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

[Alex Rosenberg]

As all of these transforms are 1-to-1, can we still support the older metadata and convert it on the fly?

Alex

> On 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.
> 
> 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, 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`.
> 
> 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, for a running total of ~2GB.
> 
>    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.
> 
>    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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