AIF layout selection

AIF can evaluate field order, flat aggregate storage, linked hot/cold splitting, and explicit data views. The purpose is not to make every type exotic; it is to choose a representation whose total construction, traversal, mutation, and destruction cost is justified by evidence.

Inputs

The layout model consumes type size and alignment, field access frequency, traversal structure, container use, and workload annotations where available. src/aif/layout.psm defines the self-hosted model, while runtime/aif_support.c contains supporting evaluation machinery.

Object and container context

A locally cheaper object split can be globally worse when it turns one flat container element into two allocations connected by a pointer. Current evidence includes exactly that failure mode. Layout work must price allocation and release traffic, flat storage eligibility, pointer chasing, copying, and vectorization together.

Explicit soa, aos, and DataView operations make selected layout changes visible to the programmer. Automatic layout remains constrained by observable identity, FFI boundaries, debug information, and safe element views.

Acceptance

A cost-model change needs a discriminating layout test, a runtime value test, a manifest assertion, and an A/B workload that measures the representation actually selected. A predicted improvement is not a measured improvement.

Type graph and exact sizing

aifRegisterTypeEdges records nominal containment/reference edges for every struct and enum. The native aif_compute_type_acyclic computes strongly connected components so recursive types are not classified from a shallow field check.

aifComputeSizes walks declarations through aifComputeSizesOf. aifComputeStructSize uses aifTypeBytes, aifStructFieldBytes, aifTypeAlign, and aifAlignUp to model the selected target. It writes exact sizes through aif_struct_set_size. The model is checked against backend ir_struct_size, ir_struct_field_offset, and ir_struct_field_size, which query LLVM target data directly.

aifFieldIsInlineExact distinguishes embedded value storage from pointer/reference fields. An optional struct is a reference edge; a non-optional struct field is embedded. Getting this wrong changes both size and lifetime traversal.

Field-order selection

aif_struct_new, aif_enum_new, and aif_struct_add_field register nominal layout inputs. aif_field_access(type, field, weight) accumulates static or measured access weights. aif_traversal_begin, aif_traversal_elem, and aif_traversal_end record sequential access patterns.

aif_layout_select evaluates candidate orders. The report API exposes aif_layout_candidates, aif_layout_best, aif_layout_cand_at_rank, aif_layout_cand_field_hot, aif_layout_cand_bytes, and aif_layout_cand_ratio. The selected field order is read with aif_layout_field and aif_layout_field_bytes; aif_layout_reordered reports whether it differs from source order.

A workload profile enters through aif_profile_load. aif_profile_source and aif_profile_is_measured distinguish observed data from static estimates. Field ranges use aif_field_range_lo, aif_field_range_hi, and aif_field_range_bytes; an observed range may guide layout but never justify a source-semantic narrowing.

Split selection and vetoes

aif_layout_split_select chooses the hot-field count. aif_layout_hot_count is consumed by ir_struct_type_split. Before selection, aifLayoutVetoInline, aifLayoutVetoListElements, and aifLayoutVetoDataViews reject layouts whose address, container, view, FFI, or full-scan behavior cannot preserve semantics.

Native veto functions include aif_layout_no_split, aif_layout_no_split_unmodelled, and aif_layout_no_split_list_full_scan. aif_layout_veto_reason gives the diagnostic explanation. aif_layout_force(type, hotCount) is a test/diagnostic assertion; the aif_layout_forced_* queries report whether the requested split was legal and applied.

Layout tests should compare source order, selected order, exact offsets, hot/cold byte counts, generated GEP paths, release of cold-owned fields, debug metadata offsets, list/DataView vetoes, and measured runtime behavior.