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Date: Tue, 1 May 2018 15:07:56 +0000
From: Toma Tabacu <>
To: "" <>
CC: Rich Fuhler <>, James Hogan <>
Subject: Introducing a nanoMIPS port for MUSL

Earlier today MIPS Tech announced the latest generation of the MIPS family of
architectures called nanoMIPS [1]. As part of the development we have been
designing all the open source tools necessary to support the architecture and,
thanks to the speed with which we were able to prototype, we have also been
using these tools to shape the architecture along the way. This has led to some
really interesting improvements in the tools, which MIPS would like to
contribute back to the community. While doing this work many of us have been
unable to contribute to the community as actively as we would have liked, we
are therefore very grateful for the community support given to the MIPS
architecture over the last 18 months. This announcement has a general
introduction at the start, so if you have already read it for one of the other
tools, you can skip down to the information specific to MUSL.

For anyone who knows the MIPS architecture you may well wonder why we are
introducing another major variant and the question is perfectly valid. We do
admittedly have quite a few: MIPS I through MIPS IV, MIPS32 and MIPS64 through
to MIPS32R6 and MIPS64R6, MIPS16e, MIPS16e2, microMIPSR3 and microMIPSR6. Each
of these serves (or served) a purpose and there is a high level of synergy
between all of them. In general, they build upon the previous and there is a
high level of compatibility, even when switching to a new encoding like moving
from MIPS to microMIPS. The switch to MIPS32R6/MIPS64R6 was a major shift in
the way the architecture innovated and drew more on the original theory of the
architecture, where evolution was not expected to be limited by binary
compatibility. MIPS Release 6 removed instructions and did create some very
minor incompatibility but is also much cleaner to implement from a
micro-architecture perspective. We have taken this idea much further with
nanoMIPS and reimagined the instruction set, by drawing on all the experience
gained from previous designs. Hopefully others will find it as interesting as
we do.

The major driving force behind the nanoMIPS architecture was to achieve
outstanding code density, while also balancing out hardware and software design
cost. As background MIPS has two compressed ISA variants: MIPS16e, which cannot
exist without also implementing MIPS32, and microMIPS, which can exist on its
own. Since MIPS16e has specific limits that cannot be engineered around, we
chose to use an approach similar to the microMIPS design.

nanoMIPS has a variable-length compressed instruction set that is completely
standalone from the other MIPS ISAs. It is designed to compress the highest
frequency instructions to 16-bits, and use 48-bit instructions to efficiently
encode 32-bit constants into the instruction stream. There is also a wider
range of 32-bit instructions, which merge carefully chosen high frequency
instruction sequences into single operations creating more flexible addressing
modes such as indexed and scaled indexed addressing, branch compare with
immediate and macro style instructions. The macro like instructions compress
prologue and epilogue sequences, as well as a small number of high frequency
instruction pairs like two move instructions or a move and function call.
nanoMIPS also totally eliminates branch delay slots which follows a precedent
set by microMIPSR6.

To get the best from a new ISA we also re-engineered the ABI and created a new
symbiotic relationship between the ISA and ABI that pushes code density and
performance further still. The ABI creates a fully link time relaxable model,
which enables us to squeeze every last byte out of the code image even when
deferring final addressing mode and layout decisions to link time. We have been
mindful of MIPS heritage and ensured that while open to any possible change, we
also have minimal impact when porting code from MIPS to nanoMIPS, and have
plenty of support to achieve source compatibility between the two.

The net effect of these changes leads to an average code size reduction of 20%
relative to microMIPSR6. This compression could well be one of the best
achieved by GNU tools for any RISC ISA. Comparing the ISA in terms of number of
instructions to issue vs microMIPS we also see a reduction of between 8% and
11% of dynamic instruction count.

Below we dig into some technical specifics for MUSL; we welcome any feedback
and questions as we start to look at rebasing this work to the trunk/master and
formally submitting it. nanoMIPS pre-built toolchains and source code tarballs
are available at:

MUSL specific details

We chose to use MUSL as the Linux user-mode C library for nanoMIPS thanks to
the incredible simplicity it offers when porting to a new architecture. With
minimal changes (and in fact removal of code compared to MIPS), we had MUSL
working as a statically linked C library within just a few days. Porting and
using the dynamic linker was equally easy, allowing us to evolve the ABI
decisions we made for dynamic executables while prototyping. Given there is so
little that needs architecture specific work in MUSL, there are only a few
technical highlights for nanoMIPS:

- added an optimized version of memcpy written in C which takes into account the
  cache subsystem of nanoMIPS architectures and allows easy modification to
  target specific implementations
- added initial experimental support for a new TLS descriptor design that
  optimises away the need for a static resolver function


- General nanoMIPS support
  Jaydeep Patil, James Hogan
- memcpy
  Rich Fuhler
- TLS/dynamic linking
  Toma Tabacu


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