runtime: implement KeepAlive using inline assembly

compiler/intrinsics.go

@@ -24,6 +24,8 @@ func (b *builder) defineIntrinsicFunction() {
 		b.createMemoryCopyImpl()
 	case name == "runtime.memzero":
 		b.createMemoryZeroImpl()
+	case name == "runtime.KeepAlive":
+		b.createKeepAliveImpl()
 	case strings.HasPrefix(name, "runtime/volatile.Load"):
 		b.createVolatileLoad()
 	case strings.HasPrefix(name, "runtime/volatile.Store"):
@@ -87,6 +89,29 @@ func (b *builder) createMemoryZeroImpl() {
 	b.CreateRetVoid()
 }
 
+// createKeepAlive creates the runtime.KeepAlive function. It is implemented
+// using inline assembly.
+func (b *builder) createKeepAliveImpl() {
+	b.createFunctionStart(true)
+
+	// Get the underlying value of the interface value.
+	interfaceValue := b.getValue(b.fn.Params[0])
+	pointerValue := b.CreateExtractValue(interfaceValue, 1, "")
+
+	// Create an equivalent of the following C code, which is basically just a
+	// nop but ensures the pointerValue is kept alive:
+	//
+	//     __asm__ __volatile__("" : : "r"(pointerValue))
+	//
+	// It should be portable to basically everything as the "r" register type
+	// exists basically everywhere.
+	asmType := llvm.FunctionType(b.ctx.VoidType(), []llvm.Type{b.i8ptrType}, false)
+	asmFn := llvm.InlineAsm(asmType, "", "r", true, false, 0, false)
+	b.createCall(asmType, asmFn, []llvm.Value{pointerValue}, "")
+
+	b.CreateRetVoid()
+}
+
 var mathToLLVMMapping = map[string]string{
 	"math.Ceil":  "llvm.ceil.f64",
 	"math.Exp":   "llvm.exp.f64",

src/runtime/runtime.go

@@ -88,12 +88,9 @@ func LockOSThread() {
 func UnlockOSThread() {
 }
 
-func KeepAlive(x interface{}) {
-	// Unimplemented.
-	// TODO: This function needs to be implemented in a way that LLVM doesn't optimize away the x
-	// parameter. This will likely need either a volatile operation or calling an assembly stub
-	// that simply returns.
-}
+// KeepAlive makes sure the value in the interface is alive until at least the
+// point of the call.
+func KeepAlive(x interface{})
 
 var godebugUpdate func(string, string)
 

testdata/gc.go

@@ -1,5 +1,7 @@
 package main
 
+import "runtime"
+
 var xorshift32State uint32 = 1
 
 func xorshift32(x uint32) uint32 {
@@ -17,6 +19,7 @@ func randuint32() uint32 {
 
 func main() {
 	testNonPointerHeap()
+	testKeepAlive()
 }
 
 var scalarSlices [4][]byte
@@ -64,3 +67,10 @@ func testNonPointerHeap() {
 	}
 	println("ok")
 }
+
+func testKeepAlive() {
+	// There isn't much we can test, but at least we can test that
+	// runtime.KeepAlive compiles correctly.
+	var x int
+	runtime.KeepAlive(&x)
+}