syscall: mark SyscallN as noescape

syscall.SyscallN is implemented by runtime.syscall_syscalln, which makes
sure that the variadic argument doesn't escape.

There is no need to worry about the lifetime of the elements of the
variadic argument, as the compiler will keep them live until the
function returns.

Fixes #70197.

Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-longtest,gotip-windows-amd64-race
Change-Id: I12991f0be12062eea68f2b103fa0a794c1b527eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/625297
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

2 files changed, 48 insertions, 0 deletions

diff --git a/src/syscall/dll_windows.go b/src/syscall/dll_windows.go
index 81134cb0bd..a7873e6ad8 100644
--- a/src/syscall/dll_windows.go
+++ b/src/syscall/dll_windows.go
@@ -42,6 +42,7 @@ func Syscall15(trap, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a
 // Deprecated: Use [SyscallN] instead.
 func Syscall18(trap, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18 uintptr) (r1, r2 uintptr, err Errno)
 
+//go:noescape
 func SyscallN(trap uintptr, args ...uintptr) (r1, r2 uintptr, err Errno)
 func loadlibrary(filename *uint16) (handle uintptr, err Errno)
 func loadsystemlibrary(filename *uint16) (handle uintptr, err Errno)
diff --git a/src/syscall/syscall_windows_test.go b/src/syscall/syscall_windows_test.go
index 8b5af9c501..63409daf04 100644
--- a/src/syscall/syscall_windows_test.go
+++ b/src/syscall/syscall_windows_test.go
@@ -227,6 +227,53 @@ func TestGetStartupInfo(t *testing.T) {
 	}
 }
 
+func TestSyscallAllocations(t *testing.T) {
+	testenv.SkipIfOptimizationOff(t)
+
+	t.Parallel()
+
+	// Test that syscall.SyscallN arguments do not escape.
+	// The function used (in this case GetVersion) doesn't matter
+	// as long as it is always available and doesn't panic.
+	h, err := syscall.LoadLibrary("kernel32.dll")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer syscall.FreeLibrary(h)
+	proc, err := syscall.GetProcAddress(h, "GetVersion")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testAllocs := func(t *testing.T, name string, fn func() error) {
+		t.Run(name, func(t *testing.T) {
+			n := int(testing.AllocsPerRun(10, func() {
+				if err := fn(); err != nil {
+					t.Fatalf("%s: %v", name, err)
+				}
+			}))
+			if n > 0 {
+				t.Errorf("allocs = %d, want 0", n)
+			}
+		})
+	}
+
+	testAllocs(t, "SyscallN", func() error {
+		r0, _, e1 := syscall.SyscallN(proc, 0, 0, 0)
+		if r0 == 0 {
+			return syscall.Errno(e1)
+		}
+		return nil
+	})
+	testAllocs(t, "Syscall", func() error {
+		r0, _, e1 := syscall.Syscall(proc, 3, 0, 0, 0)
+		if r0 == 0 {
+			return syscall.Errno(e1)
+		}
+		return nil
+	})
+}
+
 func FuzzUTF16FromString(f *testing.F) {
 	f.Add("hi")           // ASCII
 	f.Add("â")            // latin1