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WebAssembly Object File Linking

This document describes the WebAssembly object file format and the ABI used for statically linking them to produce an executable WebAssembly module. This is currently implemented in the clang/LLVM WebAssembly backend and other tools such as binaryen and wabt. As mentioned in README, this is not part of the official WebAssembly specification and other runtimes may choose to follow a different set of linking conventions.

Overview

Each translation unit is compiled into a WebAssembly object file. These files are themselves valid WebAssembly module binaries but are not expected to be directly executable and have certain additional constraints. In order to distinguish object files from executable WebAssembly modules the linker can check for the presence of the "linking" custom section which must exist in all object files.

The goal of the linker is to take one or more WebAssembly object files and merge them into a single executable module. In order to achieve this the following tasks need to be performed:

  • Merging of function sections (re-numbering functions)
  • Merging of globals sections (re-numbering globals)
  • Merging of event sections (re-numbering events)
  • Merging of table sections (re-numbering tables)
  • Merging of data segments (re-positioning data with limitations)
  • Resolving undefined external references
  • Synthesizing functions to call constructors and perform other initialization

The linking technique described here is designed to be fast, and avoids having to disassemble the code section. The extra metadata required by the linker is stored in a custom "linking" section and zero or more relocation sections whose names begin with "reloc.". For each section that requires relocation a "reloc" section will be present in the wasm file. By convention the reloc section names end with the name of the section that they refer to: e.g. "reloc.CODE" for code section relocations. However, everything after the period is ignored and the specific target section is encoded in the reloc section itself.

The linker additionally checks that linked object files were built targeting compatible feature sets. Unlike native targets, WebAssembly has no runtime feature detection, and the presence of unsupported features causes a binary to fail to validate. It is therefore important for the user to have explicit control over the features used in the output binary and for the linker to provide helpful errors when instructed to link incompatible or disallowed features. This feature information is stored in a custom "target feature section".

Relocation Sections

A relocation section is a user-defined section with a name starting with "reloc." Relocation sections start with an identifier specifying which section they apply to, and must be sequenced in the module after that section.

Relocation sections can only target code, data and custom sections. All other sections are synthetic sections: that is, rather than being memcpy'd into place as the code and data sections are, they are created from scratch by the linker.

The "reloc." custom sections must come after the "linking" custom section in order to validate relocation indices.

Any LEB128-encoded values should be maximally padded so that they can be rewritten without affecting the position of any other bytes. For instance, the function index 3 should be encoded as 0x83 0x80 0x80 0x80 0x00.

Relocations contain the following fields:

Field Type Description
section varuint32 the index of the target section
count varuint32 count of entries to follow
entries relocation_entry* sequence of relocation entries

A relocation_entry begins with:

Field Type Description
type uint8 the relocation type
offset varuint32 offset of the value to rewrite (relative to the relevant section's contents: offset zero is immediately after the id and size of the section)
index varuint32 the index of the symbol used (or, for R_WASM_TYPE_INDEX_LEB relocations, the index of the type)

A relocation type can be one of the following:

