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README.md

i686 Backend -- 32-bit x86

The i686 backend targets 32-bit x86 (IA-32) with the System V i386 ABI (cdecl). It covers the full pipeline from IR to ELF executable: code generation (instruction selection, register allocation, peephole optimization), a builtin assembler (reuses the x86-64 AT&T parser with a 32-bit encoder, producing ELFCLASS32 objects), and a builtin linker (32-bit ELF, .rel relocations, dynamic or static linking).

Directory Structure

i686/
  codegen/            Code generation and peephole optimizer
  assembler/          Builtin i686 assembler (shared x86 parser, 32-bit encoder)
  linker/             Builtin i686 linker (32-bit ELF, R_386 relocations)

Sub-Module Documentation

Module README
Code generation codegen/README.md
Assembler assembler/README.md
Linker linker/README.md

Key Characteristics

  • ABI: cdecl (all arguments on stack), with -mregparm=N and fastcall support
  • ILP32 type model: Pointers are 4 bytes, long is 4 bytes, long long requires eax:edx register pairs
  • Accumulator model: Values flow through %eax; only 3 callee-saved registers (ebx, esi, edi) plus ebp with -fomit-frame-pointer
  • 64-bit splitting: 64-bit arithmetic splits across eax:edx pairs with carry/borrow propagation; 64-bit values stored in 8-byte stack slots
  • F128 (long double): Native x87 80-bit extended precision in 12-byte stack slots
  • PIC mode: GOT-relative addressing via %ebx as GOT base register
  • Assembler: Reuses x86-64 AT&T parser, 32-bit instruction encoding
  • Linker: Dynamic and static linking with PLT/GOT, 32-bit ELF, .rel (not .rela)
  • -m16 / .code16gcc: Supports 16-bit real-mode code generation for Linux kernel early boot