libgcc_vr4300

A subset of libgcc, targetting the MIPS VR4300 CPU.

What is this?

This repository provides various functions of libgcc, targetting specifically the MIPS VR4300 CPU (the one used by the Nintendo 64).

This library is meant to be used with MIPS GCC toolchains that do not provide their own libgcc.

What is libgcc?

libgcc is a runtime library provided by GCC. This library contains functions used by GCC "whenever it needs to perform some operation that is too complicated to emit inline code for" (https://gcc.gnu.org/onlinedocs/gccint/Libgcc.html). There's no way to tell GCC to emit instructions for those operations instead of using functions from libgcc.

Most functions from libgcc are software based instructions for performing integer and float arithmetic that the target architecture can't process directly.

GCC can also emit implicit calls to standard C functions like memcmp or memset. Said functions are not part of an official libgcc.

Features

This library was made to be used with N64 decompilation projects that want to support building with modern GCC toolchains, this means not every function from an official libgcc library are implemented, only the ones that have been required by the tested games.

• libgcc implemented functions:
  • Software floating point emulation functions.
    • Float from integer conversion: __floatdidf, __floatdisf, __floatundidf, __floatundisf.
    • Integer from float conversion: __fixdfdi, __fixsfdi, __fixunsdfdi. __fixunssfdi.
    • Miscellaneous float operations: __powidf2, __powisf2.
  • Integer arithmetic functions.
    • Division operations: __divdi3, __udivdi3.
    • Reminder operations: __moddi3, __umoddi3.
• libc implemented functions:
  • This library also provides some libc functiosn because GCC is able to emit them implicitly.
  • Memory functions: memcmp, memset, memcpy, memmove.
• Linking this as a library to a project allows the linker to only link the functions that the project actually uses, contrary to simply linking all the functions and wasting precious space.
• All the functions provided by this library are marked as weak functions in the case the user provides their own implementations for some of the functions of this library and wants to use those instead.
• Compatible with various ABIs: o32, n32, o64, n64, eabi32 and eabi64.

Why I made this?

I think it is very annoying that every decompilation repository that wants to have support for a modern GCC toolchain may require copying a file that implements the required functions again and again. Some of those repositories may add new features to that file or fix bugs, but those changes won't went spreaded automatically to the other repositories.

Because of this I decided to make this library, with the idea of every new addition and fix to be placed on a centralized place that everybody can use. Just wire up the makefile to download the prebuilt libraries and link them and you are done!

Download

This repository provides prebuilt binaries in the releases page.

If you want a direct link for the latest version of a specific ABI access https://github.com/Decompollaborate/libgcc_vr4300/releases/latest/download/libgcc_vr4300-ABI.tar.gz and replace ABI with your desired ABI. For example, the download link of the o32 ABI is the following: https://github.com/Decompollaborate/libgcc_vr4300/releases/latest/download/libgcc_vr4300-o32.tar.gz. This link will always point to the latest version without needing to change it.

Building

Dependencies

There are a few build dependencies to build this library locally:

• build-essential
• gcc-mips-linux-gnu (or any other GCC MIPS cross compiler)

To install those dependencies on a Debian/Ubuntu-like distro:

apt install build-essential gcc-mips-linux-gnu

Local building

After cloning the repository, run the following command.

make TARGET_ABI=ABI

Change ABI to the desired ABI to build. The supported values for ABI are o32, n32, o64, n64, eabi32 and eabi64.

To use a different GCC cross compiler, pass CROSS= with the prefix of the toolchain. For example CROSS=mips-linux-gnu-.

The built library will be placed on build/ABI/libgcc_vr4300.a, where ABI is the ABI passed to the make command.

References

• https://en.cppreference.com/w/c/string/byte/memcmp
• https://en.cppreference.com/w/c/string/byte/memset
• https://en.cppreference.com/w/c/string/byte/memcpy
• https://en.cppreference.com/w/c/string/byte/memmove
• https://gcc.gnu.org/onlinedocs/gccint/the-gcc-low-level-runtime-library/routines-for-floating-point-emulation.html
• https://gcc.gnu.org/onlinedocs/gccint/Soft-float-library-routines.html
• https://gcc.gnu.org/onlinedocs/gccint/the-gcc-low-level-runtime-library/routines-for-integer-arithmetic.html