Incremental Compilation (IC)

The nim ic command provides incremental compilation for Nim projects. It decomposes compilation into per-module steps whose results are cached as NIF files, and uses the external nifmake build tool to re-run only the steps whose inputs changed.

This document describes how `nim ic` works today, including the edge cases that shaped the current design. The per-module backend rewrite that earlier editions of this document listed as a Plan has landed: the whole-program, reuse/redirect/def-retention backend is gone and codegen is now a set of nifmake-driven per-module rules (see The backend).

Overview

The pipeline has two halves driven by one process (nim ic, commandIc in compiler/deps.nim) that constructs a dependency graph, writes a build file, and hands it to nifmake:

1. Frontend — per module:
   • nifler parse --deps turns .nim source into a parsed NIF (.p.nif) plus a static dependency list (.deps.nif).
   • nim m (the semantic step, cmdM) reads the parsed NIF + the precompiled NIFs of the module's imports, type-checks, and writes the semmed NIF (.nif) plus invalidation sidecars (see Cookies).
2. Backend — nim nifc (cmdNifC, compiler/nifbackend.nim) reads the semmed NIFs, generates C, compiles and links.

nifmake orders the steps by their input/output files: every nim m runs before the nim nifc step that consumes its NIF, and a step re-fires only when one of its inputs is newer than its outputs. The driver invokes nifmake run --parallel by default, so independent steps at the same DAG depth fan out across cores; pass -d:icNoParallel to serialize (readable child output when debugging a build).

Artifacts (the NIF zoo)

Per module <suffix> (a content hash of the path; see NIF symbols below), under the nimcache directory:

File	Producer	Purpose
<s>.p.nif	nifler	parsed AST (syntactic)
<s>.deps.nif	nifler	static import list (syntactic imports)
<s>.s.deps.nif	nim m	real post-sem imports (incl. macro-generated); see Discovery
<s>.nif	nim m	semmed module (symbols resolved, typed)
<s>.iface.nif	nim m	iface cookie: hash of the importer-visible surface
<s>.impl.nif	nim m	impl cookie: hash of the entire content (bodies included)
<s>.edges.nif	nim m	NeedsImpl edges: modules whose bodies this sem consumed
<s>.c.nif	nim nifc	the C text as a NIF, with def/ref markers for DCE & dedup
ic_config.cfg.nif	driver	precompiled config replayed by every child (icconfig.nim)
ic.version	driver	format stamp; a mismatch wipes the cache (icFormatVersion)

NIF symbols and ownership

(See ../nifspec/doc/nif-spec.md.) A global symbol is <ident>.<disamb>.<moduleSuffix>. For a generic instantiation the <disamb> is not a counter but a content hash — setInstanceDisamb (modulegraphs.nim) MD5s the generic's identity plus the typeKey of every concrete type argument, masks it to 30 bits and tags it with InstanceDisambBit. So the only part of the name that varies between two modules making the same instantiation (seq[Foo]) is the <moduleSuffix>. Two consequences drive the backend:

• Instance names are content-addressed: the same instantiation produced in different modules yields the same <ident>.<disamb>, so a deterministic dedup is possible by the module-suffix-stripped name. The cross-TU C name (ccgtypes.sharedInstanceCName) and the merge stage's live-set/owner decision (nifbackend.computeMergeDecision) both key on this stripped form.
• The suffix names a mint-site owner. The <moduleSuffix> is the module that minted the instance (the instantiation site), so the same instance has a different full name in each module that makes it. Because every cg process emits the instances it demands (emit-everywhere), the same definition can be produced by several translation units; the merge stage then deterministically picks the single artifact allowed to embed each body (smallest claimant), which is the cross-process replacement for the old in-process single-writer machinery.

The driver: graph construction (commandIc)

1. Stamp/​wipe the cache by icFormatVersion.
2. Seed the graph with the root module and `system.nim`. system's entire import closure is folded into one node (one nim m invocation) — see single-writer below.
3. traverseDeps runs nifler per module and reads .deps.nif to add import edges.
4. SCC grouping: strongly-connected import cycles are collapsed (Tarjan). A singleton compiles as nim m <mod>; a cycle compiles as one nim m <rep> --icGroup:<member>… that builds every member from source in one process (resolving the recursion in memory) and writes each member's NIF. Only edges leaving the component become build-graph inputs.
5. Discovery fixpoint: write the build file, run nifmake; if it fails, re-derive the graph from every module's .s.deps.nif (adding nodes/edges for imports the static scanner missed), and retry. See Discovery.
6. The backend step (nim nifc) depends on every module's semmed NIF, so nifmake runs it last.

