What's wrong with o32, n32 and n64?
Linux/MIPS (up to now, Autumn 2005) has made progress using recognisable dialects of standards defined originally by MIPS Corporation and SGI:
- o32 is for 32-bit CPUs, or 64-bit CPUs running only a 32-bit subset of the instruction set.
- n32/n64 are for 64-bit CPUs only. n64 has 64-bit pointers and long integers, whereas n32 has 32-bit pointers and long integers. (Other than that they're pretty much the same and will be considered together).
o32 is fairly different from n32/n64, so we'll pull out individual problems in separate sections.
What's wrong with all of them?
- Inefficient PIC code is resistant to optimisation.
- No general-purpose register available as a thread pointer for efficient thread-local storage.
- Not enough "saved" ("callee-saved") registers, limiting the compiler's ability to keep variables in registers.
- The calling and register conventions make it very hard to build gaskets to allow 32-bit code to intercall 64-bit code and vice versa. MIPS Technologies thinks such gaskets would help embedded systems builders make the transition to using real 64-bit code.
What's wrong with o32?
o32 has been an orphan for a long time. Somewhere in the mid-1990s SGI dropped it completely, because all their systems had been using real 64-bit CPUs for some time.
- Committed to the obsolete MIPS I floating point model, which hides 16 of the FP registers.
- Only four argument registers, which means arguments are too often passed on the stack.
- Many recent improvements to the o32 standard as used by GNU tools and Linux are undocumented: DWARF debug format, for example. So a great deal of undocumented folklore is required to build a compatible implementation. In practice, few implementations are compatible at debug level, and many are incompatible even for interlinking object code.
What's wrong with n32/n64?
- Non-SGI use is really an uneasy subset of n64. The real thing would probably break GNU tools. The GNU dialect is undocumented. There are dozens of ill-documented object code section types and other magic numbers.
- n32/n64 have only ever been Irix/Linux standards. There's no established "bare-iron" version of the object code for embedded use.
- n64 as defined by SGI used their own (early, unique) extensions to DWARF debug information.
- n32/n64 are annoyingly different from each other for reasons which probably made sense at the time, but certainly don't now.
That last bullet point has led to some people asking: