So, with fixes for bug 1699686, bug 1699689, and bug 1699696 i'm to the point where the mips64 simulator runs the wasm jit-tests. I still hit some assertions (when running with MIPS_UNALIGNED=1 to filter out alignment issues):
Hit MOZ_CRASH(call) at /home/lhansen/m-u/js/src/jit/mips64/Simulator-mips64.cpp:2224 results from unsupported simulator ABI, there are very many of these errors, and after a little investigation, Clang reports that 20 signature types are not supported.
Assertion failure: rt != scratch, at /home/lhansen/m-u/js/src/jit/mips64/MacroAssembler-mips64.cpp:439 is some register management problem but I haven't looked deeper. There are very many of these too, especially when running the spec tests. They seem to be caused by the ScratchRegister being used without the ScratchRegisterScope (or the bugs would have been noticed a long time ago); this problem seems to be widespread in the MIPS macroassembler files.
There are also several segfaults.
Altogether, 674 tests failed, 1510 tests passed (69% pass rate), 16 timeouts.
It would be good to have these problems addressed so that we have a stable test bed for MIPS64 too. We try to account for MIPS in the wasm compilers, but without any way of testing what we do it's hard to say that that's worth the effort.