WIP try to use MSVCRT.DLL instead of UCRT

Very simple "hello world" style programs work.

.github/workflows/build-macos.yml

@@ -101,7 +101,7 @@ jobs:
       - name: make gen-device
         run: make -j3 gen-device
       - name: Test TinyGo
-        run: make test GOTESTFLAGS="-short"
+        run: make test GOTESTFLAGS="-only-current-os"
       - name: Build TinyGo release tarball
         run: make release -j3
       - name: Test stdlib packages

.github/workflows/windows.yml

@@ -114,7 +114,7 @@ jobs:
         run: make -j3 gen-device
       - name: Test TinyGo
         shell: bash
-        run: make test GOTESTFLAGS="-short"
+        run: make test GOTESTFLAGS="-only-current-os"
       - name: Build TinyGo release tarball
         shell: bash
         run: make build/release -j4

builder/builder_test.go

@@ -67,6 +67,7 @@ func TestClangAttributes(t *testing.T) {
 		{GOOS: "linux", GOARCH: "mipsle", GOMIPS: "softfloat"},
 		{GOOS: "darwin", GOARCH: "amd64"},
 		{GOOS: "darwin", GOARCH: "arm64"},
+		{GOOS: "windows", GOARCH: "386"},
 		{GOOS: "windows", GOARCH: "amd64"},
 		{GOOS: "windows", GOARCH: "arm64"},
 	} {

builder/builtins.go

@@ -3,6 +3,7 @@ package builder
 import (
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/tinygo-org/tinygo/compileopts"
 	"github.com/tinygo-org/tinygo/goenv"
@@ -229,6 +230,10 @@ var libCompilerRT = Library{
 			builtins = append(builtins, avrBuiltins...)
 		case "x86_64", "aarch64", "riscv64": // any 64-bit arch
 			builtins = append(builtins, genericBuiltins128...)
+		case "i386":
+			if strings.Split(target, "-")[2] == "windows" {
+				builtins = append(builtins, "i386/chkstk.S")
+			}
 		}
 		return builtins, nil
 	},

