wasm: add `//go:wasmexport` support (#4451)

This adds support for the `//go:wasmexport` pragma as proposed here:
https://github.com/golang/go/issues/65199

It is currently implemented only for wasip1 and wasm-unknown, but it is
certainly possible to extend it to other targets like GOOS=js and
wasip2.

3 files changed, 317 insertions, 38 deletions

diff --git a/compiler/compiler.go b/compiler/compiler.go
index 6756fe969..752e4a5c6 100644
--- a/compiler/compiler.go
+++ b/compiler/compiler.go
@@ -44,6 +44,7 @@ type Config struct {
 	ABI             string
 	GOOS            string
 	GOARCH          string
+	BuildMode       string
 	CodeModel       string
 	RelocationModel string
 	SizeLevel       int
@@ -1384,6 +1385,11 @@ func (b *builder) createFunction() {
 		b.llvmFn.SetLinkage(llvm.InternalLinkage)
 		b.createFunction()
 	}
+
+	// Create wrapper function that can be called externally.
+	if b.info.wasmExport != "" {
+		b.createWasmExport()
+	}
 }
 
 // posser is an interface that's implemented by both ssa.Value and
diff --git a/compiler/goroutine.go b/compiler/goroutine.go
index a23556345..701797152 100644
--- a/compiler/goroutine.go
+++ b/compiler/goroutine.go
@@ -7,6 +7,7 @@ import (
 	"go/token"
 	"go/types"
 
+	"github.com/tinygo-org/tinygo/compiler/llvmutil"
 	"golang.org/x/tools/go/ssa"
 	"tinygo.org/x/go-llvm"
 )
@@ -101,7 +102,7 @@ func (b *builder) createGo(instr *ssa.Go) {
 
 	paramBundle := b.emitPointerPack(params)
 	var stackSize llvm.Value
-	callee := b.createGoroutineStartWrapper(funcType, funcPtr, prefix, hasContext, instr.Pos())
+	callee := b.createGoroutineStartWrapper(funcType, funcPtr, prefix, hasContext, false, instr.Pos())
 	if b.AutomaticStackSize {
 		// The stack size is not known until after linking. Call a dummy
 		// function that will be replaced with a load from a special ELF
@@ -121,6 +122,147 @@ func (b *builder) createGo(instr *ssa.Go) {
 	b.createCall(fnType, start, []llvm.Value{callee, paramBundle, stackSize, llvm.Undef(b.dataPtrType)}, "")
 }
 
