Tune LLVM splitting for dev libraries (#193)

- Lower the dev-library split floor and bucket target for faster localized rebuilds.
- Preserve executable, release, sanitized, and cache-validation behavior.

packages/compiler/src/backend/llvm/split.test.ts

@@ -3,7 +3,7 @@ import { mkdtemp, readFile, writeFile } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { afterAll, expect, test } from "vitest";
-import { splitLlvmProgram } from "./split.js";
+import { splitLlvmLibraryProgram, splitLlvmProgram } from "./split.js";
 
 const scratch: string[] = [];
 afterAll(async () => {
@@ -77,6 +77,20 @@ test("thread-local program state conservatively keeps the single-TU path", () =>
   expect(splitLlvmProgram(tls, { minimumBytes: 0, targetBytes: 64 * 1024 })).toBeNull();
 });
 
+test("dev libraries split at the measured 2MB crossover while executables retain 4MB", () => {
+  const body = Array.from({ length: 700 }, (_, i) =>
+    `define internal i64 @library_pad_${i}() #0 {\nentry:\n  ; ${"x".repeat(3072)}\n  ret i64 ${i}\n}\n`,
+  ).join("\n");
+  const source = SAMPLE.replace("define i64 @public_entry", `${body}\ndefine i64 @public_entry`);
+  expect(Buffer.byteLength(source)).toBeGreaterThan(2 * 1024 * 1024);
+  expect(Buffer.byteLength(source)).toBeLessThan(4 * 1024 * 1024);
+  expect(splitLlvmProgram(source)).toBeNull();
+  expect(splitLlvmLibraryProgram(source)?.shards).toHaveLength(5);
+
+  const belowCrossover = source.slice(0, Math.floor(1.9 * 1024 * 1024));
+  expect(splitLlvmLibraryProgram(belowCrossover)).toBeNull();
+});
+
 test("every shard compiles and its merged object exposes only canonical public definitions", async () => {
   const body = Array.from({ length: 120 }, (_, i) =>
     `define internal i64 @pad_${i}() #0 {\nentry:\n  ; ${"x".repeat(2048)}\n  ret i64 ${i}\n}\n`,

packages/compiler/src/backend/llvm/split.ts

@@ -44,6 +44,14 @@ export interface LlvmProgramSplitOptions {
 
 const DEFAULT_TARGET_BYTES = 2 * 1024 * 1024;
 const DEFAULT_MINIMUM_BYTES = 4 * 1024 * 1024;
+// Library edits pay a relocatable merge/archive cost after shard compilation,
+// but their stable per-bucket object cache starts winning earlier than the
+// executable lane's whole-program posture. Node 24 Linux profiling across
+// 1.62–3.24MB generated modules found the crossover at roughly 2MB; four
+// function buckets were the best general tradeoff for the 2.15MB target and
+// remained profitable through 3.24MB. Executables retain the defaults above.
+const LIBRARY_TARGET_BYTES = 768 * 1024;
+const LIBRARY_MINIMUM_BYTES = 2 * 1024 * 1024;
 
 interface FunctionDef {
   source: string;
@@ -236,3 +244,11 @@ export function splitLlvmProgram(
     ],
   };
 }
+
+/** Dev-library policy tuned independently from executable splitting. */
+export function splitLlvmLibraryProgram(source: string): LlvmProgramSplit | null {
+  return splitLlvmProgram(source, {
+    minimumBytes: LIBRARY_MINIMUM_BYTES,
+    targetBytes: LIBRARY_TARGET_BYTES,
+  });
+}

packages/compiler/src/index.ts

@@ -3,7 +3,7 @@ import { basename, dirname, join, resolve } from "node:path";
 import { buildCacheRoot, CcCompileError, compileC, compileLibArchive, executableNativeEnvironmentFingerprint, mobileLibraryTarget, mobileTargetRefusal, prepareBuildCacheRoot, pruneBuildCache, resolveCc, targetPlatform } from "./backend/cc.js";
 import { emitModule } from "./backend/emission/emitter.js";
 import { emitLlvmModule, LlvmUnsupportedError } from "./backend/llvm/emitter.js";
-import { splitLlvmProgram } from "./backend/llvm/split.js";
+import { splitLlvmLibraryProgram, splitLlvmProgram } from "./backend/llvm/split.js";
 import { rebaseLibrarySourceComments, replaceLibraryIdentity, stripLibraryIdentity, stripLibrarySourceComments } from "./backend/library-identity.js";
 import { checkerPanicDiag, ffiNativeBuildDiag, libAsyncExportDiag, libAsyncSurfaceDiag, libExportUnresolvedDiag, libGenericExportDiag, libIntBoundaryDiag, libNpmIneligibleDiag, libSidecarDiag, libUnmappableSignatureDiag, iceDiag, isCheckerPanic, LIB_INBOUND_BYTES_TRAP_CODE, LIB_RUNTIME_TRAP_CODES, type ScrDiagnostic } from "./diagnostics/diagnostic.js";
 import { checkLibraryIntegerSlots, classSeed, hasIntSlots, numberCarrierKind, type FnIntSlots, type IntSlotConfig } from "./library/int-infer.js";
@@ -1748,7 +1748,7 @@ async function compileLibraryNative(
   }
   const llvmSplit =
     profile.emission === "llvm" && profile.optimization === "dev" && !sanitize && programSource !== undefined
-      ? splitLlvmProgram(programSource)
+      ? splitLlvmLibraryProgram(programSource)
       : null;
   await compileLibArchive({
     cPath,