builder/mingw-w64.go

@@ -32,23 +32,54 @@ var libMinGW = Library{
 		mingwDir := filepath.Join(goenv.Get("TINYGOROOT"), "lib/mingw-w64")
 		return []string{
 			"-nostdlibinc",
+			"-isystem", mingwDir + "/mingw-w64-crt/include",
 			"-isystem", mingwDir + "/mingw-w64-headers/crt",
+			"-isystem", mingwDir + "/mingw-w64-headers/include",
 			"-I", mingwDir + "/mingw-w64-headers/defaults/include",
 			"-I" + headerPath,
+			"-D__MSVCRT_VERSION__=0x700", // Microsoft Visual C++ .NET 2002
+			"-D_WIN32_WINNT=0x0501",      // target Windows XP
+			"-D__LIBMSVCRT_OS__",
+			"-D_CRTBLD",
+			"-Wno-pragma-pack",
 		}
 	},
 	librarySources: func(target string) ([]string, error) {
 		// These files are needed so that printf and the like are supported.
-		sources := []string{
+		var sources []string
-			"mingw-w64-crt/stdio/ucrt_fprintf.c",
+		if strings.Split(target, "-")[0] == "i386" {
-			"mingw-w64-crt/stdio/ucrt_fwprintf.c",
+			// Old 32-bit x86 systems use msvcrt.dll.
-			"mingw-w64-crt/stdio/ucrt_printf.c",
+			sources = []string{
-			"mingw-w64-crt/stdio/ucrt_snprintf.c",
+				"mingw-w64-crt/crt/pseudo-reloc.c",
-			"mingw-w64-crt/stdio/ucrt_sprintf.c",
+				"mingw-w64-crt/gdtoa/dmisc.c",
-			"mingw-w64-crt/stdio/ucrt_vfprintf.c",
+				"mingw-w64-crt/gdtoa/gdtoa.c",
-			"mingw-w64-crt/stdio/ucrt_vprintf.c",
+				"mingw-w64-crt/gdtoa/gmisc.c",
-			"mingw-w64-crt/stdio/ucrt_vsnprintf.c",
+				"mingw-w64-crt/gdtoa/misc.c",
-			"mingw-w64-crt/stdio/ucrt_vsprintf.c",
+				"mingw-w64-crt/misc/___mb_cur_max_func.c",
+				"mingw-w64-crt/misc/lc_locale_func.c",
+				"mingw-w64-crt/misc/mbrtowc.c",
+				"mingw-w64-crt/misc/strnlen.c",
+				"mingw-w64-crt/misc/wcrtomb.c",
+				"mingw-w64-crt/secapi/rand_s.c",
+				"mingw-w64-crt/stdio/acrt_iob_func.c",
+				"mingw-w64-crt/stdio/mingw_lock.c",
+				"mingw-w64-crt/stdio/mingw_pformat.c",
+				"mingw-w64-crt/stdio/mingw_vfprintf.c",
+				"mingw-w64-crt/stdio/mingw_vsnprintf.c",
+			}
+		} else {
+			// Anything somewhat modern (amd64, arm64) uses UCRT.
+			sources = []string{
+				"mingw-w64-crt/stdio/ucrt_fprintf.c",
+				"mingw-w64-crt/stdio/ucrt_fwprintf.c",
+				"mingw-w64-crt/stdio/ucrt_printf.c",
+				"mingw-w64-crt/stdio/ucrt_snprintf.c",
+				"mingw-w64-crt/stdio/ucrt_sprintf.c",
+				"mingw-w64-crt/stdio/ucrt_vfprintf.c",
+				"mingw-w64-crt/stdio/ucrt_vprintf.c",
+				"mingw-w64-crt/stdio/ucrt_vsnprintf.c",
+				"mingw-w64-crt/stdio/ucrt_vsprintf.c",
+			}
 		}
 		return sources, nil
 	},
@@ -63,27 +94,40 @@ var libMinGW = Library{
 func makeMinGWExtraLibs(tmpdir, goarch string) []*compileJob {
 	var jobs []*compileJob
 	root := goenv.Get("TINYGOROOT")
-	// Normally all the api-ms-win-crt-*.def files are all compiled to a single
+	var libs []string
-	// .lib file. But to simplify things, we're going to leave them as separate
+	if goarch == "386" {
-	// files.
+		libs = []string{
-	for _, name := range []string{
+			// x86 uses msvcrt.dll instead of UCRT for compatibility with old
-		"kernel32.def.in",
+			// Windows versions.
-		"api-ms-win-crt-conio-l1-1-0.def",
+			"advapi32.def.in",
-		"api-ms-win-crt-convert-l1-1-0.def.in",
+			"kernel32.def.in",
-		"api-ms-win-crt-environment-l1-1-0.def",
+			"msvcrt.def.in",
-		"api-ms-win-crt-filesystem-l1-1-0.def",
+		}
-		"api-ms-win-crt-heap-l1-1-0.def",
+	} else {
-		"api-ms-win-crt-locale-l1-1-0.def",
+		// Use the modernized UCRT on new systems.
-		"api-ms-win-crt-math-l1-1-0.def.in",
+		// Normally all the api-ms-win-crt-*.def files are all compiled to a
-		"api-ms-win-crt-multibyte-l1-1-0.def",
+		// single .lib file. But to simplify things, we're going to leave them
-		"api-ms-win-crt-private-l1-1-0.def.in",
+		// as separate files.
-		"api-ms-win-crt-process-l1-1-0.def",
+		libs = []string{
-		"api-ms-win-crt-runtime-l1-1-0.def.in",
+			"kernel32.def.in",
-		"api-ms-win-crt-stdio-l1-1-0.def",
+			"api-ms-win-crt-conio-l1-1-0.def",
-		"api-ms-win-crt-string-l1-1-0.def",
+			"api-ms-win-crt-convert-l1-1-0.def.in",
-		"api-ms-win-crt-time-l1-1-0.def",
+			"api-ms-win-crt-environment-l1-1-0.def",
-		"api-ms-win-crt-utility-l1-1-0.def",
+			"api-ms-win-crt-filesystem-l1-1-0.def",
-	} {
+			"api-ms-win-crt-heap-l1-1-0.def",
+			"api-ms-win-crt-locale-l1-1-0.def",
+			"api-ms-win-crt-math-l1-1-0.def.in",
+			"api-ms-win-crt-multibyte-l1-1-0.def",
+			"api-ms-win-crt-private-l1-1-0.def.in",
+			"api-ms-win-crt-process-l1-1-0.def",
+			"api-ms-win-crt-runtime-l1-1-0.def.in",
+			"api-ms-win-crt-stdio-l1-1-0.def",
+			"api-ms-win-crt-string-l1-1-0.def",
+			"api-ms-win-crt-time-l1-1-0.def",
+			"api-ms-win-crt-utility-l1-1-0.def",
+		}
+	}
+	for _, name := range libs {
 		outpath := filepath.Join(tmpdir, filepath.Base(name)+".lib")
 		inpath := filepath.Join(root, "lib/mingw-w64/mingw-w64-crt/lib-common/"+name)
 		job := &compileJob{
@@ -93,6 +137,9 @@ func makeMinGWExtraLibs(tmpdir, goarch string) []*compileJob {
 				defpath := inpath
 				var archDef, emulation string
 				switch goarch {
+				case "386":
+					archDef = "-DDEF_I386"
+					emulation = "i386pe"
 				case "amd64":
 					archDef = "-DDEF_X64"
 					emulation = "i386pep"

