[llvm-dev] RFC: Up front type information generation in clang and llvm

[Frédéric Riss via llvm-dev]

> On Apr 27, 2016, at 4:41 PM, Reid Kleckner <rnk at google.com> wrote:
> 
> My general feeling is that this design represents a mid-point between our current metadata design, and a future design where frontends just emit type information and LTO links it in a format-aware way.
> 
> I don't think it's an imminent priority for anyone to do this for DWARF, so I worry that if we start building infrastructure for it, it will end up overengineered.
> 
> Also, people seem to agree that in the long term, we really need a format-aware linker, and maybe LTO should just use one. Supposedly Frédéric has patches to llvm-dsymutil to make one for DWARF, but he hasn't found the time to upstream them.

There are pieces missing upstream — mostly the accelerator tables — and I’m really struggling to find time to upstream these. However, the DIE tree linking part of upstream llvm-dsymutil is complete. That’s not to say that it would be easy to use it as a generic DWARF linker. I tried to make it as agnostic to the platform as I could, but it was designed to be bit-for-bit compatible with the original dsymutil and that surely made it a lot less generic.

Would you envision the format-aware link to take place during the LTO link? This would seem pretty expensive to me (DWARF linking is not really cheap, as it’s not a format designed for this). I think it would make more sense to leave the type info in the object files and to somehow have the LTO link emit external references to it (ala module debugging). Then have the debug info link happen as an explicit step; this matches the Darwin model, but not the usual *nix model.

Fred

> Together, these reasons make me feel that we should limit the short-term scope to just CodeView, and add utilities to lib/Linker for performing basic tasks like type stream merging or type extraction, possibly with forward declaration of composite types.
> 
> In the future, when we do this work for DWARF, we can add a new DIType* stand-in similar to what you are describing.
> 
> The working patch that I have for just CodeView, all types as a single blob, is up here: http://reviews.llvm.org/D19236 <http://reviews.llvm.org/D19236> While it doesn't deal with type blobs or LTO type merging yet, I think it shows that there is surprisingly little need to bifurcate other parts of LLVM.
> 
> Thoughts?
> 
> On Tue, Mar 29, 2016 at 6:00 PM, Eric Christopher <echristo at gmail.com <mailto:echristo at gmail.com>> wrote:
> Hi All,
> 
> This is something that's been talked about for some time and it's probably time to propose it.
> 
> The "We" in this document is everyone on the cc line plus me.
> 
> Please go ahead and take a look.
> 
> Thanks!
> 
> -eric
> 
> 
> Objective (and TL;DR)
> =================
> 
> Migrate debug type information generation from the backends to the front end.
> 
> This will enable:
> 1. Separation of concerns and maintainability: LLVM shouldn’t have to know about C preprocessor macros, Obj-C properties, or extensive details about debug information binary formats.
> 2. Performance: Skipping a serialization should speed up normal compilations.
> 3. Memory usage: The DI metadata structures are smaller than they were, but are still fairly large and pointer heavy.
> 
> Motivation
> ========
> 
> Currently, types in LLVM debug info are described by the DIType class hierarchy. This hierarchy evolved organically from a more flexible sea-of-nodes representation into what it is today - a large, only somewhat format neutral representation of debug types. Making this more format neutral will only increase the memory use - and for no reason as type information is static (or nearly so). Debug formats already have a memory efficient serialization, their own binary format so we should support a front end emitting type information with sufficient representation to allow the backend to emit debug information based on the more normal IR features: functions, scopes, variables, etc.
> 
> Scope/Impact
> ===========
> 
> This is going to involve large scale changes across both LLVM and clang. This will also affect any out-of-tree front ends, however, we expect the impact to be on the order of a large API change rather than needing massive infrastructure changes.
> 
> Related work
> ==========
> 
> This is related to the efforts to support CodeView in LLVM and clang as well as efforts to reduce overall memory consumption when compiling with debug information enabled;  in particular efforts to prune LTO memory usage.
> 
> 
> Concerns
> ========
> 
> 
> We need a good story for transitioning all the debug info testcases in the backend without giving up coverage and/or readability. David believes he has a plan here.
> 
> Proposal
> =======
> 
> Short version
> -----------------
> 
> 1. Split the DIBuilder API into Types (+Macros, Imports, …) and Line Table.
> 2. Split the clang CGDebugInfo API into Types and Line Table to match.
> 3. Add a LLVM DWARF emission library similar to the existing CodeView one.
> 4. Migrate the Types API into a clang internal API taking clang AST structures and use the LLVM binary emission libraries to produce type information.
> 5. Remove the old binary emission out of LLVM.
> 
> 
> Questions/Thoughts/Elaboration
> -------------------------------------------
> 
> Splitting the DIBuilder API
> ~~~~~~~~~~~~~~~~~~~~
> Will DISubprogram be part of both?
>    * We should split it in two: Full declarations with type and a slimmed down version with an abstract origin.
> 
> How will we reference types in the DWARF blob?
>    * ODR types can be referenced by name
>    * Non-odr types by full DWARF hash
>    * Each type can be a pair(tuple) of identifier (DITypeRef today) and blob.
>    * For < DWARF4 we can emit each type as a unit, but not a DWARF Type Unit and use references and module relocations for the offsets. (See below)
> 
> How will we handle references in DWARF2 or global relocations for non-type template parameters?
>    * We can use a “relocation” metadata as part of the format.
>    * Representable as a tuple that has the DIType and the offset within the DIBlob as where to write the final relocation/offset for the reference at emission time.
> 
> Why break up the types at all?
>    * To enable non-debug format aware linking and type uniquing for LTO that won’t be huge in size. We break up the types so we don’t need to parse debug information to link two modules together efficiently.
> 
> Any other concerns there?
>    * Debug information without type units might be slightly larger in this scheme due to parents being duplicated (declarations and abstract origin, not full parents). It may be possible to extend dsymutil/etc to merge all siblings into a common parent. Open question for better ways to solve this.
> 
> How should we handle DWARF5/Apple Accelerator Tables?
>    * Thoughts:
>    * We can parse the dwarf in the back end and generate them.
>    * We can emit in the front end for the base case of non-LTO (with help from the backend for relocation aspects).
>    * We can use dsymutil on LTO debug information to generate them.
> 
> Why isn’t this a more detailed spec?
>    * Mostly because we’ve thought about the issues, but we can’t plan for everything during implementation.
> 
> 
> Future work
> ----------------
> 
> Not contained as part of this, but an obvious future direction is that the Module linker could grow support for debug aware linking. Then we can have all of the type information for a single translation unit in a single blob and use the debug aware linking to handle merging types.
> 

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