Xo 0.18 release notes

Introduction

Xo 0.18 adds a profiler, debug line tables, and race checking, moves the runtime’s stress tests to Xo, and makes arm64 code faster.

Tools

• Profiling: XO_PROF=cpu,heap, or xo run and xo test with --profile=cpu,heap, writes pprof output and a text summary, on the arm64 and LLVM backends, on macOS and Linux (decision 0238).
• Debug information: DWARF line tables from both native backends (a dSYM on macOS), so lldb stops at .xo lines and shows inlined calls, and the profiler names .xo lines. XO_TRACEBACK prints a fault’s frames, and xo test sets it (decision 0239).
• Races: xo build --backend=llvm --race builds every function, the runtime’s too, with ThreadSanitizer. The races it reported in the runtime are fixed.

Runtime

• The scheduler has a monitor thread and defers wakes for channel hand offs and spawns, bounded by the waker’s next runtime call. Real-time programs get none, XO_DEFER_WAKE=0 turns it off, and mutex wakes stay direct (decision 0077).
• The runtime’s C stress tests are Xo programs (decision 0241).

Compiler

• arm64: SHA-256 on the SHA instructions; add with carry chains with constant operands and the zero register; bic, orn, eon and extr; one more register for the allocator. RSA, P-256, P-521 and X25519 are faster on arm64.
• The WebAssembly backend passes its compiler build test again (it failed since 0.15).

Ports

• macOS arm64 links with Xo’s own linker by default again. The XProtect false positive and the stall at a program’s first launch from each path are fixed. TLS loads Security and CoreFoundation only at the first certificate check.