Invalidation: the cookie system

A dependent must re-sem only when a dependency's relevant surface changed. Two hashes per module (ast2nif.nim):

• iface cookie (.iface.nif): hashes only the importer-visible surface — exported declarations' signatures (for all routine kinds: plain procs, templates, macros, generics, inline procs alike), full content for consts/types, plus import/export/replay/hook records. Routine bodies are excluded. It also chains in the iface cookies of its own dependencies, so a surface change anywhere in the import closure propagates. A nim m rule for a module depends on its dependencies' iface cookies, so a body-only edit moves no iface cookie and stops the re-sem cascade.
• impl cookie (.impl.nif): hashes the entire serialized content (private defs and bodies included), with the module's own iface mixed in.

NeedsImpl edges (.edges.nif): if a module consumed another module's body during sem — a macro expansion, a generic instantiation, a getImpl, or a compile-time call run in the VM — it records a strong edge. The dependent is then gated on that dependency's impl cookie instead of its iface cookie, so e.g. const x = dep.foo() re-sems when foo's body changes. Recording sites: semExprs.semTemplateExpr (templates), seminst.generateInstance (generics), vmgen.genProc (VM/macros/CT procs), vm.opcGetImpl (getImpl). Inline iterators and inline procs are not tracked — they are inlined at codegen, where the backend's NIF-mtime invalidation re-codegens their users.

Discovery of macro-generated imports

The static scanner only sees syntactic imports. A macro can synthesize one (chronicles does parseStmt("import chronicles/textlines") driven by the chronicles_sinks define). Such an import is invisible until sem runs the macro. Each nim m records the imports it actually resolved (via the semdata.addImportFileDep hook → graph.importDeps → ast2nif.writeSemDeps) into <s>.s.deps.nif; a child that fails on a not-yet-built import flushes it before erroring. The driver re-derives the graph from those sidecars — adding the missing node + the importer→import edge — and reruns to a fixpoint. (This replaced an earlier icmissing.txt side channel.)

The backend: per-module nifc stages

Codegen is no longer one whole-program process. nim nifc (cmdNifC, compiler/nifbackend.nim) is invoked once per stage via --icBackendStage:<stage>; commandIc emits these as ordinary nifmake rules so "which TUs rebuild" is just "which rules nifmake re-fires from input mtimes" — exactly as the frontend already works. There are four stages:

1. `cg` (--icBackendStage:cg --icBackendModule:<suffix>) — generate C for the single named module and write only its <s>.c.nif artifact. A non-main target loads only its own import closure (loadDepClosure), so the whole program is not pulled into every parallel cg process. Codegen is still demand-driven and emit-everywhere: a cg process emits every entity it demands (generic instances, hooks, RTTI), referencing nothing extern-only. There is no whole-program DCE here — a liveness pass over all ~260 NIFs would cost ~900 MB for a result the merge stage recomputes anyway. The main module's cg is special: it loads everything (loadBackendModules), emits the whole-program method dispatchers and NimMain, and registers every other module's init/datInit from the .c.nif meta heads — so it runs last, after every other .c.nif exists. Every cg rule always leaves a .c.nif (empty if the module owns no code) so its nifmake output exists and the rule settles.
2. `merge` (--icBackendStage:merge) — a pure artifact pass, no module graph loaded. Reads every .c.nif, computes the one program-wide live set and, for each unique definition that several cg processes emitted, the single artifact allowed to embed its body; writes that to a merge-decision file (computeMergeDecision / writeMergeDecision). This is the cross-process replacement for the old in-process first-claimant + DCE coordination.
3. `emit` (--icBackendStage:emit --icBackendModule:<suffix>) — render the target module's final .c from its .c.nif and the merge decision (renderCFromArtifact, dropping globally-dead and non-owned bodies). No codegen runs; the target is loaded only so getCFile yields the path cg wrote.
4. `link` (--icBackendStage:link) — register every module's emitted .c and run extccomp.callCCompiler once (it parallelizes per-file cc and skips up-to-date objects). Per-module C compile/link directives ({.passL.} etc.) are re-collected here via replayBackendActions, since the cg processes that originally saw them are separate processes (without this, e.g. math's -lm would be lost → undefined floor/pow at link).

Because each stage is a nifmake rule keyed on file mtimes, a body-only edit to one module re-fires that module's cg+emit (and the merge/link), not the whole program — and an unchanged module's cg does not run at all.

