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WebAssembly Specifaction Release

Changes by Release

WebAssembly 1.0 (2019-07-20) — Changes since previous Working Draft

  • Syntax and notation
    • Function reference notation: anyfunc → funcref
    • Variable instructions renamed: get_local → local.get, set_local → local.set, tee_local → local.tee
    • Global instructions renamed: get_global → global.get, set_global → global.set
    • Memory instructions renamed: grow_memory → memory.grow, current_memory → memory.size
    • Numeric function names normalized (e.g., i64.extend_sx/i32 → i64.extend_i32_sx)
    • Notation for numeric conversions clarified; conversion names normalized (e.g., extend_u family normalized)
  • Validation and limits
    • Tables limited to 2^32 elements (spec §3.2.3)
    • Memory limited to 2^16 pages (64KiB pages) (spec §3.2.4)
    • Validation of external types clarified (spec §3.2.6)
  • Endianness and semantics fixes
    • Fix little-endian definition (spec §4.3.1.3)
    • Semantics for growing tables (spec §4.5.3.6) and memory (spec §4.5.3.7) made explicit
    • Stack behavior: push a dummy frame instead of raw values (spec §4.5.5)
  • Text encoding and Unicode
    • UTF-8 validation fixed to disallow surrogate code points (spec §5.2.4)
    • Use term “scalar values” instead of “code points”
  • Memory instruction immediates
    • Explicit align and offset immediates; default alignment 0
    • Alignment must be a power of two and ≤ the instruction’s natural alignment (range 0..3)
  • Appendix changes
    • Add pre- and post-conditions section (Appx A.1)
    • Rename helper functions to noun_verb form (e.g., decode_module → module_decode) (Appx A.1)
    • Each subsection may occur at most once, in order of increasing id (Appx A.4)

WebAssembly 1.1 (Draft 2021-11-16) — Integrated proposals

  • Sign-extension operations
    • New numeric instructions: i32.extend8_s, i32.extend16_s, i64.extend8_s, i64.extend16_s, i64.extend32_s
  • Non-trapping float-to-int conversions
    • New numeric instructions: i32.trunc_sat_f32_s, i32.trunc_sat_f32_u, i32.trunc_sat_f64_s, i32.trunc_sat_f64_u, i64.trunc_sat_f32_s, i64.trunc_sat_f32_u, i64.trunc_sat_f64_s, i64.trunc_sat_f64_u
  • Multiple values
    • Function types allow more than one result; blocks can have parameters and arbitrary function types
  • Reference types
    • New value types: funcref, externref
    • New reference instructions: ref.null, ref.func, ref.is_null
    • Parametric select enriched with optional type immediate
    • New declarative element segment form
  • Table instructions
    • New instructions: table.get, table.set, table.size, table.grow; table element type can be any reference type
  • Multiple tables
    • Modules may define/import/export multiple tables; table index immediates for table.* and call_indirect; element segments take a table index
  • Bulk memory and table operations
    • Memory: memory.fill, memory.init, memory.copy, data.drop
    • Table: table.fill, table.init, table.copy, elem.drop
    • Passive data/element segments; new data count section; segment boundary checks may trap at runtime
  • Fixed-width SIMD (128-bit)
    • New type: v128; loads/stores, constants, lane ops; arithmetic, logical, compare, shuffle/swizzle; conversions; tests and bitselect

WebAssembly 2.0 (Draft 2025-01-30) — Core changes

  • Incorporates Release 1.1 features: sign-extension ops, non-trapping float-to-int conversions, multiple values
  • Reference types: funcref, externref; ref.null, ref.func, ref.is_null; typed select; declarative element segments
  • Table instructions: table.get, table.set, table.size, table.grow; tables over any reference type
  • Multiple tables per module; table index immediates; element segments take a table index
  • Bulk memory and table operations as listed above
  • Fixed-width SIMD (v128) with the instruction families listed above

WebAssembly 2.0 + tracks — Deltas relative to 2.0 core

  • 2.0 + threads (Draft 2023-10-10) (Phase 4 - Standardize the Feature)
    • Shared memories; memory is declared shared
    • Atomic instructions: memory.atomic.wait32/wait64, memory.atomic.notify, atomic.fence
    • Atomic loads/stores and RMW ops: i32/64.atomic.load/store, atomic.rmw.add/sub/and/or/xor/xchg/cmpxchg
  • 2.0 + multi_memory (Draft 2024-09-04) (Phase 5 - The Feature is Standardized, merged into wasm3.0)
    • Multiple linear memories per module (define/import/export)
    • Memory index immediates for memory instructions: memory.size, memory.grow, memory.fill, memory.copy, memory.init
    • Data segments carry a memory index; loads/stores use memory index immediates
  • 2.0 + tail_calls (Draft 2023-03-01) (Phase 5 - The Feature is Standardized, merged into wasm3.0)
    • New control instructions: return_call, return_call_indirect
  • 2.0 + tail_calls + function_references (Draft 2024-12-09) (Phase 5 - The Feature is Standardized, merged into wasm3.0)
    • Typed function references and call_ref; refined typing of ref.func
    • br_on_null/br_on_non_null; tracking initialization of non-defaultable locals
  • 2.0 + tail_calls + function_references + gc (Draft 2024-12-09) (Phase 5 - The Feature is Standardized, merged into wasm3.0)
    • Integration with managed reference types and subtyping introduced by GC

WebAssembly 3.0 (Draft 2024-09-21) — Changes

  • Extended constant expressions
    • Allow basic numeric computations in const expr: iN.add, iN.sub, iN.mul, and global.get of prior immutable globals
  • Tail calls
    • Control instructions: return_call, return_call_indirect
  • Exception handling
    • Tags (define/import/export); new heap types: exn, noexn; short-hands: exnref, nullexnref
    • Control: throw, throw_ref, try_table; new tag section in binary
  • Multiple memories
    • Multiple memories per module; memory index immediates for memory ops (incl. loads/stores and SIMD loads/stores)
    • Data segments take a memory index
  • Typeful references (function references)
    • Generalized reference types: (ref null? heaptype); heap types: func, extern, typeidx
    • New/refined instructions: ref.as_non_null, br_on_null, br_on_non_null, call_ref; refined ref.func
    • Track initialization for non-defaultable locals; extended table init with optional initializer expr
  • Garbage collection (managed references)
    • Heap types: any, eq, i31, struct, array, none, nofunc, noextern
    • Shorthands: anyref, eqref, i31ref, structref, arrayref, nullref, nullfuncref, nullexternref
    • Type defs: struct, array, subtypes, recursive types; enriched subtyping
    • Generic ref ops: ref.eq, ref.test, ref.cast, br_on_cast, br_on_cast_fail
    • i31 ops: ref.i31, i31.get_s, i31.get_u
    • Struct ops: struct.new, struct.new_default, struct.get_s, struct.get_u, struct.set
    • Array ops: array.new, array.new_default, array.new_fixed, array.new_data, array.new_elem, array.get_s, array.get_u, array.set, array.len, array.fill, array.copy, array.init_data, array.init_elem
    • External conversion: any.convert_extern, extern.convert_any
    • Extended const instr: ref.i31, struct.new*, array.new*, any.convert_extern, extern.convert_any
  • Custom annotations (text format)
    • Generic annotations (@id ...); escaped identifiers @"..."
    • Built-in name annotations: (@name "...") for module/type/func/local/field; (@custom "...") for arbitrary custom sections