Dual Backend Architecture

Gat features a data-driven backend design (src/target.gat and src/codegen.gat) allowing the compiler to emit standalone native binaries for two distinct OS platforms from a single unified AST and IR representation.

1. 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 (for dynamic heap sizing)
- CreateThread, WaitForSingleObject (for concurrency)
- GetCommandLineA, ExitProcess, GetProcessId, Sleep
Zero C Runtime Dependency: Emits no references to msvcrt.dll or ucrtbase.dll.

2. Linux Backend (ELF64 Direct Syscalls)

Target Selection

gat build main.gat -o main --target=linux

Architecture & Format

Format: Executable and Linkable Format 64-bit (ELF64), static executable (ET_EXEC).
Zero Libc Dependency: Bypasses glibc, musl, and dynamic linkers completely. Emits direct x86-64 syscall instructions:

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

3. Bitwise Reproducible Self-Hosting

The compiler is written in Gat itself and can self-host on both platforms:

On Windows: fc /b gatc-stage2.exe gatc-stage3.exe produces 0 bytes difference (100% bitwise identity).
On Linux: cmp gatc-gen2 gatc-gen3 produces an exact bitwise match.