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

[Eric Christopher via cfe-dev]

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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