clang miscompiles preserve_none on aarch64: a preserve_none callee uses
x19 as its frame base without saving it, so a caller that also uses x19
as its frame base resumes with a clobbered frame
(llvm/llvm-project#70577, llvm/llvm-project#177519). ASan's larger
frames make it reliable: n_string2() is left with nulled locals and the
parser segfaults on the valid document `""`.
preserve_none is only an optimization for the musttail chain, so fall
back to the default calling convention where it is not trustworthy:
- Define PRESERVE_NONE only when musttail is available (without it the
parse() trampoline calls every continuation with an ordinary call, and
those calls returned WeaselJson_REJECT for `""`) and we are not
building for aarch64 with ASan.
- Stop annotating scan_string()/scan_string_impl() with it: n_string2()
calls them with an ordinary call and keeps buf and bufEnd live across
it, so they must preserve their caller's callee-saved registers.
Add a regression test for the minimal valid documents, parsing through
parser3::Parser3 directly so the continuations run under the flags of
the test binary; it segfaults in the aarch64+ASan build without this
change.
Closes#78.
weaselbot