Factor investigation notes

Findings from investigating the Factor VM, and experiments with the language, shared with the Factor team.

Windows ARM64: faults inside VM primitives are unrecoverable

The build-windows-arm CI job fails in core/kernel/kernel-tests.factor at [ f 0 alien-unsigned-1 ] [ vm-error? ] must-fail-with. This is not caused by the PRs it appears on.

• Cause: VM primitives are entered through the assembly stub trampoline, which has no Windows unwind data. When a primitive’s C++ code faults, Windows cannot unwind back to the Factor frame whose exception handler would turn the fault into a vm-error, so the process dies with 0xC0000005.
• Contents: the write-up has the diagnostic CI runs, unwind walks, and two candidate fixes, each tested on CI (including faults through trampoline2, used for FFI calls with stack arguments), for the team to choose between.

Read the write-up

Zig VM: differential GC fuzzing

The zig-vm-fuzzing branch adds a fuzzer for the Zig VM that runs random GC-heavy Factor programs on it and on the C++ VM, and compares the output. It also adds GC stress flags to the Zig VM (-gc-zeal, -gc-zeal-code, -nursery-budget, -verify-heap).

• Found: the three Zig VM fixes in #3211, #3212 and #3213. With each fix reverted, the fuzzer detects the bug again.
• Contents: setup, running and reading a campaign, which flags catch which bug, and pitfalls. It is not zig build --fuzz.

Read the guide

Zig VM: format-float returns an empty string on Linux

On glibc the Zig VM’s (format-float) returns an empty byte-array for every call, so sprintf with %f or %e fails, and 41 formatting tests fail. The C++ VM is unaffected.

• Cause: the Linux LC_ALL_MASK in src/primitives/math.zig is 0xFFF, which glibc’s newlocale rejects with EINVAL (glibc’s mask is 0x1FBF).
• Contents: a reproduction, the cause, and a one-line fix on the zig-vm-format-float-locale branch.

Read the bug report

Symbolic algebra: an experiment

The symbolic-algebra branch adds a computer algebra system in extra/math/symbolic: symbolic expressions and simplification, derivatives and integrals, limits, user-defined rewrite rules, and compilation of an expression back into a quotation.

• Status: an experiment, not a proposal. Names and syntax may change.
• Contents: what the four vocabularies cover, how to try the branch, and a tutorial of forty-five worked problems whose Factor blocks are all machine-checked against what Factor actually prints.

Read the write-up

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