compileopts/config.go

@@ -391,15 +391,25 @@ func (c *Config) LibcCFlags() []string {
 	case "mingw-w64":
 		root := goenv.Get("TINYGOROOT")
 		path, _ := c.LibcPath("mingw-w64")
-		return []string{
+		cflags := []string{
 			"-nostdlibinc",
 			"-isystem", filepath.Join(path, "include"),
 			"-isystem", filepath.Join(root, "lib", "mingw-w64", "mingw-w64-headers", "crt"),
 			"-isystem", filepath.Join(root, "lib", "mingw-w64", "mingw-w64-headers", "include"),
 			"-isystem", filepath.Join(root, "lib", "mingw-w64", "mingw-w64-headers", "defaults", "include"),
-			"-D_UCRT",
-			"-D_WIN32_WINNT=0x0a00", // target Windows 10
 		}
+		if c.GOARCH() == "386" {
+			cflags = append(cflags,
+				"-D__MSVCRT_VERSION__=0x700", // Microsoft Visual C++ .NET 2002
+				"-D_WIN32_WINNT=0x0501",      // target Windows XP
+			)
+		} else {
+			cflags = append(cflags,
+				"-D_UCRT",
+				"-D_WIN32_WINNT=0x0a00", // target Windows 10
+			)
+		}
+		return cflags
 	case "":
 		// No libc specified, nothing to add.
 		return nil

compileopts/target.go

@@ -430,14 +430,20 @@ func defaultTarget(options *Options) (*TargetSpec, error) {
 		spec.GC = "boehm"
 		spec.Linker = "ld.lld"
 		spec.Libc = "mingw-w64"
-		// Note: using a medium code model, low image base and no ASLR
-		// because Go doesn't really need those features. ASLR patches
-		// around issues for unsafe languages like C/C++ that are not
-		// normally present in Go (without explicitly opting in).
-		// For more discussion:
-		// https://groups.google.com/g/Golang-nuts/c/Jd9tlNc6jUE/m/Zo-7zIP_m3MJ?pli=1
 		switch options.GOARCH {
+		case "386":
+			spec.LDFlags = append(spec.LDFlags,
+				"-m", "i386pe",
+			)
+			// __udivdi3 is not present in ucrt it seems.
+			spec.RTLib = "compiler-rt"
 		case "amd64":
+			// Note: using a medium code model, low image base and no ASLR
+			// because Go doesn't really need those features. ASLR patches
+			// around issues for unsafe languages like C/C++ that are not
+			// normally present in Go (without explicitly opting in).
+			// For more discussion:
+			// https://groups.google.com/g/Golang-nuts/c/Jd9tlNc6jUE/m/Zo-7zIP_m3MJ?pli=1
 			spec.LDFlags = append(spec.LDFlags,
 				"-m", "i386pep",
 				"--image-base", "0x400000",

compiler/calls.go

@@ -76,7 +76,15 @@ func (b *builder) createCall(fnType llvm.Type, fn llvm.Value, args []llvm.Value,
 		fragments := b.expandFormalParam(arg)
 		expanded = append(expanded, fragments...)
 	}
-	return b.CreateCall(fnType, fn, expanded, name)
+	call := b.CreateCall(fnType, fn, expanded, name)
+	if !fn.IsAFunction().IsNil() {
+		if cc := fn.FunctionCallConv(); cc != llvm.CCallConv {
+			// Set a different calling convention if needed.
+			// This is needed for GetModuleHandleExA on Windows, for example.
+			call.SetInstructionCallConv(cc)
+		}
+	}
+	return call
 }
 
