llvm dev - Mar 2016 - [cfe-dev] RFC: Up front type information generation in clang and llvm

Hi Aboud,
> On Mar 31, 2016, at 11:06 AM, Aboud, Amjad via cfe-dev <cfe-dev at
lists.llvm.org> wrote:
> 
> Hi Eric,
> I can understand the need for improving the current design of debug info
representation and emission in LLVM.
> However, let’s not forget that the motivation was and still to support
CodeView debug info emission.
Well, that is *one* motivation.
 
> I am wondering if it is right to spend the huge effort needed to implement
the below proposal while knowing these facts:
> 1.      It would be more clear how to improve the design when we have a
working CodeView support.
> You said it yourself, that we still do not know what challenges we will
face while implementing this proposal.
> 2.      I understand that CodeView will need some extra extensions to
current dwarf debug info, like ‘this’ adjustment.
> However, it is doable to introduce a CodeView wrapper data structures that
can be created from current dwarf debug info IR.
> And this can be done in CodeGen (e.g. CodeViewDebug.cpp) while emitting the
code/debug info.
>  
> Again, I understand that your proposal is trying to improve a lot of things
Yes, and to give some different perspective: some of these "things"
are a lot higher priority than CodeView (for other people/use cases of course),
because DebugInfo cost is prohibitive for some use cases.
> , but it seems that we should first try support CodeView debug info with
the current debug info IR.
> The advantages:
> 1.      It works, even though you still have doubts about few issues, I
believe we can resolve them with minimum modification to the LLVM IR/Clang FE.
> 2.      It requires much smaller effort.
> 3.      It is much clean.
If it is "much cleaner" in the IR, I understand that you have insights
about Eric's proposal being "less clean", independently of adding
CodeView before or after this change. If so it's worth elaborating on this.

> 4.      We will understand more the requirements needed by CodeView that
can be used to improve the below proposal (before diving into implementing it).
Don't you forget the "Cons":

1) It is easier to perform large refactoring/changes to the debug info flow
*before* complexifying the problem.
2) This is adding more stuff that will need to go through all these changes,
wasting effort in the process.
3) It will limit forward progress for people who don't care about CodeView
but want to move forward with restructuring DI deeply, like Eric's proposal
is doing.

That is not to say that your points are not valid, but that it's not that
clear cut either.

-- 
Mehdi

>  
> I suggest that we start with:
> 1.      Define the CodeView wrapper data structure. (CodeViewDebugIR)
> 2.      Build the CodeView wrapper data structure based on dwarf debug info
IR. (CodeViewDebugBuilder)
> 3.      Emit the CodeView wrapper data structure into COFF object file.
(CodeViewDebugEmitter)
> 4.      Figure out what modification/extension need to be done to dwarf
debug info IR/Clang FE.
>  
> What do you think?
>  
> Thanks,
> Amjad
>   <>
>  <>From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org
<mailto:llvm-dev-bounces at lists.llvm.org>] On Behalf Of Eric Christopher
via llvm-dev
> Sent: Wednesday, March 30, 2016 04:01
> To: Clang Dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at
lists.llvm.org>>; llvm-dev <llvm-dev at lists.llvm.org
<mailto:llvm-dev at lists.llvm.org>>
> Subject: [llvm-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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