From 9773dccee5cacb56b90efd84895067e692a70f5e Mon Sep 17 00:00:00 2001
From: Malte Ubl <malte@vercel.com>
Date: Tue, 27 Oct 2026 09:35:44 -0700
Subject: [PATCH] Compile executable program shards as a pool

Dev executables compiled their program shards in fixed batches of up
to eight, each batch waiting for its slowest shard before the next one
started. Keep up to eight clangs busy instead, each taking the next
shard when it finishes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 .../compiler/src/backend/native/executable.ts | 27 ++++++++++++-------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/packages/compiler/src/backend/native/executable.ts b/packages/compiler/src/backend/native/executable.ts
index 60e61ff0b..18c577bb5 100644
--- a/packages/compiler/src/backend/native/executable.ts
+++ b/packages/compiler/src/backend/native/executable.ts
@@ -1485,12 +1485,18 @@ export async function compileCInternal(
             missed: false,
           };
         });
-        const shardWidth = Math.min(8, availableParallelism());
-        for (let i = 0; i < entries.length; i += shardWidth) {
-          await Promise.all(
-            entries.slice(i, i + shardWidth).map(async (entry) => {
+        // Up to eight clangs (one per available core), each taking the next
+        // shard as it finishes: shards differ in size, and a fixed batch
+        // would wait for its slowest member before starting the next. A
+        // failed shard stops the others from taking more work.
+        let nextShard = 0;
+        let failed = false;
+        await Promise.all(
+          Array.from({ length: Math.min(entries.length, 8, availableParallelism()) }, async () => {
+            while (!failed && nextShard < entries.length) {
+              const entry = entries[nextShard++]!;
               await writeFile(entry.sourcePath, entry.source());
-              if (await copyValidCachedFile(entry.cachePath, entry.staged)) return;
+              if (await copyValidCachedFile(entry.cachePath, entry.staged)) continue;
               entry.missed = true;
               await runClang([
                 ...programCompilerArgs,
@@ -1499,10 +1505,13 @@ export async function compileCInternal(
                 entry.sourcePath,
                 "-o",
                 entry.staged,
-              ]);
-            }),
-          );
-        }
+              ]).catch((error: unknown) => {
+                failed = true;
+                throw error;
+              });
+            }
+          }),
+        );
         const arArgv = isZigDriver(driver) ? [driver.argv[0]!, "ar"] : ["ar"];
         const merged = await localizeLibraryObjects(
           driver,