[PATCH] D18918: [DebugInfo] Try to make class memory layout more efficient

[David Blaikie via llvm-commits]

Certainly seems like a thing we might want to try - though it'd help to
have a reminder/description of why shrinking these fields to these specific
bit sizes is valid (are the already restricted to those sizes in other
places/ways, for example - point to where/how they're already so restricted)

On Fri, Apr 8, 2016 at 7:08 PM, Davide Italiano via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> davide created this revision.
> davide added reviewers: dexonsmith, aprantl, echristo, dblaikie.
> davide added a subscriber: llvm-commits.
>
> I started this effort after noticing that DI shows up as a considerable
> portion of memory usage during an LTO build (of clang, and others).
> This is roughly the data I got (peak usage):
> DISubprogram: 228MB
> DILocalVariable: 164MB
> The proposed patch packs field slightly more efficiently reducing the size
> of DISubprogram from 48bytes to 40bytes and of DILocalVariable from 40
> bytes to 32 bytes.
> The saving overall in memory is ~70MB.
> This is still to be considered WIP as this breaks 4 tests in the LLVM
> suite (which I'm currently investigating), but I wanted to have some eyes
> on it as DebugInfo seems to be of interest of many these days.
>
> http://reviews.llvm.org/D18918
>
> Files:
>   include/llvm/IR/DebugInfoMetadata.h
>
> Index: include/llvm/IR/DebugInfoMetadata.h
> ===================================================================
> --- include/llvm/IR/DebugInfoMetadata.h
> +++ include/llvm/IR/DebugInfoMetadata.h
> @@ -1229,21 +1229,21 @@
>
>    unsigned Line;
>    unsigned ScopeLine;
> -  unsigned Virtuality;
>    unsigned VirtualIndex;
> -  unsigned Flags;
> -  bool IsLocalToUnit;
> -  bool IsDefinition;
> -  bool IsOptimized;
> +  unsigned Virtuality:2;
> +  unsigned Flags:27;
> +  bool IsLocalToUnit:1;
> +  bool IsDefinition:1;
> +  bool IsOptimized:1;
>
>    DISubprogram(LLVMContext &C, StorageType Storage, unsigned Line,
> -               unsigned ScopeLine, unsigned Virtuality, unsigned
> VirtualIndex,
> +               unsigned ScopeLine, unsigned VirtualIndex, unsigned
> Virtuality,
>                 unsigned Flags, bool IsLocalToUnit, bool IsDefinition,
>                 bool IsOptimized, ArrayRef<Metadata *> Ops)
>        : DILocalScope(C, DISubprogramKind, Storage,
> dwarf::DW_TAG_subprogram,
>                       Ops),
> -        Line(Line), ScopeLine(ScopeLine), Virtuality(Virtuality),
> -        VirtualIndex(VirtualIndex), Flags(Flags),
> IsLocalToUnit(IsLocalToUnit),
> +        Line(Line), ScopeLine(ScopeLine), VirtualIndex(VirtualIndex),
> +        Virtuality(Virtuality), Flags(Flags),
> IsLocalToUnit(IsLocalToUnit),
>          IsDefinition(IsDefinition), IsOptimized(IsOptimized) {}
>    ~DISubprogram() = default;
>
> @@ -1858,8 +1858,8 @@
>    friend class LLVMContextImpl;
>    friend class MDNode;
>
> -  unsigned Arg;
> -  unsigned Flags;
> +  unsigned Arg:16;
> +  unsigned Flags:16;
>
>    DILocalVariable(LLVMContext &C, StorageType Storage, unsigned Line,
>                    unsigned Arg, unsigned Flags, ArrayRef<Metadata *> Ops)
>
>
>
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