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+# Lower phase — L4 and L5 status (2026-10-05)
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+
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+Branch **`ast-stage-c`**. All slices below are byte-verified
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+(`cmp gen_ssa/<Mod>.ssa gen_ssa/<Mod>L.ssa`), gated by the suite and the
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+self-hosting fixpoint.
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+
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+## Landed
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+
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+| Tag | Commit | Content |
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+| --- | --- | --- |
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+| `v3-lower4` | `53a1d15` | builtins, `CASE`, `WITH` |
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+| `v3-lower5-core` | `ab507e2` | records, arrays, designators, aggregate copies |
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+| `v3-lower5-lit` | `5e23bfe` | char/real/string/set literals, `IN`, typed relations |
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+| `v3-lower5-sets` | `081869f` | set arithmetic, string concatenation |
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+| `v3-lower5-params` | `0b22c8a` | aggregate formals, value open arrays |
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+
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+Suite **191/191**; `FIXPOINT OK` (stage2 == stage3, 3,304,549 bytes).
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+
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+## Design
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+
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+- **Unified walk.** `LowerExprFull(n, ..., asAddr)` handles both value
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+ and address contexts; `LowerExpr` is the value wrapper and
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+ `LowerDesign` the address wrapper. Selectors (`.field` / `[i]` / `^`)
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+ are walked once, then `FinalLoad` applies Fact's trailing
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+ scalar/pointer load (aggregates keep the address).
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+- **Gate.** `CanType`/`CanDesign`/`CanCaseLabel`/`CanBuiltin` mirror the
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+ walk's supported subset; `CanLower` accepts TYPE declarations and
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+ aggregate VAR/proc types.
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+- **Builtins** dispatch by callee name (before user calls):
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+ `ABS CAP CHR ORD SIZE/TSIZE MIN/MAX SHIFT ROTATE ADR`,
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+ statements `INC/DEC INCL/EXCL NEW/DISPOSE`.
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+- **CASE** is a compare-chain mirroring `CaseStat/CaseAlt/CaseLabel`
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+ (value and `lo..hi` labels, ELSE arm).
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+- **WITH** uses a `withRec`/`withTop` stack shared by the gate and the
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+ walk; bare field idents resolve through `WithField` + `QbeGen.TopWith`.
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+- **Relations** dispatch on operand type: `IN`, pointer (`CmpL`), set
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+ (`CmpSet`), string (`StrEq`), long (`CmpLong`), else `Cmp`.
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+- **Arithmetic** covers set `+ - * /` (`SetBinOp`) and string `+`
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+ (`StrFold`/`StrCat`), plus proper long/real result types.
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+- **Open arrays**: a value `ARRAY OF T` formal is passed by descriptor
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+ address (`FuncParam(..., isV OR IsOpenArray(t))`), matching the
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+ grammar.
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+
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+## Tests
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+
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+`t_lower4` (builtins/sets/pointers), `t_lower4case`, `t_lower4with`,
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+`t_lower5` (records/arrays), `t_lower5arr` (open-array + VAR record
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+params), `t_lower5lit` (literals/IN), `t_lower5set` (set arithmetic +
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+string concat).
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+
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+## Remaining (documented, not blocking)
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+
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+- **Nested arrays** (`ARRAY OF ARRAY`) — the known codegen bug
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+ (`docs/wip/nested-array-field-bug.mod`); Lower does not add support.
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+- **UString** literals/ops.
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+- **Aggregate constants** (record/array CONST values).
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+- **Qualified names / imports / class methods** — L6 territory.
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+
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+## Not touched
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+
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+`QbeGen` remains at its committed state (stride fix only); the general
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+array-of-record array-field fix stays deferred
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+(`docs/summary_l3_selfhost_fix.md`).
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