RFC #0001

[MotherOfProgrammers]

[RFC-0001] M4 Native Codegen Lowering Strategy & System V AMD64 Stack Contract

• Status: Proposed
• Author: MotherOfProgrammers (MOP)
• Target Milestone: M4/Lithon JIT Backend

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1. Summary

This RFC formalizes the execution contract and lowering strategy for converting verified ir::Module basic blocks into bare-metal x86-64 machine instructions.

With M1–M3 complete (PEP 526 parser, 11/11 untyped regression tests, and 12/12 typed range-verified regression tests passing), M4 establishes how lithonc lowers static $0\text{ malloc}$ basic blocks directly into executable memory (mmap/PROT_EXEC) or ELF static binaries.

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2. Core Architectural Invariants

Every backend lowering pass in Lithon/Boa must enforce the following four hardware guarantees:

1. Zero Runtime Safety Guards: Hot loops MUST NOT emit bounds-checking or type-assertion branches. All safety contracts ($[a, b]$ interval bounds) are proven at compile time by typecheck.cpp.
2. Zero Dynamic Allocation ($0\text{ malloc}$): Stack frames are pre-computed during basic block layout. All scalar variables sit at fixed negative offsets relative to rbp.
3. System V AMD64 Calling Convention Compliance:
   • Arguments passed in: rdi, rsi, rdx, rcx, r8, r9.
   • Scratch registers: rax, r10, r11.
   • Callee-saved registers: rbx, rsp, rbp, r12, r13, r14, r15.
   • Return values in: rax.
4. 16-Byte Stack Frame Alignment: Frame allocations (sub rsp, N) must be rounded up to 16-byte boundaries to maintain ABI alignment requirements for nested function calls.

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3. Stack Frame Layout Specification

For a basic function block, stack frame allocation is laid out strictly at negative offsets from rbp:

               +-----------------------+
               |  Caller Stack Arguments|
               +-----------------------+
               |  Return Address (8B)  |
     rbp ----> +-----------------------+
               |  Saved Frame Pointer  |  [rbp - 0]
               +-----------------------+
               |  Local Variable 1     |  [rbp - 8]
               |  Local Variable 2     |  [rbp - 16]
               |  ...                  |  ...
     rsp ----> +-----------------------+  (16-byte aligned)