  • 0 / R_WASM_FUNCTION_INDEX_LEB (since LLVM 10.0) - a function index encoded as a 5-byte varuint32. Used for the immediate argument of a call instruction.
  • 1 / R_WASM_TABLE_INDEX_SLEB (since LLVM 10.0) - a function table index encoded as a 5-byte varint32. Used to refer to the immediate argument of a i32.const instruction, e.g. taking the address of a function.
  • 2 / R_WASM_TABLE_INDEX_I32 (since LLVM 10.0) - a function table index encoded as a uint32, e.g. taking the address of a function in a static data initializer.
  • 3 / R_WASM_MEMORY_ADDR_LEB (since LLVM 10.0) - a linear memory index encoded as a 5-byte varuint32. Used for the immediate argument of a load or store instruction, e.g. directly loading from or storing to a C++ global.
  • 4 / R_WASM_MEMORY_ADDR_SLEB (since LLVM 10.0) - a linear memory index encoded as a 5-byte varint32. Used for the immediate argument of a i32.const instruction, e.g. taking the address of a C++ global.
  • 5 / R_WASM_MEMORY_ADDR_I32 (since LLVM 10.0) - a linear memory index encoded as a uint32, e.g. taking the address of a C++ global in a static data initializer.
  • 6 / R_WASM_TYPE_INDEX_LEB (since LLVM 10.0) - a type index encoded as a 5-byte varuint32, e.g. the type immediate in a call_indirect.
  • 7 / R_WASM_GLOBAL_INDEX_LEB (since LLVM 10.0) - a global index encoded as a 5-byte varuint32, e.g. the index immediate in a get_global.
  • 8 / R_WASM_FUNCTION_OFFSET_I32 (since LLVM 10.0) - a byte offset within code section for the specific function encoded as a uint32. The offsets start at the actual function code excluding its size field.
  • 9 / R_WASM_SECTION_OFFSET_I32 (since LLVM 10.0) - a byte offset from start of the specified section encoded as a uint32.
  • 10 / R_WASM_EVENT_INDEX_LEB (since LLVM 10.0) - an event index encoded as a 5-byte varuint32. Used for the immediate argument of a throw and if_except instruction.
  • 13 / R_WASM_GLOBAL_INDEX_I32 (since LLVM 11.0) - a global index encoded as uint32.
  • 14 / R_WASM_MEMORY_ADDR_LEB64 (since LLVM 11.0) - the 64-bit counterpart of R_WASM_MEMORY_ADDR_LEB. A 64-bit linear memory index encoded as a 10-byte varuint64, Used for the immediate argument of a load or store instruction on a 64-bit linear memory array.
  • 15 / R_WASM_MEMORY_ADDR_SLEB64 (since LLVM 11.0) - the 64-bit counterpart of R_WASM_MEMORY_ADDR_SLEB. A 64-bit linear memory index encoded as a 10-byte varint64. Used for the immediate argument of a i64.const instruction.
  • 16 / R_WASM_MEMORY_ADDR_I64 (since LLVM 11.0) - the 64-bit counterpart of R_WASM_MEMORY_ADDR. A 64-bit linear memory index encoded as a uint64, e.g. taking the 64-bit address of a C++ global in a static data initializer.
  • 18 / R_WASM_TABLE_INDEX_SLEB64 (in LLVM 12.0) - the 64-bit counterpart of R_WASM_TABLE_INDEX_SLEB. A function table index encoded as a 10-byte varint64. Used to refer to the immediate argument of a i64.const instruction, e.g. taking the address of a function in Wasm64.
  • 19 / R_WASM_TABLE_INDEX_I64 (in LLVM 12.0) - the 64-bit counterpart of R_WASM_TABLE_INDEX_I32. A function table index encoded as a uint64, e.g. taking the address of a function in a static data initializer.
  • 20 / R_WASM_TABLE_NUMBER_LEB (in LLVM 12.0) - a table number encoded as a 5-byte varuint32. Used for the table immediate argument in the table.* instructions.
  • 22 / R_WASM_FUNCTION_OFFSET_I64 (in LLVM 12.0) - the 64-bit counterpart of R_WASM_FUNCTION_OFFSET_I32. A byte offset within code section for the specific function encoded as a uint64.
  • 23 / R_WASM_MEMORY_ADDR_LOCREL_I32 (in LLVM 13.0) - a byte offset between the relocating address and a linear memory index encoded as a uint32. Used for pointer-relative addressing.
  • 24 / R_WASM_TABLE_INDEX_REL_SLEB64 (in LLVM 13.0) - the 64-bit counterpart of R_WASM_TABLE_INDEX_REL_SLEB. A function table index encoded as a 10-byte varint64.
  • 26 / R_WASM_FUNCTION_INDEX_I32 (in LLVM 17.0) - a function index encoded as a uint32. Used in custom sections for function annotations (__attribute__((annotate(<name>)))).

Note: Please note that the 64bit relocations are not yet stable and therefore, subject to change.

For R_WASM_MEMORY_ADDR_*, R_WASM_FUNCTION_OFFSET_I32, and R_WASM_SECTION_OFFSET_I32 relocations (and their 64-bit counterparts) the following field is additionally present:

Field Type Description
addend varint32 addend to add to the address

Note that for all relocation types, the bytes being relocated:

  • from offset to offset + 5 for LEB/SLEB relocations;
  • from offset to offset + 10 for LEB64/SLEB64 relocations;
  • from offset to offset + 4 for I32 relocations;
  • or from offset to offset + 8 for I64;

must lie within the section to which the relocation applies (as offsets are relative to the section's contents, this means that they cannot be too large). In addition, the bytes being relocated may not overlap the boundary between the section's chunks, where such a distinction exists (it may not for custom sections). For example, for relocations applied to the CODE section, a relocation cannot straddle two functions, and for the DATA section relocations must lie within a data element's body.

Linking Metadata Section

A linking metadata section is a user-defined section with the name "linking".

The "linking" custom section must be after the data section in order to validate data symbols.

A linking metadata section begins with a version number which is then followed by a series of sub-sections laid out in the same way as the "names" section:

Field Type Description
version varuint32 the version of linking metadata contained in this section. Currently: 2
subsections subsection* sequence of subsection

This version allows for breaking changes to be made to the format described here. Tools can then choose to reject inputs containing unexpected versions.

Each subsection is encoded as:

Field Type Description
type uint8 code identifying type of subsection
payload_len varuint32 size of this subsection in bytes
payload_data bytes content of this subsection, of length payload_len

The current list of valid type codes are:

  • 5 / WASM_SEGMENT_INFO - Extra metadata about the data segments.

  • 6 / WASM_INIT_FUNCS - Specifies a list of constructor functions to be called at startup. These constructors will be called in priority order after memory has been initialized.

  • 7 / WASM_COMDAT_INFO - Specifies the COMDAT groups of associated linking objects, which are linked only once and all together.

  • 8 / WASM_SYMBOL_TABLE - Specifies extra information about the symbols present in the module.

For WASM_SEGMENT_INFO the following fields are present in the subsection:

Field Type Description
count varuint32 number of segment in segments
segments segment* sequence of segment

where a segment is encoded as:

Field Type Description
name_len varuint32 length of name_data in bytes
name_data bytes UTF-8 encoding of the segment's name
alignment varuint32 The required alignment of the segment, encoded as a power of 2
flags varuint32 a bitfield containing flags for this segment

The current set of valid flag for segments are:

  • 1 / WASM_SEGMENT_FLAG_STRINGS - Signals that the segment contains only null terminated strings allowing the linker to perform merging.
  • 2 / WASM_SEGMENT_FLAG_TLS - The segment contains thread-local data. This means that a unique copy of this segment will be created for each thread.
  • 4 / WASM_SEG_FLAG_RETAIN - If the object file is included in the final link, the segment should be retained in the final output regardless of whether it is used by the program.

For WASM_INIT_FUNCS the following fields are present in the subsection:

Field Type Description
count varuint32 number of init functions that follow
functions init_func* sequence of init_func

where an init_func is encoded as:

Field Type Description
priority varuint32 priority of the init function
symbol_index varuint32 the symbol index of init function (not the function index)

The WASM_INIT_FUNC subsection must come after the WASM_SYMBOL_TABLE subsection.

For WASM_SYMBOL_TABLE the following fields are present in the subsection:

Field Type Description
count varuint32 number of syminfo in infos
infos syminfo* sequence of syminfo

where a syminfo is encoded as:

Field Type Description
kind uint8 The symbol type. One of:
0 / SYMTAB_FUNCTION
1 / SYMTAB_DATA
2 / SYMTAB_GLOBAL
3 / SYMTAB_SECTION
4 / SYMTAB_EVENT
5 / SYMTAB_TABLE
flags varuint32 a bitfield containing flags for this symbol

For functions, globals, events and tables, we reference an existing Wasm object, which is either an import or a defined function/global/event/table (recall that the operand of a Wasm call instruction uses an index space consisting of the function imports followed by the defined functions, and similarly get_global for global imports and definitions and throw for event imports and definitions).

