druntime: stop hiding lr/d8-d15 from GC in AArch64 fiber switch - #15
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ljmf00-wekaio merged 2 commits intoAug 11, 2026
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fiber_switchContext saves x19-x28, fp, lr and d8-d15 on the fiber's stack, but reported sp + 9*8 as the stack top (tstack), deliberately hiding the saved lr and d8-d15 from the GC scan. On AArch64, d8-d15 are callee-saved, so the compiler is free to keep a live GC pointer in one of them across a suspension point (e.g. a cheap fmov d8, x0 spill). If that saved register holds the only reference to a heap object, a collection while the fiber is suspended frees memory that is still reachable, corrupting the program when the fiber resumes (WEKAPP-581124: weka stats SwitchError on ARM clients). Report the true sp instead, so the whole save area is scanned, and adjust initStack to the new convention: reserve the lr/d8-d15 words below tstack and zero them (they are now GC-scanned, so stale data on fiber reuse must not be misread as pointers). Co-Authored-By: Claude <noreply@anthropic.com> Signed-off-by: Luís Ferreira <luis.ferreira@weka.io>
Parks the sole reference to a canary object in d8 (all other copies XOR-obfuscated, stack spills and scratch registers scrubbed), yields, forces collections plus heap churn from the main context, then resumes and asserts the canary survived. Fails against the old fiber_switchContext convention (canary collected because the saved d8 was hidden from the GC scan) and passes with the true-sp convention. No-op on non-AArch64 or non-LDC builds. Co-Authored-By: Claude <noreply@anthropic.com> Signed-off-by: Luís Ferreira <luis.ferreira@weka.io>
BaruchWeka
approved these changes
Aug 11, 2026
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