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

Robinson, Paul via cfe-dev cfe-dev at lists.llvm.org
Tue Mar 29 23:20:24 PDT 2016


Skipping a serialization and doing something clever about LTO uniquing sounds awesome.  I'm guessing you achieve this by extracting types out of DI metadata and packaging them as lumps-o-DWARF that the back-end can then paste together?  Reading between the lines a bit here.
Can you share data about how much "pure" types dominate the size of debug info?  Or at least the current metadata scheme?  (Channeling Sean Silva here: show me the data!)  Does this hold for C as well as C++?
Not much discussion of data objects and code objects (other than concrete subprograms), is that because they basically aren't changing?  Still defined in the metadata and still managed/emitted by the back-end?
Please say something about types (which you're thinking of as a front-end thing) defined within scopes (which it looks like you're thinking of as a back-end thing).  Not seeing how to get the scoping right.

Thanks!
--paulr

From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of Eric Christopher via cfe-dev
Sent: Tuesday, March 29, 2016 6:01 PM
To: Clang Dev; llvm-dev
Subject: [cfe-dev] RFC: Up front type information generation in clang and llvm

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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