If a symbols refers to an import, and the WASM_SYM_EXPLICIT_NAME flag is not set, then the name is taken from the import; otherwise the syminfo specifies the symbol's name.

Field Type Description
index varuint32 the index of the Wasm object corresponding to the symbol, which references an import if and only if the WASM_SYM_UNDEFINED flag is set
name_len varuint32 ? the optional length of name_data in bytes, omitted if index references an import
name_data bytes ? UTF-8 encoding of the symbol name, omitted if index references an import

For data symbols:

Field Type Description
name_len varuint32 the length of name_data in bytes
name_data bytes UTF-8 encoding of the symbol name
index varuint32 ? the index of the data segment; provided if the symbol is defined
offset varuint32 ? the offset within the segment; provided if the symbol is defined; must be <= the segment's size
size varuint32 ? the size (which can be zero); provided if the symbol is defined; offset + size must be <= the segment's size

For section symbols:

Field Type Description
section varuint32 the index of the target section

The current set of valid flags for symbols are:

  • 1 / WASM_SYM_BINDING_WEAK - Indicating that this is a weak symbol. When linking multiple modules defining the same symbol, all weak definitions are discarded if any strong definitions exist; then if multiple weak definitions exist all but one (unspecified) are discarded; and finally it is an error if more than one definition remains.
  • 2 / WASM_SYM_BINDING_LOCAL - Indicating that this is a local symbol (this is exclusive with WASM_SYM_BINDING_WEAK). Local symbols are not to be exported, or linked to other modules/sections. The names of all non-local symbols must be unique, but the names of local symbols are not considered for uniqueness. A local function or global symbol cannot reference an import.
  • 4 / WASM_SYM_VISIBILITY_HIDDEN - Indicating that this is a hidden symbol. Hidden symbols are not to be exported when performing the final link, but may be linked to other modules.
  • 0x10 / WASM_SYM_UNDEFINED - Indicating that this symbol is not defined. For non-data symbols, this must match whether the symbol is an import or is defined; for data symbols, determines whether a segment is specified.
  • 0x20 / WASM_SYM_EXPORTED - The symbol is intended to be exported from the wasm module to the host environment. This differs from the visibility flags in that it effects the static linker.
  • 0x40 / WASM_SYM_EXPLICIT_NAME - The symbol uses an explicit symbol name, rather than reusing the name from a wasm import. This allows it to remap imports from foreign WebAssembly modules into local symbols with different names.
  • 0x80 / WASM_SYM_NO_STRIP - The symbol is intended to be included in the linker output, regardless of whether it is used by the program.
  • 0x100 / WASM_SYM_TLS - The symbol resides in thread local storage.
  • 0x200 / WASM_SYM_ABSOLUTE - The symbol represents an absolute address. This means it's offset is relative to the start of the wasm memory as opposed to being relative to a data segment.

For WASM_COMDAT_INFO the following fields are present in the subsection:

Field Type Description
count varuint32 Number of Comdat in comdats
comdats comdat* Sequence of Comdat

where a comdat is encoded as:

Field Type Description
name_len varuint32 length of name_str in bytes
name_str bytes UTF-8 encoding of the name
flags varuint32 Must be zero, no flags currently defined
count varuint32 Number of comdat_sym in comdat_syms
comdat_syms comdat_sym* Sequence of comdat_sym

and where a comdat_sym is encoded as:

Field Type Description
kind uint8 Type of symbol, one of:
* 0 / WASM_COMDAT_DATA, a data segment
* 1 / WASM_COMDAT_FUNCTION
* 2 / WASM_COMDAT_GLOBAL
* 3 / WASM_COMDAT_EVENT
* 4 / WASM_COMDAT_TABLE
* 5 / WASM_COMDAT_SECTION
index varuint32 Index of the data segment/function/global/event/table in the Wasm module (depending on kind). The function/global/event/table must not be an import.

