The contiguous-stack varargs walk depends on SelectionDAGBuilder's
argument-copy elision making &last_fixed the incoming stack slot. The
elision requires the fixed object and the parameter alloca to have equal
size, so an i8/i16 anchor silently stays a private local and the walk
reads the wrong slot. Reject a variadic definition unless its last fixed
argument is i32 or a pointer, and pin the i32 anchor's FP-64 slot and the
first variadic at one slot below in m4-varargs-anchor.ll.
arti
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