Multi-Backend Architecture

Gat features a data-driven backend design (src/target.gat, src/codegen.gat, and src/codegen_arm64.gat) allowing the compiler to emit standalone native binaries across Windows PE32+, Linux ELF64 (x86-64 and AArch64), and macOS Apple Silicon Mach-O from a single unified AST and IR representation.

1. macOS Apple Silicon Backend (Mach-O ARM64)

Target Selection

Default behavior when running on macOS Apple Silicon, or explicitly via:

gat build main.gat -o main --target=macos-arm64

Architecture & Format

Format: Mach-O 64-bit Relocatable Object (MH_OBJECT, CPU_TYPE_ARM64).
ABI & Syscalls: Darwin AArch64 ABI calling convention:
- Register argument passing: X0 through X7.
- Direct kernel syscall invocation via SVC #0x80, syscall number in X16:
  - sys_mmap (197) and sys_munmap (73)
  - sys_write (4), sys_read (3), sys_open (5), sys_close (6), sys_lseek (199)
  - sys_getpid (20)
Native Linker Integration: The compiler driver natively links standalone Mach-O executables (MH_EXECUTE) directly in memory with internal symbol/relocation resolution and ad-hoc code signing (codesign), removing external clang/ld64 dependencies. Relocatable object files continue to be supported when -o file.o is specified.

2. Linux Backend (ELF64 Direct Syscalls: x86-64 & ARM64)

Target Selection

# x86-64 Linux
gat build main.gat -o main --target=linux

# ARM64 / AArch64 Linux
gat build main.gat -o main --target=linux-arm64

Architecture & Format

Format: Executable and Linkable Format 64-bit (ELF64), static executable (ET_EXEC).
Zero Libc Dependency: Emits direct syscall (x86-64) or svc #0 (ARM64) kernel instructions:

Syscall Name	x86-64 (`rax`)	ARM64 (`x8`)	Purpose in Gat Runtime
`sys_read`	0	63	File and stdin reading (`read_file`, `fs_read_all`)
`sys_write`	1	64	Console output and file writing (`print`, `write_file`)
`sys_open`	2	56 (openat)	File descriptor acquisition
`sys_close`	3	57	Resource cleanup
`sys_stat`	4	80 (fstat)	File metadata and size queries (`file_size`)
`sys_mmap`	9	222	Heap page allocation (`alloc_mem`)
`sys_munmap`	11	215	Heap page reclamation (`free_mem`)
`sys_nanosleep`	35	101	Process sleep (`proc_sleep`)
`sys_getpid`	39	172	Process ID query (`proc_get_pid`)
`sys_clone`	56	220	Native kernel thread spawning (`thread_spawn`)
`sys_exit_group`	231	94	Clean multi-threaded process termination

3. Windows Backend (PE32+)

Target Selection

Default behavior when running on Windows, or explicitly via:

gat build main.gat -o main.exe --target=windows

Architecture & Format

Format: Portable Executable (PE32+) 64-bit console subsystem.
Import Address Table (IAT): Direct linking against kernel32.dll APIs:
- GetStdHandle, WriteFile, ReadFile, CreateFileA, CloseHandle
- VirtualAlloc, VirtualFree (dynamic heap sizing)
- CreateThread, WaitForSingleObject (concurrency)
- GetCommandLineA, ExitProcess, GetProcessId, Sleep
Zero C Runtime Dependency: Emits no references to msvcrt.dll or ucrtbase.dll.

4. Bitwise Reproducible Self-Hosting

The compiler is written in Gat itself (src/compiler.gat) and self-hosts across all supported platforms:

Windows: fc /b gatc-stage2.exe gatc-stage3.exe produces 0 byte differences (100% bitwise identity).
Linux: cmp gatc-gen2 gatc-gen3 produces 0 byte differences.
macOS ARM64: cmp bin/gatc_stage2.o bin/gatc_stage3.o produces 0 byte differences.