Target Features Section

The target features section is an optional custom section with the name "target_features". The target features section must come after the "producers" section.

The contents of the target features section is a vector of entries:

Field Type Description
prefix byte See below.
feature bytes The name of the feature. Must be unique.

The recognized prefix bytes and their meanings are below. When the user does not supply a set of allowed features explicitly, the set of allowed features is taken to be the set of features used or required. Any feature not mentioned in an object's target features section is not used by that object, but is not necessarily prohibited in the final binary.

Prefix Meaning
0x2b (+) This object uses this feature, and the link fails if this feature is not in the allowed set.
0x2d (-) This object does not use this feature, and the link fails if this feature is in the allowed set.
0x3d (=) This object uses this feature, and the link fails if this feature is not in the allowed set or if any object does not use this feature.

The generally accepted features are:

  1. atomics
  2. bulk-memory
  3. exception-handling
  4. multivalue
  5. mutable-globals
  6. nontrapping-fptoint
  7. sign-ext
  8. simd128
  9. tail-call

Merging Global Sections

Merging of the global sections requires the re-numbering of globals. This follows the normal rules for defining symbols: if two object files provide the same global symbol strongly, there is a link error; if two object files provide the symbol weakly, one is chosen.

When creating non-relocatable output, the Wasm output shall have an import for each undefined strong symbol, and an export for each defined symbol with non-local linkage and non-hidden visibility.

The linker may provide certain symbols itself, even if not defined by any object file. For example, the __stack_pointer symbol may be provided at link-time.

Merging Event Sections

Events are meant to represent various control-flow changing constructs of wasm. Currently, we have a proposal for one kind of events: exceptions, but the event section can be used to support other kinds of events in future as well. The event section is a list of declared events associated with the module.

Merging of the event sections requires the re-numbering of events. This follows the normal rules for defining symbols: if two object files provide the same event symbol strongly, there is a link error; if two object files provide the symbol weakly, one is chosen.

When creating non-relocatable output, the Wasm output shall have an import for each undefined strong symbol, and an export for each defined symbol with non-local linkage and non-hidden visibility.

Merging Function Sections

Merging of the function sections requires the re-numbering of functions. This requires modification to code sections at each location where a function index is embedded. There are currently two ways in which function indices are stored in the code section:

  1. Immediate argument of the call instruction (calling a function)
  2. Immediate argument of the i32.const instruction (taking the address of a function).

The immediate argument of all such instructions are stored as padded LEB128 such that they can be rewritten without altering the size of the code section. For each such instruction a R_WASM_FUNCTION_INDEX_LEB or R_WASM_TABLE_INDEX_SLEB reloc entry is generated pointing to the offset of the immediate within the code section.

The same technique applies for all function calls whether the function is imported or defined locally.

When creating non-relocatable output, the Wasm output shall have an import for each undefined strong symbol, and an export for each defined symbol with non-local linkage and non-hidden visibility.

Merging Data Sections

Merging of data sections is performed by creating a new data section from the data segments in the object files. Data symbols (e.g. C/C+ globals) are represented in the object file as Wasm data segments with an associated data symbol, so each linked data symbol pulls its associated data segment into the linked output.

Segments are merged according their type: segments with a common prefix such as .data or .rodata are merged into a single segment in the output data section. It is an error if this behavior would merge shared and unshared segments.

The output data section is formed, essentially, by concatenating the data sections of the input files. Since the final location in linear memory of any given symbol is not known until link time, all references to data addresses with the code and data sections generate R_WASM_MEMORY_ADDR_* relocation entries, which reference a data symbol.

Segments are linked as a whole, and a segment is either entirely included or excluded from the link.

Merging Custom Sections

Merging of custom sections is performed by concatenating all payloads for the customs sections with the same name. The section symbol will refer the resulting section, this means that the relocation entries addend that refer the referred custom section fields shall be adjusted to take new offset into account.

COMDATs

A COMDAT group may contain one or more functions, data segments, and/or custom sections. The linker will include all of these elements with a given group name from one object file, and will exclude any element with this group name from all other object files.

