Xo 0.21 release notes

Introduction

Xo 0.21 builds Intel Mac programs with Xo’s own backend and no C toolchain, makes native binaries smaller, brings dbg to every backend, adds Int word methods and lookup commands for agents, and makes TLS and HTTP cheaper.

Changes to the language

• dbg(expr) works on every backend and in --no-gc builds, and --release strips it (decision 0265).
• Int’s word operations are methods that compile to inline operations: wrapping_shl, shr_logical, rotate_right, rotate_right32, swap_bytes, mul_hi_unsigned, lt_unsigned and le_unsigned, next to the existing wrapping_add, wrapping_sub and wrapping_mul (decision 0260).

Tools

• Lookups for agents: xo def, xo refs, xo type, xo doc and xo symbols (decision 0261).
• Miscompile triage: XO_PASSES turns optional optimization passes off by name, and xo dev bisect-pass finds the first pass whose build behaves differently. Property tests reduce a failing case automatically.
• xo test --std-module runs a standard library module’s own tests (decision 0262).
• XO_POISON_FRAMES for finding reads of stale stack memory.
• Test programs no longer outlive their tests: each runs in its own process group.
• xo vet warns about large copies with XO0530 (decision 0263).

Compiler

• Native builds drop every function nothing calls. hello is 26.5% smaller, and the JSON, HTTP and TLS servers 13.5 to 16.7% smaller on an Apple silicon Mac, smaller than LLVM’s builds. xo itself went from 57 MB to 42 MB.
• Tuple folding after inlining: HTTP/2 11.5% and TLS 8.9% fewer instructions per request.
• Fixes to range analysis, including a crash on an empty interval.

Standard library

• Bytes from outside the program that are not valid UTF-8 become a valid Str at every boundary (decision 0259).
• TLS handshakes are 13.9% cheaper on arm64 and 22% on x86-64; HTTP/2 is 11.6% cheaper, and more from moving reference counted payloads. P-256 and X25519 are faster.

Ports

• Intel Macs: Xo’s own x86-64 backend now targets darwin/amd64, with its own Mach-O assembler (objects byte identical to clang’s) and linker. All 193 test programs pass on an Intel Mac with no C toolchain (decision 0254).
• Windows: Xo’s own calls on Windows will use the System V convention, decided for the coming windows/amd64 port.