unix: avoid allocations for common uses of Readv, Writev, etc.

Fixes golang/go#57296

Change-Id: Ifd57487122a590df467e97e2d35f388a58cc080d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/457815
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

unix/syscall_linux_test.go

@@ -1132,3 +1132,44 @@ func TestPwritevOffsets(t *testing.T) {
 		t.Fatalf("expected size to be %d, got %d", want, info.Size())
 	}
 }
+
+func TestReadvAllocate(t *testing.T) {
+	f, err := os.Create(filepath.Join(t.TempDir(), "test"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { f.Close() })
+
+	test := func(name string, fn func(fd int)) {
+		n := int(testing.AllocsPerRun(100, func() {
+			fn(int(f.Fd()))
+		}))
+		if n != 0 {
+			t.Errorf("%q got %d allocations, want 0", name, n)
+		}
+	}
+
+	iovs := make([][]byte, 8)
+	for i := range iovs {
+		iovs[i] = []byte{'A'}
+	}
+
+	test("Writev", func(fd int) {
+		unix.Writev(fd, iovs)
+	})
+	test("Pwritev", func(fd int) {
+		unix.Pwritev(fd, iovs, 0)
+	})
+	test("Pwritev2", func(fd int) {
+		unix.Pwritev2(fd, iovs, 0, 0)
+	})
+	test("Readv", func(fd int) {
+		unix.Readv(fd, iovs)
+	})
+	test("Preadv", func(fd int) {
+		unix.Preadv(fd, iovs, 0)
+	})
+	test("Preadv2", func(fd int) {
+		unix.Preadv2(fd, iovs, 0, 0)
+	})
+}