Sigil 0.5.0

February 17, 2026release

This release is all about stability and correctness. A comprehensive audit of the garbage collector uncovered and fixed a critical write barrier gap, and GC stress testing now runs across the entire compiler pipeline. The HAMT dict implementation is complete, arbitrary-precision integers (bignums) are fully integrated, and every open issue has been resolved. Performance improves with inline caching and superinstructions, and the multimedia packages have been reorganized for flexibility.

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GC Correctness

The most impactful change in this release: the tri-color write barrier was implemented but never called. Every mutation site that writes a heap pointer into an existing object could violate the tri-color invariant during incremental marking. This was the root cause of several hard-to-reproduce crashes.

Write barriers: Added sigil__gc_write_barrier() calls to all 13 heap mutation sites across vm.c and natives.c
Compiler rooting: Removed the SIGIL_GC_DISABLE_DURING_COMPILATION workaround entirely. Fixed 46+ unrooted allocation sites in the compiler through systematic audit
GC stress testing: SIGIL_GC_STRESS=1 forces GC on every allocation, maximizing the window for bugs. All tests pass under stress

Performance

Inline caching: Module binding lookups now use per-code cache arrays. On benchmarks, 3.3M hash lookups reduced to ~38K cold-start misses, with 100% steady-state cache hit rate
Superinstructions: Peephole optimizer fuses 8 common opcode patterns (e.g., PUSH_LOCAL+PUSH_LOCAL, PUSH_CONST+ADD). Compiler-level fusion merges source location tracking with call opcodes

Arbitrary-Precision Integers

Integers larger than 2^47 now automatically promote to bignums instead of raising an error:

Full arithmetic operations (add, subtract, multiply, divide, modulo, GCD)
Comparison and bitwise operations
Transparent promotion and demotion (bignum results that fit in a fixnum automatically demote)
Serialization support in bytecode files

Dict Completeness

The HAMT (Hash Array Mapped Trie) implementation is now feature-complete:

Printing: HAMT dicts display all entries with display and write
Equality: equal? walks HAMT nodes for structural comparison
Collision handling: Fixed silent key dropping when hash collisions occur at the same node
Removal: Proper structural dict-remove with sharing, branch collapse, and HAMT-to-array demotion when count drops below 8

Error Handling

Three exit(1) calls in the module system replaced with sigil__vm_error() so the Scheme-level exception system handles them gracefully
OP_ABORT_TO_PROMPT now extracts real error messages from exception objects
Macro expansion errors propagate properly to the compiler
match raises an error instead of printing a warning when no clauses match

Stack Traces

Procedures now track where they were defined. Stack traces show definition locations, making it easier to identify which function failed:

Stack trace (most recent call first):
  0: my-helper
     at utils.sgl:42:5

Multimedia Packages

sigil-studio has been split into three independent packages for more flexible dependency management:

Package	Description
`sigil-app`	Windowing, input, application lifecycle (sokol_app)
`sigil-graphics`	2D rendering, image loading, font rendering (sokol_gfx, stb)
`sigil-audio`	Sound effects and music streaming (sokol_audio, stb_vorbis)

sigil-studio remains as a meta-package that pulls in all three. New: playlist support with wait-music-end and play-playlist.

Build System

Test harness generation for packages with native C dependencies
c-flags field on library definitions
--profile flag for opcode-level performance analysis
Build safety: warns when replacing the running binary, errors on 0-byte output or missing target package

Test Coverage

Added test suites for 15+ packages that previously had none, including sigil-crypto, sigil-tls, sigil-web (routing, middleware, cookies, SSE, security), sigil-socket, sigil-process, sigil-mcp, sigil-jwt, sigil-websocket, sigil-markdown, sigil-format, and sigil-sxml. Error path tests added across the standard library.

Bug Fixes

Fixed assq-ref to return values instead of aliasing assq; added assv-ref
Fixed number->string precision for exact IEEE 754 roundtrip (was losing digits on large flonums)
Fixed REPL string-length error after exception recovery
Fixed cross-module symbol resolution corruption
Fixed bundle directory listing for bundled applications (VFS)
Fixed sigil-run re-import of (sigil core) before calling entry main
Fixed process port caching for stable repeated access
Removed dead opcodes (OP_LOAD_BYTECODE, OP_MAKE_CLOSURE_RT, OP_WRITE_BYTECODE_FILE)