Building Dreamcast programs With GCC spec files

May 25, 2023
gcc
dreamcast
kallistios

In the KallistiOS ecosystem, there are basically two ways to build Dreamcast programs - use the CMake toolchain support, or use the compiler wrapper scripts from the KallistiOS source tree. I was interested in the latter, but they end up depending on a large number of environment variables, which are traditionally sourced through $KOS_BASE/environ.sh. Although it’s not really a big issue to have all those environment variables, I wondered whether there was a cleaner approach.

GCC has a spec file feature, which is a custom DSL that describes how the compiler driver should process flags for the various programs in the compilation toolchain. Since the KallistiOS compiler wrappers are basically injecting compiler flags, spec files seemed like a promising approach.

It turns out that this approach works pretty nicely, and we only need to depend on the single $KOS_BASE environment variable being set. I’m pretty naive about GCC internals, but the spec file below worked for me.

*kos_header_paths:
-isystem %:getenv(KOS_BASE /include) -isystem %:getenv(KOS_BASE /kernel/arch/dreamcast/include) -isystem %:getenv(KOS_BASE /addons/include)

*kos_library_paths:
-L%:getenv(KOS_BASE /lib/dreamcast) -L%:getenv(KOS_BASE /addons/lib/dreamcast)

*kos_linker_script:
-T%:getenv(KOS_BASE /utils/ldscripts/shlelf.xc)

*kos_libs:
--start-group -lkallisti -lc %G --end-group

*kos_defines:
-D_arch_dreamcast

*kos_cc_options:
-ffunction-sections -fdata-sections -fno-builtin

*cc1_options:
+ %(kos_header_paths) %(kos_defines) %(kos_cc_options)

*cc1plus_options:
+ %(kos_header_paths) %(kos_defines) %(kos_cc_options)

*cpp_options:
+ %(kos_header_paths) %(kos_defines)

*subtarget_link_spec:
%(kos_linker_script) -Ttext=0x8c010000 --gc-sections

*link_gcc_c_sequence:
%(kos_library_paths) %(kos_libs)

I think that this makes it a lot simpler to build Dreamcast programs. In the trivial case, it can be as easy as this:

$ /usr/local/Cellar/dc-toolchain-testing/2023.05.19/sh-elf/bin/sh-elf-g++ -specs=kos-dreamcast.specs -o hello.elf hello.cpp
$ file hello.elf
hello.elf: ELF 32-bit LSB executable, Renesas SH, version 1 (SYSV), statically linked, with debug_info, not stripped

The final benefit is that this is easy to use in any old build system. You just have to add the -specs flag to your compiler flags, in what ever way the build system supports.