+// Create an exported wrapper function for functions with the //go:wasmexport
+// pragma. This wrapper function is quite complex when the scheduler is enabled:
+// it needs to start a new goroutine each time the exported function is called.
+func (b *builder) createWasmExport() {
+	pos := b.info.wasmExportPos
+	if b.info.exported {
+		// //export really shouldn't be used anymore when //go:wasmexport is
+		// available, because //go:wasmexport is much better defined.
+		b.addError(pos, "cannot use //export and //go:wasmexport at the same time")
+		return
+	}
+
+	const suffix = "#wasmexport"
+
+	// Declare the exported function.
+	paramTypes := b.llvmFnType.ParamTypes()
+	exportedFnType := llvm.FunctionType(b.llvmFnType.ReturnType(), paramTypes[:len(paramTypes)-1], false)
+	exportedFn := llvm.AddFunction(b.mod, b.fn.RelString(nil)+suffix, exportedFnType)
+	b.addStandardAttributes(exportedFn)
+	llvmutil.AppendToGlobal(b.mod, "llvm.used", exportedFn)
+	exportedFn.AddFunctionAttr(b.ctx.CreateStringAttribute("wasm-export-name", b.info.wasmExport))
+
+	// Create a builder for this wrapper function.
+	builder := newBuilder(b.compilerContext, b.ctx.NewBuilder(), b.fn)
+	defer builder.Dispose()
+
+	// Define this function as a separate function in DWARF
+	if b.Debug {
+		if b.fn.Syntax() != nil {
+			// Create debug info file if needed.
+			pos := b.program.Fset.Position(pos)
+			builder.difunc = builder.attachDebugInfoRaw(b.fn, exportedFn, suffix, pos.Filename, pos.Line)
+		}
+		builder.setDebugLocation(pos)
+	}
+
+	// Create a single basic block inside of it.
+	bb := llvm.AddBasicBlock(exportedFn, "entry")
+	builder.SetInsertPointAtEnd(bb)
+
+	// Insert an assertion to make sure this //go:wasmexport function is not
+	// called at a time when it is not allowed (for example, before the runtime
+	// is initialized).
+	builder.createRuntimeCall("wasmExportCheckRun", nil, "")
+
+	if b.Scheduler == "none" {
+		// When the scheduler has been disabled, this is really trivial: just
+		// call the function.
+		params := exportedFn.Params()
+		params = append(params, llvm.ConstNull(b.dataPtrType)) // context parameter
+		retval := builder.CreateCall(b.llvmFnType, b.llvmFn, params, "")
+		if b.fn.Signature.Results() == nil {
+			builder.CreateRetVoid()
+		} else {
+			builder.CreateRet(retval)
+		}
+
+	} else {
+		// The scheduler is enabled, so we need to start a new goroutine, wait
+		// for it to complete, and read the result value.
+
+		// Build a function that looks like this:
+		//
+		//   func foo#wasmexport(param0, param1, ..., paramN) {
+		//       var state *stateStruct
+		//
+		//       // 'done' must be explicitly initialized ('state' is not zeroed)
+		//       state.done = false
+		//
+		//       // store the parameters in the state object
+		//       state.param0 = param0
+		//       state.param1 = param1
+		//       ...
+		//       state.paramN = paramN
+		//
+		//       // create a goroutine and push it to the runqueue
+		//       task.start(uintptr(gowrapper), &state)
+		//
+		//       // run the scheduler
+		//       runtime.wasmExportRun(&state.done)
+		//
+		//       // if there is a return value, load it and return
+		//       return state.result
+		//   }
+
+		hasReturn := b.fn.Signature.Results() != nil
+
+		// Build the state struct type.
+		// It stores the function parameters, the 'done' flag, and reserves
+		// space for a return value if needed.
+		stateFields := exportedFnType.ParamTypes()
+		numParams := len(stateFields)
+		stateFields = append(stateFields, b.ctx.Int1Type()) // 'done' field
+		if hasReturn {
+			stateFields = append(stateFields, b.llvmFnType.ReturnType())
+		}
+		stateStruct := b.ctx.StructType(stateFields, false)
+
+		// Allocate the state struct on the stack.
+		statePtr := builder.CreateAlloca(stateStruct, "status")
+
+		// Initialize the 'done' field.
+		doneGEP := builder.CreateInBoundsGEP(stateStruct, statePtr, []llvm.Value{
+			llvm.ConstInt(b.ctx.Int32Type(), 0, false),
+			llvm.ConstInt(b.ctx.Int32Type(), uint64(numParams), false),
+		}, "done.gep")
+		builder.CreateStore(llvm.ConstNull(b.ctx.Int1Type()), doneGEP)
+
+		// Store all parameters in the state object.
+		for i, param := range exportedFn.Params() {
+			gep := builder.CreateInBoundsGEP(stateStruct, statePtr, []llvm.Value{
+				llvm.ConstInt(b.ctx.Int32Type(), 0, false),
+				llvm.ConstInt(b.ctx.Int32Type(), uint64(i), false),
+			}, "")
+			builder.CreateStore(param, gep)
+		}
+
+		// Create a new goroutine and add it to the runqueue.
+		wrapper := b.createGoroutineStartWrapper(b.llvmFnType, b.llvmFn, "", false, true, pos)
+		stackSize := llvm.ConstInt(b.uintptrType, b.DefaultStackSize, false)
+		taskStartFnType, taskStartFn := builder.getFunction(b.program.ImportedPackage("internal/task").Members["start"].(*ssa.Function))
+		builder.createCall(taskStartFnType, taskStartFn, []llvm.Value{wrapper, statePtr, stackSize, llvm.Undef(b.dataPtrType)}, "")
+
+		// Run the scheduler.
+		builder.createRuntimeCall("wasmExportRun", []llvm.Value{doneGEP}, "")
+
+		// Read the return value (if any) and return to the caller of the
+		// //go:wasmexport function.
+		if hasReturn {
+			gep := builder.CreateInBoundsGEP(stateStruct, statePtr, []llvm.Value{
+				llvm.ConstInt(b.ctx.Int32Type(), 0, false),
+				llvm.ConstInt(b.ctx.Int32Type(), uint64(numParams)+1, false),
+			}, "")
+			retval := builder.CreateLoad(b.llvmFnType.ReturnType(), gep, "retval")
+			builder.CreateRet(retval)
+		} else {
+			builder.CreateRetVoid()
+		}
+	}
+}
+
 // createGoroutineStartWrapper creates a wrapper for the task-based
 // implementation of goroutines. For example, to call a function like this:
 //
@@ -144,7 +286,7 @@ func (b *builder) createGo(instr *ssa.Go) {
 // to last parameter of the function) is used for this wrapper. If hasContext is
 // false, the parameter bundle is assumed to have no context parameter and undef
 // is passed instead.
-func (c *compilerContext) createGoroutineStartWrapper(fnType llvm.Type, fn llvm.Value, prefix string, hasContext bool, pos token.Pos) llvm.Value {
+func (c *compilerContext) createGoroutineStartWrapper(fnType llvm.Type, fn llvm.Value, prefix string, hasContext, isWasmExport bool, pos token.Pos) llvm.Value {
 	var wrapper llvm.Value
 
