125 lines
4.3 KiB
Markdown
125 lines
4.3 KiB
Markdown
# PBS IRVM Optimization Pipeline Specification
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Status: Draft v1 (Pipeline Baseline)
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Applies to: placement and invariants of the `OptimizeIRVM` stage between lowering and bytecode emission
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## 1. Purpose
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This document defines the normative placement and safety invariants of the `OptimizeIRVM` stage.
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Its purpose is to allow optimization in the backend pipeline without transferring semantic optimization responsibility to runtime.
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## 2. Scope
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This document defines:
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- canonical stage placement for `OptimizeIRVM`,
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- input/output contracts of optimization stage boundaries,
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- semantic-preservation invariants for optimization passes,
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- profile-compatibility obligations (`vm_profile`) across optimization and emission,
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- and minimum validation expectations for optimization-stage regressions.
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This document does not define:
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- a mandatory list of optimization passes,
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- pass scheduling heuristics,
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- or runtime dynamic optimization/JIT policies.
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## 3. Authority and Precedence
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Normative precedence:
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1. Runtime authority (`docs/specs/hardware/topics/chapter-2.md`, `chapter-3.md`, `chapter-9.md`, `chapter-12.md`, `chapter-16.md`)
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2. Bytecode authority (`docs/specs/bytecode/ISA_CORE.md`)
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3. `20. IRBackend to IRVM Lowering Specification.md`
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4. `15. Bytecode and PBX Mapping Specification.md`
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5. This document
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If a rule here conflicts with higher-precedence authorities, it is invalid.
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## 4. Normative Inputs
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This document depends on:
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- `20. IRBackend to IRVM Lowering Specification.md`
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- `15. Bytecode and PBX Mapping Specification.md`
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- `19. Verification and Safety Checks Specification.md`
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- `docs/pbs/decisions/IRVM Optimization Stage Placement Decision.md`
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## 5. Canonical Backend Pipeline Order
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For executable backends, the canonical order is:
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1. `LowerToVM`: `IRBackend -> IRVM`,
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2. `OptimizeIRVM`: `IRVM -> IRVM` optimized,
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3. `EmitBytecode`: optimized `IRVM -> BytecodeModule` pre-load artifact.
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`OptimizeIRVM` must not be skipped in a pipeline that claims this profile unless explicitly configured as a no-op stage that still preserves contracts.
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## 6. Input and Output Contracts
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### 6.1 Input contract
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`OptimizeIRVM` input must:
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1. satisfy lowering obligations from `20`,
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2. carry selected `vm_profile`,
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3. and remain structurally valid for optimizer analyses.
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### 6.2 Output contract
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`OptimizeIRVM` output must:
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1. be semantically equivalent to input under the selected `vm_profile`,
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2. preserve callable/slot/return contracts,
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3. preserve deterministic mapping behavior required by emission,
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4. and be valid for bytecode emission and subsequent runtime verification.
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## 7. Optimization Invariants
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Optimization passes must satisfy all of:
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1. semantic preservation of observable program behavior,
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2. determinism for the same admitted input graph and selected profile,
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3. profile compatibility (no introduction of out-of-profile executable opcode forms),
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4. preservation of host-backed vs VM-owned call-boundary classification,
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5. and preservation of diagnostics/source-attribution hooks required at v1 minimum.
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## 8. Runtime Responsibility Boundary
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1. Semantic/structural program optimization belongs to compile-time backend stages.
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2. Runtime remains responsible for load, patch, verify, and execute responsibilities.
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3. Runtime-internal micro-optimizations are allowed only when they do not alter bytecode-observable semantics or contract behavior.
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## 9. Validation Expectations
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At minimum, optimization-stage validation must include:
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1. equivalence-oriented regression fixtures across optimized vs non-optimized pipeline outputs,
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2. preservation of known negative loader/verifier rejection behavior,
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3. and deterministic reproduction of artifact-level invariants required by `15`.
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## 10. Explicit Deferrals
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The following are intentionally deferred:
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- target-specific optimization profitability models,
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- advanced interprocedural pipelines beyond v1 baseline,
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- and formal equivalence proof frameworks.
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## 11. Non-Goals
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- Defining runtime JIT or adaptive execution.
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- Replacing loader/verifier contracts.
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- Mandating one optimizer architecture or pass framework.
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## 12. Exit Criteria
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This document is healthy when:
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1. stage placement is explicit and implemented in pipeline order,
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2. optimization invariants are explicit and test-backed,
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3. backend/runtime responsibility boundary is explicit,
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4. and integration safety evidence references this stage concretely.
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