Edge cases (and why the machinery exists)

• Single-writer. Instance type-ids are minted in process-local order, so if two nim m processes both write a module's NIF (e.g. a stdlib module pulled into system's from-source closure and given its own rule), the second overwrites with different ids and every module checked against the first carries dangling refs ("symbol has no offset"). Fixed by folding system's closure into one SCC and by forwarding the project's defines to every child so their when bodies (hence import sets and NIF contents) match the scanner's.
• `when … else: import`. nifler emits else-branch imports unguarded, so a dead else: import would be scheduled. The compiler's own sources were rewritten to explicit negated whens; the vendored nifler later learned to negate prior conditions for the else.
• `nil` sons of loaded ASTs. NIF dot-tokens load as nil where from-source ASTs have nkEmpty; several passes gained nil guards.
• Sealed loaded types. Loaded types are Sealed; sem/transform mutate via unsealForTransform/exactReplica(idgen) (the latter mints a fresh uniqueId so serialized replicas don't collapse).
• Methods/RTTI ownership. RTTI and type-bound hooks are emit-everywhere at cg and deduplicated by the merge stage, like generic instances; the main module's cg owns the whole-program method dispatchers.
• Config cost. Each child re-parsing nim.cfg + re-running config.nims in the VM was ~80 ms; replaced by a precompiled ic_config.cfg.nif replayed in loadConfigs (compiler/icconfig.nim).
• `koch bootic` bootstraps the compiler through nim ic (a 3-iteration fixed-point check). It writes its binary to bin/nim_ic and never clobbers bin/nim.

Resolved by the rewrite

The whole-program backend's hand-rolled mini-nifmake — computeModuleReuse, enforceDefRetention, redirectToLiveModule, the cached-defs/claim bookkeeping and the standalone dce.nim — is gone. Reuse is now just per-rule nifmake mtime checks, and the single-writer decision is the merge stage. The old cross-mm / `--force` `var not init` hazard dissolved with it: every codegen rule's config (including --mm) is a declared nifmake input, so a stale-config TU is simply rebuilt rather than mixed in. koch bootic is green under both orc and --mm:refc.

Known residual hack

• deps.runNifler still uses setLastModificationTime to mark its scan up-to-date and deletes a stale parsed file to coordinate with the nifmake nifler rule — the driver duplicating nifmake's freshness logic. It is explicitly flagged in the source and folds away with a full frontend/nifler split.

Status and performance

nim ic self-builds the compiler (koch bootic's byte-identical fixed-point check) under both orc and --mm:refc, and passes the external-package CI set.

Cold full bootstrap on a 32-core box (-d:release, no edits — IC's worst case, since incremental reuse is not exercised):

| wall | notes |

• | ---- | ----- |

koch boot (classic) | ~1m00s | reference |
koch bootic (nim ic) | ~1m39s | ~1.66× |

This is down from ~7.5× in the whole-program-backend era. IC does modestly more aggregate work (more processes, NIF re-parsing of imports per process), but on a many-core box that overhead is absorbed by the parallel nim m/nifc fan-out, and the C compile+link floor is shared with the classic backend. On few-core machines the cold gap is correspondingly wider — IC trades single-build latency for incremental latency.

The cold number is the least favourable comparison: it pays IC's full per-process overhead while using none of its incremental machinery. Warm rebuilds — the actual point of IC — recompile only the modules whose inputs changed (a body-only edit re-fires one module's cg+emit, not the program), so an edit-driven rebuild is a small fraction of either full build.

The strategic direction (decided 2026-06-13) is to make this NIF backend (cmdNifC) the default code generator. The per-module pipeline above is the realization of that direction; remaining work is promotion + deletion of the classic path, not new machinery.

Design notes and open decisions

The per-module backend (above) mirrors Nimony's src/nimony/deps.nim: the backend stopped re-implementing nifmake; each stage is a build rule, so reuse is just mtime checks and the merge stage is the only cross-module coordination.

Settled vs. open:

• Ownership. Emittable entities (generic instances, type-bound hooks, RTTI, lifted procs) are emit-everywhere at cg time and deduplicated at merge time (smallest claimant owns each unique body). The earlier idea of a static per-suffix owner computed before codegen was not needed — content-addressed names make the merge decision deterministic. The precise owner rule (minting module vs. root-type's module) can still be tuned where it would force a downstream package to own stdlib code.
• Remaining cleanup. The runNifler setLastModificationTime coordination (above) folds away with a full frontend/nifler split; dead when imports could also be pruned during the .s.deps re-derivation.

Validation bar (held on every change): koch bootic must reach its byte-identical fixed point, and binary size must not regress (DCE parity), across the external-package CI set.

Further possible improvements

A warm-edit profiling pass (2026-07-02, self-compiling the compiler into a dedicated --nimcache, editing one private proc body — internalErrorImpl — in the hub module compiler/msgs.nim) surfaced where a hub-module warm rebuild actually spends its time. The result refines the "a body-only edit re-fires one module" claim above: that holds for the backend, but the frontend can still cascade.

Measured: no-op 0.05s; hub body edit ~15s, split ~13s frontend / ~1.6s backend. Editing a body in a leaf (few importers) is fast; editing a body in a widely-imported module is not, and the cost is almost entirely frontend re-sem.

