Spilling a weak ref forces a spill slot, but omits the restore.
Spill slots for snapshot refs override the register, anyway.
Marking snapshot refs weak avoids pointless restores.
Typically reduces storage overhead for snapshot maps by 60%.
The extensible format is a prerequisite for the next redesign steps:
Eliminate IR_FRAME and implement return-to-lower-frame.
Only works with DWARF2-enabled GCC 4.x (not the default MinGW GCC).
Fix fastcall symbol names for COFF assembler output.
Add DWARF2 unwind info to COFF assembler output.
Use COFF assembler mode for MinGW builds.
Always enable the DWARF2 handler if compiled with GCC.
To build the x64 interpreter open a "Windows SDK Command Shell".
Then set the compiler environment: setenv /release /x64
Then cd to the src directory and run msvcbuild.bat.
All upvalue objects hold a disambiguation hash value now.
It's built from the parent prototype and the slot number.
Different hash values imply the upvalues cannot alias.
Same hash values don't imply anything (collision or different closures).
Upvalue disambiguation makes use of a reduced hash due to IR contraints.
Modify unwinding to always return _ff or _c unwind type.
Generate PE object .pdata/.xdata sections for x64 interpreter.
Can drop r12-r15 saves in Windows/x64 interpreter now.
Make external unwinding the default on x64.
It's mandatory on WIN64 due to the abundance of callee-saved regs.
Allow piecewise internal frame unwinding and optional cleanup.
Store ERRMEM, ERRERR and ERRCPP early and copy down later.
Use FRAME_CP for lj_vm_resume.
Add lj_vm_unwind_*_eh variants as landing pads for external unwinder.
Use fastcall for lj_vm_unwind_*.
Can drop r12/r13 saves in POSIX/x64 interpreter now.
Get number of multiple results from C frame.
Add lj_cont_hook: restores multres and dispatch to static ins.
Can use fastcall for lj_dispatch_ins() now.