 // createInvoke is like createCall but continues execution at the landing pad if

compiler/llvm.go

@@ -452,6 +452,21 @@ func (b *builder) readStackPointer() llvm.Value {
 	return b.CreateCall(stacksave.GlobalValueType(), stacksave, nil, "")
 }
 
+// writeStackPointer emits a LLVM intrinsic call that updates the current stack
+// pointer.
+func (b *builder) writeStackPointer(sp llvm.Value) {
+	name := "llvm.stackrestore.p0"
+	if llvmutil.Version() < 18 {
+		name = "llvm.stackrestore" // backwards compatibility with LLVM 17 and below
+	}
+	stackrestore := b.mod.NamedFunction(name)
+	if stackrestore.IsNil() {
+		fnType := llvm.FunctionType(b.ctx.VoidType(), []llvm.Type{b.dataPtrType}, false)
+		stackrestore = llvm.AddFunction(b.mod, name, fnType)
+	}
+	b.CreateCall(stackrestore.GlobalValueType(), stackrestore, []llvm.Value{sp}, "")
+}
+
 // createZExtOrTrunc lets the input value fit in the output type bits, by zero
 // extending or truncating the integer.
 func (b *builder) createZExtOrTrunc(value llvm.Value, t llvm.Type) llvm.Value {

compiler/symbol.go

@@ -208,6 +208,12 @@ func (c *compilerContext) getFunction(fn *ssa.Function) (llvm.Type, llvm.Value)
 			// > circumstances, and should not be exposed to source languages.
 			llvmutil.AppendToGlobal(c.mod, "llvm.compiler.used", llvmFn)
 		}
+	case "GetModuleHandleExA", "GetProcAddress", "GetSystemInfo", "GetSystemTimeAsFileTime", "LoadLibraryExW", "QueryUnbiasedInterruptTime", "SetEnvironmentVariableA", "Sleep", "VirtualAlloc":
+		// On Windows we need to use a special calling convention for some
+		// external calls.
+		if c.GOOS == "windows" && c.GOARCH == "386" {
+			llvmFn.SetFunctionCallConv(llvm.X86StdcallCallConv)
+		}
 	}
 
 	// External/exported functions may not retain pointer values.

compiler/syscall.go

@@ -268,6 +268,8 @@ func (b *builder) createSyscall(call *ssa.CallCommon) (llvm.Value, error) {
 		// The signature looks like this:
 		//   func Syscall(trap, nargs, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
 
+		isI386 := strings.HasPrefix(b.Triple, "i386-")
+
 		// Prepare input values.
 		var paramTypes []llvm.Type
 		var params []llvm.Value
@@ -285,11 +287,17 @@ func (b *builder) createSyscall(call *ssa.CallCommon) (llvm.Value, error) {
 		if setLastError.IsNil() {
 			llvmType := llvm.FunctionType(b.ctx.VoidType(), []llvm.Type{b.ctx.Int32Type()}, false)
 			setLastError = llvm.AddFunction(b.mod, "SetLastError", llvmType)
+			if isI386 {
+				setLastError.SetFunctionCallConv(llvm.X86StdcallCallConv)
+			}
 		}
 		getLastError := b.mod.NamedFunction("GetLastError")
 		if getLastError.IsNil() {
 			llvmType := llvm.FunctionType(b.ctx.Int32Type(), nil, false)
 			getLastError = llvm.AddFunction(b.mod, "GetLastError", llvmType)
+			if isI386 {
+				getLastError.SetFunctionCallConv(llvm.X86StdcallCallConv)
+			}
 		}
 
 		// Now do the actual call. Pseudocode:
@@ -300,9 +308,24 @@ func (b *builder) createSyscall(call *ssa.CallCommon) (llvm.Value, error) {
 		// Note that SetLastError/GetLastError could be replaced with direct
 		// access to the thread control block, which is probably smaller and
 		// faster. The Go runtime does this in assembly.
-		b.CreateCall(setLastError.GlobalValueType(), setLastError, []llvm.Value{llvm.ConstNull(b.ctx.Int32Type())}, "")
+		// On windows/386, we also need to save/restore the stack pointer. I'm
+		// not entirely sure why this is needed, but without it these calls
+		// change the stack pointer leading to a crash soon after.
+		setLastErrorCall := b.CreateCall(setLastError.GlobalValueType(), setLastError, []llvm.Value{llvm.ConstNull(b.ctx.Int32Type())}, "")
+		var sp llvm.Value
+		if isI386 {
+			setLastErrorCall.SetInstructionCallConv(llvm.X86StdcallCallConv)
+			sp = b.readStackPointer()
+		}
 		syscallResult := b.CreateCall(llvmType, fnPtr, params, "")
+		if isI386 {
+			syscallResult.SetInstructionCallConv(llvm.X86StdcallCallConv)
+			b.writeStackPointer(sp)
+		}
 		errResult := b.CreateCall(getLastError.GlobalValueType(), getLastError, nil, "err")
+		if isI386 {
+			errResult.SetInstructionCallConv(llvm.X86StdcallCallConv)
+		}
 		if b.uintptrType != b.ctx.Int32Type() {
 			errResult = b.CreateZExt(errResult, b.uintptrType, "err.uintptr")
 		}