Processing Relocations

The final code and data sections are written out with relocations applied.

R_WASM_TYPE_INDEX_LEB relocations cannot fail. The output Wasm file shall contain a newly-synthesised type section which contains entries for all functions and type relocations in the output. R_WASM_TABLE_INDEX_* relocations cannot fail. The output Wasm file shall contain a newly-synthesised table, which contains an entry for all defined or imported symbols referenced by table relocations. The output table elements shall begin at a non-zero offset within the table, so that a call_indirect 0 instruction is guaranteed to fail. Finally, when processing table relocations for symbols which have neither an import nor a definition (namely, weakly-undefined function symbols), the value 0 is written out as the value of the relocation.

R_WASM_FUNCTION_INDEX_LEB relocations may fail to be processed, in which case linking fails. This occurs if there is a weakly-undefined function symbol, in which case there is no legal value that can be written as the target of any call instruction. The frontend must generate calls to undefined weak symbols via a call_indirect instruction.

R_WASM_GLOBAL_INDEX_LEB relocations may fail to be processed, in which case linking fails. This occurs if there is a weakly-undefined global symbol, in which case there is no legal value that can be written as the target of any get_global or set_global instruction. (This means the frontend must not generate weak globals which may not be defined; a definition or import must exist for all global symbols in the linked output.)

R_WASM_MEMORY_* relocations cannot fail. The relocation's value is the offset within the linear memory of the symbol within the output segment, plus the symbol's addend. If the symbol is undefined (whether weak or strong), the value of the relocation shall be 0.

R_WASM_FUNCTION_OFFSET_I32 relocations cannot fail. The values shall be adjusted to reflect new offsets in the code section.

R_WASM_SECTION_OFFSET_I32 relocation cannot fail. The values shall be adjusted to reflect new offsets in the combined sections.

R_WASM_EVENT_INDEX_LEB relocations may fail to be processed, in which case linking fails. This occurs if there is a weakly-undefined event symbol, in which case there is no legal value that can be written as the target of any throw and if_except instruction. (This means the frontend must not generate weak events which may not be defined; a definition or import must exist for all event symbols in the linked output.)

Start Section

By default the static linker should not output a WebAssembly start section. Constructors are instead called from a synthetic function __wasm_call_ctors that the runtime and embedder should arrange to have called after instantiation. __wasm_call_ctors is not exported by default because it may be called by some other startup function defined by the runtime. For the embedder to call it directly it should be exported like any other function.

Rationale: Use of the WebAssembly start function was considered for running static constructors and/or the main entry point to the program. However, running arbitrary code in the start section is currently problematic due to the fact the module exports not available to the embedder at the time when the start function runs. A common example is the module memory itself. If the code in the start function wants to transfer any data to the embedder (e.g. printf) this will not work as the embedded cannot yet access the modules memory. This extends to all embedder functions that might want to call back into the module.

If some future version of the WebAssembly spec allows for module exports to be available during execution of the start function it will make sense to reconsider this.

When shared memory is requested, a start function will be emitted to initialize memory as described below.

Experimental Threading Support

By default all atomics and TLS are currently lowered to WebAssembly MVP and threads are not supported. However, when enabled, llvm does support an exprimental multithreading ABI based on the WebAssembly threads proposal. These features are used to support threading in Emscripten.

The next section describes compatibility between threading features and MVP, and the following sections describe shared memory and TLS implementation based on bulk memory and experimental threading support.

Lowering Atomics and TLS to MVP

MVP WebAssembly does not include support for atomic operations or the bulk memory operations necessary to implement thread-local storage. As a result, any atomics or TLS present at the source level must be lowered to non-atomic operations and normal storage when targeting MVP WebAssembly. This is safe as long as the resulting MVP object files are not used in a multi-threaded context.

To enforce this safety guarantee, the linker will error out if a shared memory is requested but the atomics target feature is disallowed in the target features section of any input objects. The compiler is responsible for marking atomics disallowed and thereby preventing thread-unsafe linking if either atomic operations or TLS are stripped during compilation. If the compiler removes either one of atomic operations or TLS, the resulting object may only be used with a single thread with an unshared memory, so the other one should be removed as well.

