Lazy standard-unit emission / routine-level dead-code elimination

• Type: feature (compiler / codegen size)
• Status: backlog
• Track: A
• Owner: —
• Opened: 2026-06-21
• Relation: follow-up to feature-default-standard-units (done). That change default-loads textfile + builtin into every non-ESP program; this ticket reclaims the size cost so the default surface can broaden safely.

Problem

feature-default-standard-units made textfile (and its builtin numeric-format backing) load by default. PXX has no dead-code elimination, so every non-ESP program now emits the full textfile + builtin routine bodies whether or not it does any file I/O. Measured cost:

• test/hello.pas: 29,086 → ~42,661 bytes (+~13.5 KB, +47%).

This was an accepted tradeoff (user chose "always include textfile" over a fragile per-use token scan), but it breaks the size guard in the parent ticket and scales badly as the default standard surface grows (System, more RTL).

Direction

• Add routine-level reachability: emit a unit's proc bodies only if reached from the program (transitively), not merely because the unit was uses-loaded. The proc/call graph already exists (Procs[], CallFix*, ProcAddr*); the missing piece is a mark-and-sweep over reachable procs before emission, with the entry body + InitProcs as roots.
• Keep it deterministic (reachability derives from the stable call graph, not address/map iteration order) so make bootstrap/cross-bootstrap stay byte-identical. -g/debug paths unaffected.
• Watch the landmines: floating IR_CALLs need IRIVal:=1 to emit; some builtin helpers are registered (hence "called") indirectly — those must count as roots, not be swept.
• Re-verify the size guard afterward: test/hello.pas should return to ~29 KB (no textfile routines emitted when no file I/O is used), with explicit textfile programs still complete.

Acceptance

• test/hello.pas (no file I/O) emits no textfile/builtin routine bodies and is back at or near the 29,086-byte baseline.
• A program that uses Text/Assign/WriteLn still links and runs identically.
• make test green; make bootstrap + cross-bootstrap byte-identical.

Log

• 2026-06-21 - Opened from feature-default-standard-units: default-loading textfile grew hello.pas 29,086 → ~42,661 bytes with no DCE. File this before broadening the default standard surface further.

MERGED (2026-06-30 triage)

Redundant with the sibling emission-size ticket — both are reachability-gated DCE. Merged into [[feature-emission-size-dce]]. Rejected here to avoid duplication.