• Frontend over-invalidation (the dominant hub-edit cost). Editing any body in a module — even a private routine that is only ever called — flips that module's whole-module impl cookie (writeImplCookie hashes the entire serialized module). Every module carrying a NeedsImpl edge on it then re-sems, even though the symbol it actually consumed is unchanged (e.g. a dependent that expanded the internalError template needs the template body, which is untouched; it does not need internalErrorImpl's body). In the msgs edit this re-fires 57 nim m processes. A .s.bif mtime diff hides this — .s.bif is content-stable, so a re-semmed-but-identical module keeps its timestamp; count actual nim m PIDs to see the fan-out.

  The precise fix is per-symbol NeedsImpl gating: record which symbols' bodies a dependent consumed (the recording site modulegraphs.recordIcImplDep already receives the PSym; it currently coarsens to module(s.itemId)) and gate the dependent on only those. The obstacle is that nifmake gates on file mtimes, so per-symbol granularity needs either many cookie files or a bucketing scheme, and "which bodies are compile-time-consumable" is entangled with getImpl and the CT call graph (a macro that runs a private helper at CT does consume its body). A conservative narrowing — keep template/generic/macro/sfCompileTime bodies (plus getImpl targets) in the impl cookie but drop ordinary runtime routine bodies — captures the common "edit a private implementation proc" case, at the cost of proving the exclusion is complete.

• Serial re-sem chains. The 57 re-sems above run essentially one at a time despite --parallel, because the core modules they belong to form a deep import chain and nifmake's depth-barriered scheduler runs one depth level at a time (≈1 node per level). This is independent of the invalidation problem: even perfect per-symbol precision leaves a serial tail whenever the re-sem set is a chain. Mitigations live in the scheduler (content-stability already stops the cascade at one level, but does not flatten the chain).
• Emit stage need not load the module graph (done). generateEmitStage used to loadDepClosure/loadBackendModules — materializing a module's whole transitive import closure as BModules — solely to reach getCFile(bmod) for the output path. renderCFromArtifact is pure text filtering over the .c.nif plus the merge decision; it needs none of that. Deriving the .c path directly from the suffix (the same pure computation deps.backendCFile uses to declare the stage's output) lets an emit process load nothing. Under the fire-all-every-edit emit barrier (see below) this halved backend CPU (user-time 51s → 24s on the msgs edit); wall-clock barely moved because the frontend dominates, but the reduced CPU/RAM contention matters when an editor is running alongside. koch ic stays byte-identical.
• Do NOT make the merge decision content-stable. A tempting frontend to the above: emit re-fires for every live module whenever merge rewrites the decision file's mtime (deliberate — a decision change must re-render every .c consistently). Writing the decision OnlyIfChanged (with a stamp output so the merge rule is not perpetually stale) makes a warm no-op instant, but a real edit then fires emit only for the modules whose .c.nif changed — and that produces multiple-definition link errors even when the decision is byte-identical. Fire-all emit is a correctness invariant, not just insurance (see the comment at generateEmitStage): partial emit leaves inconsistent ownership across the .c set. This path was tried and reverted; do not retry.

Code, logic & debugging

Core modules:

• `compiler/deps.nim` — graph construction, SCC grouping, discovery fixpoint, build-file generation; commandIc.
• `compiler/ast2nif.nim` — AST↔NIF, the cookie hashes (cookieSd, writeIfaceCookie, writeImplCookie, writeEdgesFile, writeSemDeps).
• `compiler/nifbackend.nim` — the per-module backend stages (generateCgStage, generateMergeStage, generateEmitStage, generateLinkStage).
• `compiler/cnif.nim` — .c.nif artifact read/write, computeMergeDecision, renderCFromArtifact.
• `compiler/icconfig.nim` — precompiled config.
• `compiler/pipelines.nim` / `modulegraphs.nim` — pipeline integration and the graph state (importDeps, icImplDeps, icCnifFiles, instDisambs, …).

Manual workflow:

• Frontend a module: nim m --nimcache:nifcache path/to/mod.nim (writes .nif + cookies + .s.deps).
• Backend is stage-based (a bare nim nifc main.nim errors — there is no whole-program fallback). The exact per-stage commands nifmake runs are in the *.backend.build.nif build file; rerun one directly against an existing cache, e.g. nim nifc --nimcache:nifcache --icBackendStage:cg --icBackendModule:<suffix> main.nim to regenerate one module's .c.nif, then --icBackendStage:merge / :emit / :link.
• NIF and .c.nif files are text — open/grep them directly; diff two successive .nif to see why a module rebuilt.
• Force a re-sem: delete the module's .nif and rerun nim m.
• A stale-cache crash after editing the serialization layout means bumping icFormatVersion (compiler/options.nim).

See also

• NIF format spec: nifspec/doc/nif-spec.md
• NIFC (C-like target) spec: dist/nimony/doc/nifc-spec.md