From c80945ec21ddbd208f7191b2547ad217d6b76987 Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Sun, 25 Oct 2026 06:50:19 -0500
Subject: [PATCH] Leave timing and cache placement variables out of the
 frontend cache key

The native frontend cache key included every SCRIPTC_* variable, so
turning SCRIPTC_TIMING on or off, or changing SCRIPTC_CACHE_DIR,
SCRIPTC_CACHE_MAX_MB, SCRIPTC_EMIT_WORKERS or
SCRIPTC_EMIT_WORKER_FUNCTIONS, reran the frontend on an unchanged
program. The frontend's output does not depend on these variables, so
the key now leaves them out. Every other SCRIPTC_* variable still keys
the entry.
---
 packages/compiler/src/native/prepare.test.ts | 27 +++++++++++++++++++-
 packages/compiler/src/native/prepare.ts      | 26 ++++++++++++++-----
 2 files changed, 46 insertions(+), 7 deletions(-)

diff --git a/packages/compiler/src/native/prepare.test.ts b/packages/compiler/src/native/prepare.test.ts
index 5d0f9faae..07698820e 100644
--- a/packages/compiler/src/native/prepare.test.ts
+++ b/packages/compiler/src/native/prepare.test.ts
@@ -1,7 +1,7 @@
 import { mkdirSync, mkdtempSync, readdirSync, readFileSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { afterEach, expect, test } from "vitest";
+import { afterEach, expect, test, vi } from "vitest";
 import {
   LINUX_ARM64_GNU_TARGET,
   nativeCodegenTarget,
@@ -23,6 +23,7 @@ function text(input: ReturnType<ReturnType<typeof fixture>["prepare"]>) {
 
 const scratch: string[] = [];
 afterEach(() => {
+  vi.unstubAllEnvs();
   for (const path of scratch.splice(0)) rmSync(path, { recursive: true, force: true });
 });
 
@@ -197,6 +198,30 @@ test("structurally invalid cached features cause a rebuild even with a valid dig
   expect(f.loads()).toBe(2);
 });
 
+test("timing, cache placement and emission worker variables keep the frontend output", () => {
+  for (const name of [
+    "SCRIPTC_CACHE_DIR",
+    "SCRIPTC_CACHE_MAX_MB",
+    "SCRIPTC_EMIT_WORKERS",
+    "SCRIPTC_EMIT_WORKER_FUNCTIONS",
+    "SCRIPTC_TIMING",
+    "SCRIPTC_TEST_FRONTEND_KEY",
+  ])
+    vi.stubEnv(name, undefined);
+  const f = fixture();
+  expect(f.prepare().ok).toBe(true);
+  vi.stubEnv("SCRIPTC_CACHE_DIR", f.cache.root);
+  vi.stubEnv("SCRIPTC_CACHE_MAX_MB", "8192");
+  vi.stubEnv("SCRIPTC_EMIT_WORKERS", "1");
+  vi.stubEnv("SCRIPTC_EMIT_WORKER_FUNCTIONS", "100");
+  vi.stubEnv("SCRIPTC_TIMING", "0");
+  expect(f.prepare().ok).toBe(true);
+  expect(f.loads()).toBe(1);
+  vi.stubEnv("SCRIPTC_TEST_FRONTEND_KEY", "1");
+  expect(f.prepare().ok).toBe(true);
+  expect(f.loads()).toBe(2);
+});
+
 test("serialized IR travels as bounded parts through the frontend cache", () => {
   const f = fixture();
   const first = f.prepare({ emitIr: true });
diff --git a/packages/compiler/src/native/prepare.ts b/packages/compiler/src/native/prepare.ts
index a10d16585..11b8f9dd1 100644
--- a/packages/compiler/src/native/prepare.ts
+++ b/packages/compiler/src/native/prepare.ts
@@ -211,6 +211,24 @@ function implementationIdentity(toolchain: NativeToolchain): string {
   );
 }
 
+/** SCRIPTC_* variables that report on a build, place its cache or set
+ * backend emission parallelism. The frontend's output does not depend on
+ * them, so they are left out of its cache key. */
+const FRONTEND_INDEPENDENT_VARIABLES = new Set([
+  "SCRIPTC_CACHE_DIR",
+  "SCRIPTC_CACHE_MAX_MB",
+  "SCRIPTC_EMIT_WORKERS",
+  "SCRIPTC_EMIT_WORKER_FUNCTIONS",
+  "SCRIPTC_TIMING",
+]);
+
+/** The SCRIPTC_* variables a frontend cache entry depends on, sorted by name. */
+function frontendEnvironment(): [string, string | undefined][] {
+  return Object.entries(process.env)
+    .filter(([name]) => name.startsWith("SCRIPTC_") && !FRONTEND_INDEPENDENT_VARIABLES.has(name))
+    .sort(([a], [b]) => a.localeCompare(b));
+}
+
 function validFeatures(value: unknown): value is NativeLinkFeatures {
   if (value === null || typeof value !== "object" || Array.isArray(value)) return false;
   const features = value as Record<string, unknown>;
@@ -363,9 +381,7 @@ export function prepareNativeExecutable(
           implementation: implementationIdentity(toolchain),
           target: toolchain.target.name,
           declarations: toolchain.declarationsRoot ?? null,
-          environment: Object.entries(process.env)
-            .filter(([name]) => name.startsWith("SCRIPTC_"))
-            .sort(([a], [b]) => a.localeCompare(b)),
+          environment: frontendEnvironment(),
         }),
       );
       const cached = readFrontendCache(cache, key, exclusions);
@@ -493,9 +509,7 @@ export function prepareNativeLibrary(
           implementation: implementationIdentity(toolchain),
           target: toolchain.target.name,
           declarations: toolchain.declarationsRoot ?? null,
-          environment: Object.entries(process.env)
-            .filter(([name]) => name.startsWith("SCRIPTC_"))
-            .sort(([a], [b]) => a.localeCompare(b)),
+          environment: frontendEnvironment(),
         }),
       );
       const cached = readFrontendCache(cache, key, exclusions);