prometeu-studio/docs/general/specs/20. IRBackend to IRVM Lowering Specification.md

PBS IRBackend to IRVM Lowering Specification

Status: Draft v1 (Backend Baseline)
Applies to: executable-backend lowering from IRBackend handoff into IRVM

1. Purpose

This document defines the normative lowering contract from executable IRBackend to IRVM.

Its purpose is to make backend lowering deterministic, reviewable, and compatible with runtime ISA/verification authority.

2. Scope

This document defines:

• required backend preconditions before IRBackend -> IRVM starts,
• IRVM structural obligations needed before optimization/emission,
• deterministic function-id assignment and control-flow lowering obligations,
• callsite lowering obligations across function, host-backed, and VM-owned intrinsic paths,
• mandatory compiler-side structural pre-verification before bytecode emission,
• and deterministic rejection obligations for backend-originated failures.

This document does not define:

• binary PBX layout details,
• loader patching internals,
• runtime execution internals,
• or one mandatory optimizer implementation.

3. Authority and Precedence

Normative precedence:

1. Runtime authority (docs/specs/hardware/topics/chapter-2.md, chapter-3.md, chapter-9.md, chapter-12.md, chapter-16.md)
2. Bytecode authority (docs/specs/bytecode/ISA_CORE.md)
3. docs/pbs/specs/13. Lowering IRBackend Specification.md
4. This document

If a rule here conflicts with higher-precedence authorities, it is invalid.

4. Normative Inputs

This document depends on:

• docs/pbs/specs/13. Lowering IRBackend Specification.md
• docs/pbs/specs/6.1. Intrinsics and Builtin Types Specification.md
• docs/pbs/specs/6.2. Host ABI Binding and Loader Resolution Specification.md
• 15. Bytecode and PBX Mapping Specification.md
• 21. IRVM Optimization Pipeline Specification.md

5. Backend Entry Preconditions

Lowering from IRBackend to IRVM may start only when:

1. frontend admission to IRBackend is complete for the active slice,
2. required callsite categories (CALL_FUNC/CALL_HOST/CALL_INTRINSIC) are available,
3. required canonical host/intrinsic metadata is available for admitted callsites,
4. dependency-scoped fail-fast exclusions have already been applied at the IRBackend boundary,
5. and target vm_profile is selected deterministically.

6. IRVM Model Obligations

IRVM v1 is a quasi-ISA backend form:

1. it may contain Core ISA ops and internal extension ops (IRVM_EXT),
2. each IRVM_EXT must declare structural metadata equivalent to opcode-spec needs:
   • pops,
   • pushes,
   • is_branch,
   • is_terminator,
3. and IRVM_EXT must be eliminable before bytecode emission.

IRVM must preserve, per function:

1. param_slots,
2. local_slots,
3. return_slots,
4. max_stack_slots,
5. function entry mapping and deterministic identity.

7. Control-Flow Lowering Obligations

Control-flow lowering must satisfy:

1. symbolic labels are allowed internally in lowering,
2. final jump immediates must be resolved to u32 offsets relative to function start before emission,
3. jump targets must be instruction-boundary-valid within function ranges,
4. and reachable fallthrough beyond function end without terminator is rejected.

8. Deterministic Function-ID Assignment

Function indexing must be deterministic:

1. entrypoint function id is 0,
2. remaining functions are ordered by (module_key, callable_name, source_start),
3. and identical admitted input graphs must produce identical function-id assignments.

9. Callsite Lowering Obligations

9.1 Function calls

CALL_FUNC lowers to CALL <func_id> with valid index and arity-consistent signature contracts.

9.2 Host-backed calls

CALL_HOST lowers to pre-load HOSTCALL <sysc_index>, not raw SYSCALL.

9.3 VM-owned intrinsic calls

CALL_INTRINSIC lowers to VM-owned intrinsic path and must remain distinct from host-binding paths.

9.4 Closure and coroutine constraints (v1)

1. closure calls with unknown static callee are rejected in v1,
2. SPAWN argument count must match callee param_slots,
3. and YIELD with non-empty operand stack is rejected.

10. Mandatory Compiler-Side Pre-Verification

Before bytecode emission, backend must run structural pre-verification on lowered IRVM (after resolving to Core-ISA-compatible form), covering at minimum:

1. jump target validity,
2. stack-depth join consistency,
3. underflow/overflow against declared limits,
4. return-shape consistency,
5. function-id validity,
6. callsite arity consistency,
7. and structural validity of host/intrinsic call forms.

This pre-verification does not replace runtime verifier authority.

11. Deterministic Rejection Policy

When backend lowering fails at this boundary:

1. rejection must be deterministic,
2. diagnostics identity and phase must remain stable,
3. and source attribution must be preserved whenever the failure remains source-attributable and user-actionable.

12. Explicit Deferrals

The following are intentionally deferred:

• optimizer pass catalog and tuning policy,
• richer internal IR analysis models beyond v1 obligations,
• and advanced closure/lifetime optimizations beyond v1 constraints.

13. Non-Goals

• Defining one in-memory IR implementation graph.
• Redefining runtime verifier/loader internals.
• Defining PBX binary chunk policy in this document.

14. Exit Criteria

This document is healthy when:

1. IRBackend -> IRVM obligations are explicit and testable,
2. deterministic function/call/control-flow lowering rules are explicit,
3. mandatory pre-verification boundary is explicit,
4. and scope boundaries with optimization/emission specs are explicit.