From d39fe580fcd369e83e9e4804ce02ee7b950e2d06 Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cdo@wip.sh>
Date: Thu, 19 Mar 2026 22:03:36 +0000
Subject: [PATCH] Implement sampling profiler

---
 prof.js | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)
 create mode 100644 prof.js

diff --git a/prof.js b/prof.js
new file mode 100644
index 0000000..f612a74
--- /dev/null
+++ b/prof.js
@@ -0,0 +1,48 @@
+const INTERVAL_MS = 1;
+const RS_TOP_ADDR = 0x10000;
+const PROF_DATA_ADDR = 0x58;
+const PROF_DATA_IDX = PROF_DATA_ADDR / 8;
+
+let mem_8;
+let mem_64;
+let sampler;
+
+const samples = [];
+
+function sample() {
+	const data = Atomics.load(mem_64, PROF_DATA_IDX);
+	const ip = Number(data & 0xffffffffn);
+	const rsp = Number(data >> 32n);
+	samples.push({ ip, rs_bytes: mem_8.slice(rsp, RS_TOP_ADDR) });
+}
+
+function i32(bytes) {
+	return bytes[0]
+		| (bytes[1] << 8)
+		| (bytes[2] << 16)
+		| (bytes[3] << 24);
+}
+function postproc({ ip, rs_bytes }) {
+	const rs = [];
+	for (let i = 0; i < rs_bytes.length; i += 4)
+		rs.push(i32(rs_bytes.slice(i, i + 4)));
+	rs.reverse();
+	return { ip, rs };
+}
+
+self.onmessage = (e) => {
+	switch (e.data.type) {
+	case "start":
+		console.log("Starting profiler");
+		mem_8 = new Uint8Array(e.data.mem.buffer);
+		mem_64 = new BigUint64Array(e.data.mem.buffer);
+		ip = e.data.ip;
+		rsp = e.data.rsp;
+		sampler = setInterval(sample, INTERVAL_MS);
+		break;
+	case "stop":
+		clearInterval(sample);
+		console.log("Stopped profiler");
+		self.postMessage(samples.map(postproc));
+	}
+};