Introduction

This is a C/C++ Game Boy Advance toolchain based on LLVM.

Currently there are binary packages available for Linux (x86_64, AArch64) and macOS (arm64). See the https://github.com/stuij/gba-llvm-devkit/releases page to grab the latest packages.

The CMakefile.txt file in the source repo will build all the LLVM and GBA tools and components from scratch. See the Building from source section below for instructions.

Why

No good reason but 'seemed like a fun idea at the time'. Mostly to make sure that LLVM would not loose the ability to compile for ARMv4T. For example compiler-rt and LLD didn't support ARMv4T before this was fixed as part of this project.

Also, the more choice the better. It's good to have an alternative to GCC. And the GBA is a fun platform to try LLVM concepts on.

Components

• LLVM compiler suite tools: clang/clang++, lld, lldb, etc..
• compiler-rt: LLVM builtins runtime support
• LLVM libcxx: C++ library
• ETL: embedded alternative to common C++ STL constructs, can be used in conjunction with libcxx
• picolibc: embedded C library
• gba_cart.ld: linker script for GBA cart executables (as opposed to multiboot)
• gba_crt0.s: startup code for the GBA
• libtonc: GBA library
• Apex Audio System: play music and sound effects on the GBA
• Grit: GBA graphics swiss army knife
• gbafix: fix up GBA headers

Usage

command-line invocation

TLDR: use the following invocations to create a GBA program:

<root>/bin/clang --config armv4t-gba.cfg -Wl,-T,gba_cart.ld program.c -o program.elf
<root>/bin/llvm-objcopy -O binary program.elf program.gba
<root>/bin/gbafix program.gba

These are essentially the same commands you'd execute when using DevkitArm, except that -specs=gba.specs is replaced with --config armv4t-gba.cfg.

See the below sections for a breakdown of what's involved.

armv4t-gba.cfg

Specifying --config armv4t-gba.cfg will include cmdline arguments found in the armv4t-gba.cfg file found in the same directory as clang.

Which are:

• --target=armv4-none-unknown-eabi
• -mfpu=none
• -fno-exceptions
• -fno-rtti
• --sysroot <CFGDIR>/../lib/clang-runtimes/arm-none-eabi/armv4t
• -lcrt0-gba
• -D_LIBCPP_AVAILABILITY_HAS_NO_VERBOSE_ABORT

Breakdown:

For cross-compiling, one of the main differences between GCC and LLVM is that LLVM can target multiple architectures from the same binary. So LLVM needs to be told this on the command-line by using what is called a target triple.

The GBA sports an ARM7TDMI chip, which is from the ARMv4T architecture family. We're targeting the Arm Embedded ABI in a bare-metal environment, so the target triple becomes:

--target=armv4-none-unknown-eabi

And we have no fpu:

-mfpu=none

We currently don't support C++ exceptions or RTTI, so for C++ programs we should disable those:

-fno-exceptions
-fno-rtti

Furthermore the compiler should understand where it can find the correct libraries and headers. As Clang can potentially target multiple architectures and multiple library variants, we need to supply a --sysroot argument. the libraries for ARMv4T can be found at <root>/lib/clang-runtimes/arm-none-eabi/armv4t:

--sysroot <CFGDIR>/../lib/clang-runtimes/arm-none-eabi/armv4t

Where <CFGDIR> is a magic variable with points to the config file directory. This should be handled by Clang's multiboot, but there's currently an outstanding bug when targetting Thumb, so for now this works too.

We also need to include the GBA startup code library, which is resolved relative to sysroot.

-lcrt0-gba

Lastly as we're on an embedded platform, and we usually don't have stderr and friends available. Recently in verbose aborting has been enabled by default in libcxx, which means that on an abort, an error message is printed. To stop the compiler linking in symbols that don't exist, add:

-D_LIBCPP_AVAILABILITY_HAS_NO_VERBOSE_ABORT

gba_cart.ld

Speaks somewhat for itself.

The linker file can be found at <root>/lib/clang-runtimes/arm-none-eabi/armv4t/lib/gba_cart.ld. As we've set --sysroot, the linker can resolve gba_cart.ld.

In our example above, as we need to pass the linker file from the driver to the linker, the cmdline argument becomes:

-Wl,-T,gba_cart.ld

This will set up the correct data regions for the GBA, which the startup code uses to put code and data in the right place.

Targeting memory regions

There are three memory regions that code and data could reside in:

• iwram (internal working ram)
• ewram (external working ram)
• the default is cartidge rom

iwram and ewram are mapped to linker sections iwram and ewram, so use the appropriate C or asm declarations:

• C: extern void foobar(void) __attribute__((section("iwram")));
• arm: .section iwram

Alternately, filenames of the form *.iwram.c or *.ewram.c will be placed in the appropriate section.

Env variable

After unpacking the archive to the desired location, point the GBA_LLVM environmental variable to the root of the installation:

export GBA_LLVM=/root/of/your/installation

This variable will be used by the gba-llvm-devkit examples and some compatible software packages to configure themselves correctly. Although the examples will use the relative path to the toolchain if they can't find GBA_LLVM.

