Sprint 18: HELLO WORLD MILESTONE (Executable)

Prerequisites

Sprints 0–17 — full read, resolve, atomize, layout, reloc-apply, dyld-info, function-starts, unwind pipeline.

Goals

Produce a runnable arm64 PIE executable. afs-ld takes hello.o + libarmfortas_rt.a + libSystem.tbd + -e _main and emits a binary that, when executed on an M-series Mac, prints "Hello, World!". Not a demo — an exit criterion for everything that came before.

Deliverables

1. Staging fixture

tests/corpus/hello/ contains:

• hello.f90: trivial Fortran program with print *, "Hello, World!".
• hello.o: assembled by afs-as from armfortas's hello.s output.
• Expected output on run: Hello, World!\n (Fortran print adds a leading blank on some paths — match what armfortas currently produces).

2. End-to-end link

Invocation:

afs-ld hello.o libarmfortas_rt.a \
  -lSystem -syslibroot "$(xcrun --show-sdk-path)" \
  -e _main -no_uuid -platform_version macos 11.0 14.0 \
  -o hello

Expected:

• Output file passes file hello → Mach-O 64-bit executable arm64.
• otool -lV hello accepts without errors and shows LC_MAIN, expected segments, dylibs.
• codesign -dv hello reports no signature (Sprint 22 adds ad-hoc signing — binaries still run unsigned on Apple Silicon from Xcode or a trusted source, but ./hello from a Terminal prompt requires signing. Document this caveat.).
• ./hello (after the Sprint 22 signature) prints the expected string and exits 0.

3. Differential gate

tests/hello_world.rs:

let ours = link_with_afs_ld(&inputs, &args);
let theirs = link_with_system_ld(&inputs, &args);
let diff = diff_macho(&ours, &theirs);
assert!(diff.critical.is_empty(), "critical diffs: {:#?}", diff.critical);

Allowed tolerated diffs:

• UUID bytes (we emit zero with -no_uuid, ld may or may not — both should honor -no_uuid).
• String table ordering within a partition as long as every symbol resolves to the same address.
• LC_DYLD_INFO vs LC_DYLD_CHAINED_FIXUPS if defaults disagree — gate with the same -fixup_chains / -no_fixup_chains flag.

4. Load-command otool -lV parity

Run otool -lV on both outputs; diff should be empty after normalizing absolute file offsets (ld and afs-ld may interleave __LINKEDIT regions slightly differently — document and justify any remaining diffs).

5. Execution gate (requires Sprint 22's ad-hoc signing, but staged here)

Two cases:

• Unsigned path: ./hello fails with "killed: 9" (correct Apple Silicon behavior); codesign -s - hello && ./hello works.
• Once Sprint 22 lands, afs-ld's own output is signed ad-hoc and ./hello works directly.

This sprint declares success as soon as codesign -s - hello && ./hello prints the expected string.

6. Audit

Brutal audit after Sprint 18. Same rules as armfortas audits:

• No "placeholder" or "stub" explanations that hide wrong output.
• Test every claim. Wrong output from a linker is critical.
• If the binary runs but produces extra or missing newlines, investigate — don't rationalize.

Testing Strategy

• tests/hello_world.rs runs the differential gate.
• tests/hello_world_run.rs executes the binary (gated on CI-locally-on-Mac) and asserts stdout.
• Regression fixtures: any hello-world variant that once broke gets its own test.

Definition of Done

• tests/hello_world.rs passes — zero critical diffs vs ld.
• Binary runs and produces correct output (after codesign -s - hello pre-Sprint-22).
• otool -lV diff empty after documented normalizations.
• Audit passes.