From d267eaa3b94ae8efece6ae32ee889d08012097a8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bartek=20Iwa=C5=84czuk?= <biwanczuk@gmail.com>
Date: Mon, 17 Jul 2023 23:17:28 +0200
Subject: [PATCH] fix(bench): run warmup benchmark to break JIT bias (#19844)

Closes https://github.com/denoland/deno/issues/15277

This commit adds a single "warmup" run of empty function when running
`deno bench`.
This change will break so-called "JIT bias" which makes V8 optimize the
first function
and then bail out of optimization on second function. In essence the
"warmup" function
is getting optimized and then all user benches are bailed out of
optimization.
---
 cli/js/40_testing.js | 22 ++++++++++++++++++++++
 cli/ops/bench.rs     |  3 +++
 cli/tools/bench.rs   |  9 +++++++++
 3 files changed, 34 insertions(+)

diff --git a/cli/js/40_testing.js b/cli/js/40_testing.js
index 3058fcee38..740e950a31 100644
--- a/cli/js/40_testing.js
+++ b/cli/js/40_testing.js
@@ -685,6 +685,8 @@ function test(
   });
 }
 
+let registeredWarmupBench = false;
+
 // Main bench function provided by Deno.
 function bench(
   nameOrFnOrOptions,
@@ -695,6 +697,25 @@ function bench(
     return;
   }
 
+  if (!registeredWarmupBench) {
+    registeredWarmupBench = true;
+    const warmupBenchDesc = {
+      name: "<warmup>",
+      fn: function warmup() {},
+      async: false,
+      ignore: false,
+      baseline: false,
+      only: false,
+      sanitizeExit: true,
+      permissions: null,
+      warmup: true,
+    };
+    warmupBenchDesc.fn = wrapBenchmark(warmupBenchDesc);
+    const { id, origin } = ops.op_register_bench(warmupBenchDesc);
+    warmupBenchDesc.id = id;
+    warmupBenchDesc.origin = origin;
+  }
+
   let benchDesc;
   const defaults = {
     ignore: false,
@@ -777,6 +798,7 @@ function bench(
   const AsyncFunction = (async () => {}).constructor;
   benchDesc.async = AsyncFunction === benchDesc.fn.constructor;
   benchDesc.fn = wrapBenchmark(benchDesc);
+  benchDesc.warmup = false;
 
   const { id, origin } = ops.op_register_bench(benchDesc);
   benchDesc.id = id;
diff --git a/cli/ops/bench.rs b/cli/ops/bench.rs
index f569a8cbb4..0f75097510 100644
--- a/cli/ops/bench.rs
+++ b/cli/ops/bench.rs
@@ -101,6 +101,8 @@ struct BenchInfo<'s> {
   group: Option<String>,
   ignore: bool,
   only: bool,
+  #[serde(default)]
+  warmup: bool,
 }
 
 #[derive(Debug, Serialize)]
@@ -128,6 +130,7 @@ fn op_register_bench<'a>(
     group: info.group,
     ignore: info.ignore,
     only: info.only,
+    warmup: info.warmup,
   };
   let function: v8::Local<v8::Function> = info.function.v8_value.try_into()?;
   let function = v8::Global::new(scope, function);
diff --git a/cli/tools/bench.rs b/cli/tools/bench.rs
index f926cec5a9..fd686a938d 100644
--- a/cli/tools/bench.rs
+++ b/cli/tools/bench.rs
@@ -95,6 +95,7 @@ pub struct BenchDescription {
   pub group: Option<String>,
   pub ignore: bool,
   pub only: bool,
+  pub warmup: bool,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -194,6 +195,10 @@ impl BenchReporter for JsonReporter {
   fn report_output(&mut self, _output: &str) {}
 
   fn report_result(&mut self, desc: &BenchDescription, result: &BenchResult) {
+    if desc.warmup {
+      return;
+    }
+
     let maybe_bench = self.0.benches.iter_mut().find(|bench| {
       bench.origin == desc.origin
         && bench.group == desc.group
@@ -326,6 +331,10 @@ impl BenchReporter for ConsoleReporter {
   }
 
   fn report_result(&mut self, desc: &BenchDescription, result: &BenchResult) {
+    if desc.warmup {
+      return;
+    }
+
     let options = self.options.as_ref().unwrap();
 
     match result {