context: use atomic operation in ctx.Err

oos: darwin
goarch: arm64
pkg: context
cpu: Apple M1 Pro
               │ /tmp/bench.0.mac │          /tmp/bench.1.mac           │
               │      sec/op      │   sec/op     vs base                │
ErrOK-10             13.750n ± 1%   2.080n ± 0%  -84.87% (p=0.000 n=10)
ErrCanceled-10       13.530n ± 1%   3.248n ± 1%  -76.00% (p=0.000 n=10)
geomean               13.64n        2.599n       -80.94%

goos: linux
goarch: amd64
pkg: context
cpu: Intel(R) Xeon(R) CPU @ 2.30GHz
               │ /tmp/bench.0.linux │         /tmp/bench.1.linux          │
               │       sec/op       │   sec/op     vs base                │
ErrOK-16               21.435n ± 0%   4.243n ± 0%  -80.21% (p=0.000 n=10)
ErrCanceled-16         21.445n ± 0%   5.070n ± 0%  -76.36% (p=0.000 n=10)
geomean                 21.44n        4.638n       -78.37%

Fixes #72040

Change-Id: I3b337ab1934689d2da4134492ee7c5aac8f92845
Reviewed-on: https://go-review.googlesource.com/c/go/+/653795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/src/context/benchmark_test.go b/src/context/benchmark_test.go
index 144f473a44..d10950d258 100644
--- a/src/context/benchmark_test.go
+++ b/src/context/benchmark_test.go
@@ -188,3 +188,23 @@ func BenchmarkDeepValueSameGoRoutine(b *testing.B) {
 		})
 	}
 }
+
+func BenchmarkErrOK(b *testing.B) {
+	ctx, cancel := WithCancel(Background())
+	defer cancel()
+	for b.Loop() {
+		if err := ctx.Err(); err != nil {
+			b.Fatalf("ctx.Err() = %v", err)
+		}
+	}
+}
+
+func BenchmarkErrCanceled(b *testing.B) {
+	ctx, cancel := WithCancel(Background())
+	cancel()
+	for b.Loop() {
+		if err := ctx.Err(); err == nil {
+			b.Fatalf("ctx.Err() = %v", err)
+		}
+	}
+}