[LLVMdev] [RFC] Less memory and greater maintainability for debug info IR

David Blaikie dblaikie at gmail.com
Mon Oct 13 15:23:00 PDT 2014

On Mon, Oct 13, 2014 at 3:02 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

> In r219010, I merged integer and string fields into a single header
> field.  By reducing the number of metadata operands used in debug info,
> this saved 2.2GB on an `llvm-lto` bootstrap.  I've done some profiling
> of DW_TAGs to see what parts of PR17891 and PR17892 to tackle next, and
> I've concluded that they will be insufficient.

Could you explain what your end-goal here looked like and what data you
used to evaluate its insufficiency?

Just to be clear, what I was picturing was that, starting with your initial
improvement, we'd string-ify more data in the records but eventually we'd
start stringifying across records (eg: rolling a DW_TAG_structure_type's
members into the structure type itself, one big string). In the end we'd
just pull out the non-metadata references (like the llvm::Function* in the
DW_TAG_subroutine_type metadata) into a table kept separately from a
handful of big strings of debug info (I say a handful, as we'd keep the
types separate so they could be easily deduplicated).

> Instead, I'd like to implement a more aggressive plan, which as a
> side-effect cleans up the much "loved" debug info IR assembly syntax.
> At a high-level, the idea is to create distinct subclasses of `Value`
> for each debug info concept,

My concern with this is baking parts of our current debug info
representation into IR constructs seems rather heavyweight. If we need to
add first class IR constructs to cope with debug info I'd hope to find,
ideally, one, general purpose extension we can use for this (& possibly for
other things). But maybe the bar for adding first class IR constructs is
lower than I've imagined it to be.

> starting with line table entries and moving
> on to the DIDescriptor hierarchy.  By leveraging the use-list
> infrastructure for metadata operands -- i.e., only using value handles
> for non-metadata operands -- we'll improve memory usage and increase
> RAUW speed.
> My rough plan follows.  I quote some numbers for memory savings below
> based on an -flto -g bootstrap of `llvm-lto` (i.e., running `llvm-lto`
> on `llvm-lto.lto.bc`, an already-linked bitcode file dumped by ld64's
> -save-temps option) that currently peaks at 15.3GB.
>  1. Introduce `MDUser`, which inherits from `User`, and whose `Use`s
>     must all be metadata.  The cost per operand is 1 pointer, vs. 4
>     pointers in an `MDNode`.

Perhaps a generic MD-only-node might be a sufficiently generically valuable
IR construct.

A similar alternative: A schematized metadata node. Much like DWARF, being
able to say "this node is of some type T, defined elsewhere in the module -
string, int, string, string, etc... ". Heck, this could even be just a
generic improvement to llvm IR, maybe? (the textual representation might
not need to change at all - IR Generation would just do much like DWARF
generation in LLVM does - create abbreviation/type descriptions on the fly
and share them rather than having every metadata node include its own

>  2. Create `MDLineTable` as the first subclass of `MDUser`.  Use normal
>     fields (not `Value`s) for the line and column, and use `Use`
>     operands for the metadata operands.
>     On x86-64, this will save 104B / line table entry.  Linking
>     `llvm-lto` uses ~7M line-table entries, so this on its own saves
>     ~700MB.
>     Sketch of class definition:
>         class MDLineTable : public MDUser {
>           unsigned Line;
>           unsigned Column;
>         public:
>           static MDLineTable *get(unsigned Line, unsigned Column,
>                                   MDNode *Scope);
>           static MDLineTable *getInlined(MDLineTable *Base, MDNode *Scope);
>           static MDLineTable *getBase(MDLineTable *Inlined);
>           unsigned getLine() const { return Line; }
>           unsigned getColumn() const { return Column; }
>           bool isInlined() const { return getNumOperands() == 2; }
>           MDNode *getScope() const { return getOperand(0); }
>           MDNode *getInlinedAt() const { return getOperand(1); }
>         };
>     Proposed assembly syntax:
>         ; Not inlined.
>         !7 = metadata !MDLineTable(line: 45, column: 7, scope: metadata !9)
>         ; Inlined.
>         !7 = metadata !MDLineTable(line: 45, column: 7, scope: metadata !9,
>                                    inlinedAt: metadata !10)
>         ; Column defaulted to 0.
>         !7 = metadata !MDLineTable(line: 45, scope: metadata !9)
>     (What colour should that bike shed be?)
>  3. (Optional) Rewrite `DebugLoc` lookup tables.  My profiling shows
>     that we have 3.5M entries in the `DebugLoc` side-vectors for 7M line
>     table entries.  The cost of these is ~180B each, for another
>     ~600MB.
>     If we integrate a side-table of `MDLineTable`s into its uniquing,
>     the overhead is only ~12B / line table entry, or ~80MB.  This saves
>     520MB.

>     This is somewhat perpendicular to redesigning the metadata format,
>     but IMO it's worth doing as soon as it's possible.
>  4. Create `GenericDebugMDNode`, a transitional subclass of `MDUser`
>     through an intermediate class `DebugMDNode` with an
>     allocation-time-optional `CallbackVH` available for referencing
>     non-metadata.  Change `DIDescriptor` to wrap a `DebugMDNode` instead
>     of an `MDNode`.
>     This saves another ~960MB,

960 from what?

> for a running total of ~2GB.

~2GB is the total of what? (you mention a lot of numbers in this post, but
it's not always clear what they're relative to/out of/subtracted from)

>     Proposed assembly syntax:
>         !7 = metadata !GenericDebugMDNode(tag: DW_TAG_compile_unit,
>                                           fields: "0\00clang 3.6\00...",
>                                           operands: { metadata !8, ... })
>         !7 = metadata !GenericDebugMDNode(tag: DW_TAG_variable,
>                                           fields: "global_var\00...",
>                                           operands: { metadata !8, ... },
>                                           handle: i32* @global_var)
>     This syntax pulls the tag out of the current header-string, calls
>     the rest of the header "fields", and includes the metadata operands
>     in "operands".
>  5. Incrementally create subclasses of `DebugMDNode`, such as
>     `MDCompileUnit` and `MDSubprogram`.  Sub-classed nodes replace the
>     "fields" and "operands" catch-alls with explicit names for each
>     operand.

I wouldn't mind seeing how expensive it would be if these schema
descriptions were within the module itself - so we didn't have to bake them
into the IR spec, but could still share them between every usage within a

>     Proposed assembly syntax:
>         !7 = metadata !MDSubprogram(line: 45, name: "foo", displayName:
> "foo",
>                                     linkageName: "_Z3foov", file: metadata
> !8,
>                                     function: i32 (i32)* @foo)
>  6. Remove the dead code for `GenericDebugMDNode`.
>  7. (Optional) Refactor `DebugMDNode` sub-classes to minimize RAUW
>     traffic during bitcode serialization.  Now that metadata types are
>     known, we can write debug info out in an order that makes it cheap
>     to read back in.
>     Note that using `MDUser` will make RAUW much cheaper, since we're
>     using the use-list infrastructure for most of them.  If RAUW isn't
>     showing up in a profile, I may skip this.
> Does this direction seem reasonable?  Any major problems I've missed?
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