fix(runtime/darwin): rewrite aarch64 _start in mach's inline-asm dialect#321
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the aarch64 darwin _start was written in standard ARM syntax that mach's inline-asm dialect rejects, so darwin-aarch64 could not link: '#'-prefixed immediates (mach treats '#' as the comment char, stripped at lower time, so 'add x1, sp, #8' became 'add x1, sp,'), a 4-operand shifted-register add 'add x2, x1, x2, lsl #3', and a bitmask-immediate 'and sp, sp, #-16'. mirror the CI-tested linux aarch64 _start instruction-form style — bare immediates, an explicit lsl+add for the envp scale, and no explicit stack realignment (sp is 16-byte aligned on arm64 kernel entry) — while preserving the darwin semantics: entry symbol 'start', the _rt_argc/_rt_argv/_rt_envp capture, _rt_init then main, and the darwin 'svc 0x80' exit. closes #320
This was referenced Jun 28, 2026
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Closes #320.
The aarch64 darwin
_startwas written in standard ARM assembler syntax thatmach's inline-asm dialect rejects, so
mach build . --target darwin-aarch64couldnot link the runtime. Rewrites it in the dialect every other (working) runtime uses,
with zero behavior change.
Offending forms → fix
#-prefixed immediates (add x1, sp, #8,mov x16, #1,svc #0x80, ...) —#is mach's inline-asm comment character (stripped at lower time), so these became
malformed operands. → bare immediates (as the x86_64
_startalready uses).add x2, x1, x2, lsl #3— unsupported. → explicitlslthen 3-operandadd.and sp, sp, #-16— unsupported (and the register form can'ttarget SP). → dropped: sp is 16-byte aligned on arm64 kernel entry (an
architectural invariant the CI-tested linux aarch64
_startalready relies on).Mirrors the instruction-form style of the tested
src/runtime/linux/aarch64.mach_startwhile preserving the darwin semantics: entry symbolstart, the_rt_argc/_rt_argv/_rt_envpcapture,_rt_init→main, and the darwinsvc 0x80exit. The file header doc is updated to match.Validation
Built end-to-end with a mach compiler whose lock points at this commit:
mach build . --target darwin-aarch64 --profile releaselinks cleanly (nocset/undefined symbol/malformederrors) to a valid arm64 Mach-O executable —MH_NOUNDEFS, full segment set (__PAGEZERO/__TEXT/__DATA/__DATA_CONST/__LINKEDIT),LC_UNIXTHREADentry, and an ad-hocLC_CODE_SIGNATURE— verifiedwith llvm-otool. (Execution on macOS is gated on mach #1680's runners.)
Pairs with mach #1714 (the mach-side
csetinline-asm dispatch); both areprerequisites for the aarch64-darwin half of mach #1680.