Known command-line differences between GCC and LLVM

There are of course a lot of command line differences between GCC and LLVM. But if you've used DevkitArm, these are the ones you might have a good chance to run into:

• LLVM has no support for -mthumb-interwork. This is fine because these days, this option does nothing (AAPCS calling convention is the default).
• LLVM does not support the long-call function attribute. Which is also fine, because these days the linker automatically places range extention thunks for long calls.
• LLVM does not support spec files. It copes by using multilib and config files.

Let us know if you encounter others worth mentioning.

Debugging

See the debugging document.

Building from source

repo

https://github.com/stuij/gba-llvm-devkit

Prerequisites

Recent versions of the following tools are pre-requisites:

• A toolchain such as GCC/Clang & binutils
• git
• cmake
• make
• meson (Use pip for latest version. 1.2.1 works.)
• ninja
• python3
• autoconf
• automake suite of tools
• libtoolize
• libfreeimage (for Grit)
• qemu to run general LLVM tests

Linux:

sudo apt-get install python3 git make ninja-build qemu-system-arm libfreeimage-dev libtool automake autoconf cmake clang
pip install meson

macOS:

• install Xcode from the app store to get Clang

• then use your favorite package manager (homebrew in the below example) to install the following dependencies:

brew install python3 git make ninja qemu libfreeimage libtool automake autoconf cmake clang meson

Windows:

This toolchain hasn't been tested on Windows yet at all, but hopefully not too much tweaking should be necessary

Before you build on macOS:

Some tweaks are necesary for macOS to compile Grit:

• for macOs silicon you to explicitly set the include and lib paths of your package manager so Grit can find libfreeimage. This is a known issue: Homebrew/brew#13481.
• GNU libtool has a name clashes with the macOS libtool which is different. The conventional workaround is to name GNU libtool glibtool, and consequently name libtoolize glibtoolize.
• The system/Xcode clang is stuck on c++98 by default. But Grit needs at minimum c++14.

To overcome this, CMakeList.txt sets some cache variables, so they can be overwritten from the command line. The lib/include paths default to the Homebrew default install location:

LIBTOOLIZE=glibtoolize
GRIT_LDFLAGS="LDFLAGS=-L/opt/homebrew/lib"
GRIT_CPPFLAGS="CPPFLAGS=-I/opt/homebrew/include"
GRIT_COMPILER="CXX=clang -std=c++14"

On non-macOS platforms LIBTOOLIZE is set to libtoolize and the rest of the vars are empty.

Commands to build:

mkdir build
cd build
cmake .. -GNinja -DFETCHCONTENT_QUIET=OFF
ninja

ninja install
# or
ninja package-gba-llvm

This will (should) create a functional toolchain in the 'build' dir:

• ninja package-gba-llvm will create a package with the general form: gba-llvm-devkit-1-Linux-AArch64.tar.xz
• ninja install will install all the needed files to the install directory that will contains all the install targets of all the CMAKE subtargets, some of which `ninja package-gba-llvm omits. So this includes a lot more LLVM tools for example, which might be useful for development of the toolchain itself.

Testing the toolchain/examples

ninja check-llvm
ninja check-picolib
ninja check-compiler-rt
ninja check-cxxabi
ninja check-unwind
ninja check-cxx

The below will check everything, but we're failing some expected llvm tests at the moment, and after that the picolibc tests won't be run.

ninja check-llvm-toolchain

Just checking the runtimes:

ninja check-llvm-toolchain-runtimes

The toolchain doesn't have automated GBA tests yet, but you can run the examples in the 'examples' dir. See below.

Examples

The distribution package comes with a couple of examples.

Two very basic ones to just test compiling a GBA program with C and C++ and another basic one, called ubsan, triggers an undefined behaviour sanitizer.

A more involved one is the slides example, which is a poor-persons integration test of sorts, which uses Tonc, Apex Audio System and Grit.

If the GBA_LLVM environmental variable is set, the examples will use this to find the root of the toolchain. If not set, the examples will use the relative path from the example dir to to the root dir of the toolchain.

Type make within an example directory to build.

Issues

For bug reports or any questions, create an issue on the source repo: https://github.com/stuij/gba-llvm-devkit/issues

Todo

• convert/include Maxmod sound library.
• Include posprintf printf replacement.
• The GBFS tools are included in the distribution, but not the GBFS GBA library and header file.
• linker script for multiboot
• some kind of GBA-specific test suite
• add tonc examples repo
• support stdout/stdwarn/stderr
• debug why memcpy seems to be so slow
• macOS universal libraries
• LLVM currently does no ARMv4T-specific optimizations. Should be a bunch of low-hanging fruit (for example loop unrolling).

Resources

If you want to learn how to do Game Boy Advance development, but don't know where to start, this page will get you on your way:

https://gbadev.net/getting-started.html

Changelog

v1:

Initial release. Packages for Linux and macOS. Tested by compiling a relatively involved rom.

No immediately known issues exept for that memcpy seems to be really slow. Needs an investigation. Workaround: use memcpy16/memcpy32.