If atomics or bulk-memory is not available during compilation but the source does not contain atomic operations or TLS, then the result is a "thread-agnostic" object that neither uses nor disallows the atomics feature. Thread-agnostic objects can be safely linked with objects that do or do not use atomics, although not both at the same time.

Shared Memory and Passive Segments

When shared memory is enabled, all data segments will be emitted as passive segments to prevent each thread from reinitializing memory. In a web context, using active segments would cause memory to be reinitialized every time the module is instantiated on a new WebWorker as part of spawning a new thread. The memory.init instructions that initialize these passive segments and the data.drop instructions that mark them collectible will be emitted into a synthetic function __wasm_init_memory that is made the WebAssembly start function and called automatically on instantiation but is not exported. __wasm_init_memory shall perform any synchronization necessary to ensure that no thread returns from instantiation until memory has been fully initialized, even if a module is instantiated on multiple threads simultaneously. This synchronization may involve waiting, but waiting is disallowed in some Web contexts such as on the main thread or in Audio worklets. For instantiation to succeed on all threads, the embedder must guarantee for each thread in a context that disallows waiting that the thread either wins the race and becomes responsible for initializing memory or that it is initialized after memory has already been initialized[1]. To make the memory.init and data.drop instructions valid, a DataCount section will also be emitted.

Note that memory.init and the DataCount section are features of the bulk-memory proposal, not the atomics proposal, so any engine that supports threads needs to support both of these proposals.

[1] In LLVM 13 and earlier, embedders had to guarantee that threads on contexts that disallow waiting had to win the race to initialize memory. That meant that there could only be one such thread in the system.

Thread Local Storage

Currently, thread-local storage is only supported in the main WASM module and cannot be accessed outside of it. This corresponds to the ELF local exec TLS model.

Additionally, thread local storage depends on bulk memory instructions, and therefore support depends on the bulk memory proposal.

All thread local variables will be merged into one passive segment called .tdata. This section contains the starting values for all TLS variables. The thread local block of every thread will be initialized with this segment.

In a threaded build, the linker will create:

  • an immutable global variable of type i32 called __tls_size. Its value is the total size of the thread local block for the module, i.e. the sum of the sizes of all thread local variables plus padding. This value will be 0 if there are no thread-local variables.
  • an immutable global variable of type i32 called __tls_align. Its value is the alignment requirement of the thread local block, in bytes, and will be a power of 2. The value will be 1 if there are no thread-local variables.
  • a mutable global i32 called __tls_base, with a i32.const 0 initializer.
  • a global function called __wasm_init_tls with signature (i32) -> ().

To initialize thread-local storage, a thread should do the equivalent of the following pseudo-code upon startup:

(if (global.get __tls_size) (then
  (call __wasm_init_tls
    (call aligned_alloc
      (global.get __tls_align)
      (call roundUpToMultipleOf
        (global.get __tls_align)
        (global.get __tls_size))))))

__wasm_init_tls takes a pointer argument containing the memory block to use as the thread local storage block of the current thread. It should do nothing if there are no thread-local variables. Otherwise, the memory block will be initialized with the passive segment .tdata via the memory.init instruction. It will then set __tls_base to the address of the memory block passed to __wasm_init_tls.

Note that __tls_size is not necessarily a multiple of __tls_align. In order to use aligned_alloc, we must round the size up to be a multiple of __tls_align.

The relocations for thread local variables shall resolve into offsets relative to the start of the TLS block. As such, adding the value of __tls_base yields the actual address of the variable. For example, a variable called tls_var would have its address computed as follows:

(i32.add (global.get __tls_base) (i32.const tls_var))

The variable can then be used as normal. Upon thread exit, the runtime should free the memory allocated for the TLS block.

Limitations

  • There is currently no support for passive data segments. The relocation types necessary for referencing such segments (e.g. in data.drop or memory.init instruction) do not yet exist.
  • There is currently no support for table element segments, either active or passive.