I think all scopes have a
file/directory) reference the MDFile directly?


>
> 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.)
>
>
>
>
>
>
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