 	b := &builder{
@@ -162,14 +304,18 @@ func (c *compilerContext) createGoroutineStartWrapper(fnType llvm.Type, fn llvm.
 	if !fn.IsAFunction().IsNil() {
 		// See whether this wrapper has already been created. If so, return it.
 		name := fn.Name()
-		wrapper = c.mod.NamedFunction(name + "$gowrapper")
+		wrapperName := name + "$gowrapper"
+		if isWasmExport {
+			wrapperName += "-wasmexport"
+		}
+		wrapper = c.mod.NamedFunction(wrapperName)
 		if !wrapper.IsNil() {
 			return llvm.ConstPtrToInt(wrapper, c.uintptrType)
 		}
 
 		// Create the wrapper.
 		wrapperType := llvm.FunctionType(c.ctx.VoidType(), []llvm.Type{c.dataPtrType}, false)
-		wrapper = llvm.AddFunction(c.mod, name+"$gowrapper", wrapperType)
+		wrapper = llvm.AddFunction(c.mod, wrapperName, wrapperType)
 		c.addStandardAttributes(wrapper)
 		wrapper.SetLinkage(llvm.LinkOnceODRLinkage)
 		wrapper.SetUnnamedAddr(true)
@@ -199,23 +345,110 @@ func (c *compilerContext) createGoroutineStartWrapper(fnType llvm.Type, fn llvm.
 			b.SetCurrentDebugLocation(uint(pos.Line), uint(pos.Column), difunc, llvm.Metadata{})
 		}
 
-		// Create the list of params for the call.
-		paramTypes := fnType.ParamTypes()
-		if !hasContext {
-			paramTypes = paramTypes[:len(paramTypes)-1] // strip context parameter
-		}
-		params := b.emitPointerUnpack(wrapper.Param(0), paramTypes)
-		if !hasContext {
-			params = append(params, llvm.Undef(c.dataPtrType)) // add dummy context parameter
-		}
+		if !isWasmExport {
+			// Regular 'go' instruction.
 
-		// Create the call.
-		b.CreateCall(fnType, fn, params, "")
+			// Create the list of params for the call.
+			paramTypes := fnType.ParamTypes()
+			if !hasContext {
+				paramTypes = paramTypes[:len(paramTypes)-1] // strip context parameter
+			}
 
