Why does the MIPS Architecture need a new ABI?
An ABI (Application Binary Interface, though that hardly helps) is a set of rules governing compiled programs which - if followed - make the programs able to be linked together (for calling and to share data) and to be comprehensible to various useful bits of software - that includes debuggers, the Linux kernel, and run-time loaders.
Earlier MIPS ABIs were interpreted as machine-specific extensions to the cross-architecture [SVR4]; but the definition of OS services in a unix-like system now falls to POSIX and (specifically) Linux. This specification does not include the machine-indepedent parts of SVR4 ABI by reference or otherwise. You might like to brush up on your MIPS ABI History.
The existing MIPS ABIs were evolved substantially by Silicon Graphics Inc (SGI) for various versions of their Irix OS; they are fairly typical of ABIs for Linux and other sophisticated operating systems. At least to date most MIPS embedded systems have got by using a subset of SGI's complicated ABIs - o32, n32 and n64. So What's wrong with o32, n32 and n64?.
Specific Goals for NUBI
- Replace multiple existing ABIs : a new family should be good enough for all the applications we can make contact with, and a seed for consolidation on a single standard.
- Make better use of registers : o32's limit of four argument registers causes unnecessary stack shuffling, and programs would run slightly better if we reserved more. Eight argument registers has been tried with success, notably by n64/n32.
We will define more saved registers. Not only is this common for other architectures, but it is evident that the GNU C compiler would quite often generate better code with a few more of these.
- Add a thread pointer : a per-thread pointer in a reserved register makes for efficient thread-local storage.
- Avoid unnecessary trouble with MIPS16e: the MIPS16 compact-code standard uses half-size (16-bit) instructions, and one of the trade-offs made means it only has first-class access to eight general-purpose registers. We want to ensure the ABI's register use does not cause avoidable pain to MIPS16e programs (1).
- Better position-independent code (PIC) : all Linux shared libraries and applications are built PIC, so PIC efficiency matters. The general PIC code sequences for external data access and subroutine linkage are quite slow: we want to permit more optimisations of PIC calling and data referencing sequences.
- Reduce 32-/64-bit incompatibility : 64-bit hardware is already with us. It will be easier for applications to migrate to 64-bit computing if we can offer transition tools which can build applications from a mixture of 32- and 64-bit modules. This will never be seamless, but we believe it's worth making it practicable in controlled circumstances.
Want to know more? See Introducing NUBI.
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