main_test.go

@@ -35,6 +35,8 @@ const TESTDATA = "testdata"
 
 var testTarget = flag.String("target", "", "override test target")
 
+var testOnlyCurrentOS = flag.Bool("only-current-os", false, "")
+
 var supportedLinuxArches = map[string]string{
 	"AMD64Linux": "linux/amd64",
 	"X86Linux":   "linux/386",
@@ -158,20 +160,35 @@ func TestBuild(t *testing.T) {
 		return
 	}
 
-	t.Run("EmulatedCortexM3", func(t *testing.T) {
+	if !*testOnlyCurrentOS {
-		t.Parallel()
+		t.Run("EmulatedCortexM3", func(t *testing.T) {
-		runPlatTests(optionsFromTarget("cortex-m-qemu", sema), tests, t)
+			t.Parallel()
-	})
+			runPlatTests(optionsFromTarget("cortex-m-qemu", sema), tests, t)
+		})
 
-	t.Run("EmulatedRISCV", func(t *testing.T) {
+		t.Run("EmulatedRISCV", func(t *testing.T) {
-		t.Parallel()
+			t.Parallel()
-		runPlatTests(optionsFromTarget("riscv-qemu", sema), tests, t)
+			runPlatTests(optionsFromTarget("riscv-qemu", sema), tests, t)
-	})
+		})
 
-	t.Run("AVR", func(t *testing.T) {
+		t.Run("AVR", func(t *testing.T) {
-		t.Parallel()
+			t.Parallel()
-		runPlatTests(optionsFromTarget("simavr", sema), tests, t)
+			runPlatTests(optionsFromTarget("simavr", sema), tests, t)
-	})
+		})
+
+		t.Run("WebAssembly", func(t *testing.T) {
+			t.Parallel()
+			runPlatTests(optionsFromTarget("wasm", sema), tests, t)
+		})
+		t.Run("WASI", func(t *testing.T) {
+			t.Parallel()
+			runPlatTests(optionsFromTarget("wasip1", sema), tests, t)
+		})
+		t.Run("WASIp2", func(t *testing.T) {
+			t.Parallel()
+			runPlatTests(optionsFromTarget("wasip2", sema), tests, t)
+		})
+	}
 
 	if runtime.GOOS == "linux" {
 		for name, osArch := range supportedLinuxArches {
@@ -191,18 +208,13 @@ func TestBuild(t *testing.T) {
 			options := optionsFromOSARCH("linux/mipsle/softfloat", sema)
 			runTest("cgo/", options, t, nil, nil)
 		})
-		t.Run("WebAssembly", func(t *testing.T) {
+	} else if runtime.GOOS == "windows" {
-			t.Parallel()
+		if runtime.GOARCH != "386" {
-			runPlatTests(optionsFromTarget("wasm", sema), tests, t)
+			t.Run("Windows386", func(t *testing.T) {
-		})
+				t.Parallel()
-		t.Run("WASI", func(t *testing.T) {
+				runPlatTests(optionsFromOSARCH("windows/386", sema), tests, t)
-			t.Parallel()
+			})
-			runPlatTests(optionsFromTarget("wasip1", sema), tests, t)
+		}
-		})
-		t.Run("WASIp2", func(t *testing.T) {
-			t.Parallel()
-			runPlatTests(optionsFromTarget("wasip2", sema), tests, t)
-		})
 	}
 }
 

src/crypto/rand/rand_windows.go

@@ -21,6 +21,7 @@ func (r *reader) Read(b []byte) (n int, err error) {
 		// Call rand_s every four bytes because it's a C int (always 32-bit in
 		// Windows).
 		if i%4 == 0 {
+			// TODO: use RtlGenRandom on GOARCH=386.
 			errCode := libc_rand_s(&randomByte)
 			if errCode != 0 {
 				// According to the documentation, it can return an error.

