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

[Adrian Prantl via cfe-dev]

> On Mar 29, 2016, at 6:00 PM, Eric Christopher <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.

This is a bit of an overstatement: This proposal is only about the debug *type* information. The back end still needs to know about line tables, subprograms, etc., and in order to produce the Apple/DWARF5 accelerator tables it will even need to have a basic understanding of the type info.

> 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.

We should back up this claim with some numbers, but the idea is that the expected savings come from the “type units” being variable-length records with abbreviations not unlike LLVM bitcode. In contrast to LLVM metadata, however, there is also some additional overhead due to each “type unit” containing a redundant declcontext, the support for relocations, and potentially for supporting the accelerator tables.

> 
> 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.

David, can you elaborate on this?

> 
> 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.

I realized that the last two bullet points would not work well with ThinLTO. One of its selling points is (fast) incremental rebuilds, and requiring dsymutil to make a second pass over all the object files is negating this benefit.

-- adrian
> 
> 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.