src/internal/task/task_stack_386.S

@@ -1,7 +1,12 @@
+#ifdef _WIN32
+.global  _tinygo_startTask
+_tinygo_startTask:
+#else // Linux etc
 .section .text.tinygo_startTask
 .global  tinygo_startTask
 .type    tinygo_startTask, %function
 tinygo_startTask:
+#endif
     .cfi_startproc
     // Small assembly stub for starting a goroutine. This is already run on the
     // new stack, with the callee-saved registers already loaded.
@@ -24,12 +29,21 @@ tinygo_startTask:
     addl $4, %esp
 
     // After return, exit this goroutine. This is a tail call.
+    #ifdef _WIN32
+    jmp _tinygo_pause
+    #else
     jmp tinygo_pause
+    #endif
     .cfi_endproc
 
+#ifdef _WIN32
+.global _tinygo_swapTask
+_tinygo_swapTask:
+#else
 .global tinygo_swapTask
 .type tinygo_swapTask, %function
 tinygo_swapTask:
+#endif
     // This function gets the following parameters:
     movl 4(%esp), %eax // newStack uintptr
     movl 8(%esp), %ecx // oldStack *uintptr

src/runtime/asm_386.S

@@ -1,7 +1,12 @@
+#ifdef _WIN32
+.global _tinygo_scanCurrentStack
+_tinygo_scanCurrentStack:
+#else
 .section .text.tinygo_scanCurrentStack
 .global tinygo_scanCurrentStack
 .type tinygo_scanCurrentStack, %function
 tinygo_scanCurrentStack:
+#endif
     // Sources:
     //   * https://stackoverflow.com/questions/18024672/what-registers-are-preserved-through-a-linux-x86-64-function-call
     //   * https://godbolt.org/z/q7e8dn
@@ -15,7 +20,11 @@ tinygo_scanCurrentStack:
     // Scan the stack.
     subl $8, %esp // adjust the stack before the call to maintain 16-byte alignment
     pushl %esp
+    #ifdef _WIN32
+    calll _tinygo_scanstack
+    #else
     calll tinygo_scanstack
+    #endif
 
     // Restore the stack pointer. Registers do not need to be restored as they
     // were only pushed to be discoverable by the GC.
@@ -23,9 +32,14 @@ tinygo_scanCurrentStack:
     retl
 
 
+#ifdef _WIN32
+.global _tinygo_longjmp
+_tinygo_longjmp:
+#else
 .section .text.tinygo_longjmp
 .global tinygo_longjmp
 tinygo_longjmp:
+#endif
     // Note: the code we jump to assumes eax is set to a non-zero value if we
     // jump from here.
     movl 4(%esp), %eax

src/runtime/runtime_windows.go

@@ -239,11 +239,17 @@ func procUnpin() {
 }
 
 func hardwareRand() (n uint64, ok bool) {
-	var n1, n2 uint32
+	if GOARCH == "386" {
-	errCode1 := libc_rand_s(&n1)
+		// Use the old RtlGenRandom, introduced in Windows XP.
-	errCode2 := libc_rand_s(&n2)
+		ok = RtlGenRandom(unsafe.Pointer(&n), 8)
-	n = uint64(n1)<<32 | uint64(n2)
+	} else {
-	ok = errCode1 == 0 && errCode2 == 0
+		// Use the newer secure rand_s function that's part of UCRT.
+		var n1, n2 uint32
+		errCode1 := libc_rand_s(&n1)
+		errCode2 := libc_rand_s(&n2)
+		n = uint64(n1)<<32 | uint64(n2)
+		ok = errCode1 == 0 && errCode2 == 0
+	}
 	return
 }
 
@@ -253,3 +259,10 @@ func hardwareRand() (n uint64, ok bool) {
 //
 //export rand_s
 func libc_rand_s(randomValue *uint32) int32
+
+// This function is part of advapi32.dll, and is called SystemFunction036 for
+// some reason. It's available on Windows XP and newer.
+// See: https://learn.microsoft.com/en-us/windows/win32/api/ntsecapi/nf-ntsecapi-rtlgenrandom
+//
+//export SystemFunction036
+func RtlGenRandom(buf unsafe.Pointer, len int) bool