-		if c.Scheduler == "asyncify" {
-			b.CreateCall(deadlockType, deadlock, []llvm.Value{
-				llvm.Undef(c.dataPtrType),
-			}, "")
+			params := b.emitPointerUnpack(wrapper.Param(0), paramTypes)
+			if !hasContext {
+				params = append(params, llvm.Undef(c.dataPtrType)) // add dummy context parameter
+			}
+
+			// Create the call.
+			b.CreateCall(fnType, fn, params, "")
+
+			if c.Scheduler == "asyncify" {
+				b.CreateCall(deadlockType, deadlock, []llvm.Value{
+					llvm.Undef(c.dataPtrType),
+				}, "")
+			}
+		} else {
+			// Goroutine started from a //go:wasmexport pragma.
+			// The function looks like this:
+			//
+			//   func foo$gowrapper-wasmexport(state *stateStruct) {
+			//       // load values
+			//       param0 := state.params[0]
+			//       param1 := state.params[1]
+			//
+			//       // call wrapped functions
+			//       result := foo(param0, param1, ...)
+			//
+			//       // store result value (if there is any)
+			//       state.result = result
+			//
+			//       // finish exported function
+			//       state.done = true
+			//       runtime.wasmExportExit()
+			//   }
+			//
+			// The state object here looks like:
+			//
+			//   struct state {
+			//       param0
+			//       param1
+			//       param* // etc
+			//       done bool
+			//       result returnType
+			//   }
+
+			returnType := fnType.ReturnType()
+			hasReturn := returnType != b.ctx.VoidType()
+			statePtr := wrapper.Param(0)
+
+			// Create the state struct (it must match the type in createWasmExport).
+			stateFields := fnType.ParamTypes()
+			numParams := len(stateFields) - 1
+			stateFields = stateFields[:numParams:numParams]     // strip 'context' parameter
+			stateFields = append(stateFields, c.ctx.Int1Type()) // 'done' bool
+			if hasReturn {
+				stateFields = append(stateFields, returnType)
+			}
+			stateStruct := b.ctx.StructType(stateFields, false)
+
+			// Extract parameters from the state object, and call the function
+			// that's being wrapped.
+			var callParams []llvm.Value
+			for i := 0; i < numParams; i++ {
+				gep := b.CreateInBoundsGEP(stateStruct, statePtr, []llvm.Value{
+					llvm.ConstInt(b.ctx.Int32Type(), 0, false),
+					llvm.ConstInt(b.ctx.Int32Type(), uint64(i), false),
+				}, "")
+				param := b.CreateLoad(stateFields[i], gep, "")
+				callParams = append(callParams, param)
+			}
+			callParams = append(callParams, llvm.ConstNull(c.dataPtrType)) // add 'context' parameter
+			result := b.CreateCall(fnType, fn, callParams, "")
+
+			// Store the return value back into the shared state.
+			// Unlike regular goroutines, these special //go:wasmexport
+			// goroutines can return a value.
+			if hasReturn {
+				gep := b.CreateInBoundsGEP(stateStruct, statePtr, []llvm.Value{
+					llvm.ConstInt(c.ctx.Int32Type(), 0, false),
+					llvm.ConstInt(c.ctx.Int32Type(), uint64(numParams)+1, false),
+				}, "result.ptr")
+				b.CreateStore(result, gep)
+			}
+
+			// Mark this function as having finished executing.
+			// This is important so the runtime knows the exported function
+			// didn't block.
+			doneGEP := b.CreateInBoundsGEP(stateStruct, statePtr, []llvm.Value{
+				llvm.ConstInt(c.ctx.Int32Type(), 0, false),
+				llvm.ConstInt(c.ctx.Int32Type(), uint64(numParams), false),
+			}, "done.gep")
+			b.CreateStore(llvm.ConstInt(b.ctx.Int1Type(), 1, false), doneGEP)
+
+			// Call back into the runtime. This will exit the goroutine, switch
+			// back to the scheduler, which will in turn return from the
+			// //go:wasmexport function.
+			b.createRuntimeCall("wasmExportExit", nil, "")
 		}
 
 	} else {
@@ -297,5 +530,5 @@ func (c *compilerContext) createGoroutineStartWrapper(fnType llvm.Type, fn llvm.
 	}
 
 	// Return a ptrtoint of the wrapper, not the function itself.
-	return b.CreatePtrToInt(wrapper, c.uintptrType, "")
+	return llvm.ConstPtrToInt(wrapper, c.uintptrType)
 }
diff --git a/compiler/symbol.go b/compiler/symbol.go
index 32eb55107..4a080cbbc 100644
--- a/compiler/symbol.go
+++ b/compiler/symbol.go
@@ -23,15 +23,17 @@ import (
 // The linkName value contains a valid link name, even if //go:linkname is not
 // present.
 type functionInfo struct {
-	wasmModule string     // go:wasm-module
-	wasmName   string     // wasm-export-name or wasm-import-name in the IR
-	linkName   string     // go:linkname, go:export - the IR function name
-	section    string     // go:section - object file section name
-	exported   bool       // go:export, CGo
-	interrupt  bool       // go:interrupt
-	nobounds   bool       // go:nobounds
-	variadic   bool       // go:variadic (CGo only)
-	inline     inlineType // go:inline
+	wasmModule    string     // go:wasm-module
+	wasmName      string     // wasm-export-name or wasm-import-name in the IR
+	wasmExport    string     // go:wasmexport is defined (export is unset, this adds an exported wrapper)
+	wasmExportPos token.Pos  // position of //go:wasmexport comment
+	linkName      string     // go:linkname, go:export - the IR function name
+	section       string     // go:section - object file section name
+	exported      bool       // go:export, CGo
+	interrupt     bool       // go:interrupt
+	nobounds      bool       // go:nobounds
+	variadic      bool       // go:variadic (CGo only)
+	inline        inlineType // go:inline
 }
 
 type inlineType int
@@ -241,8 +243,22 @@ func (c *compilerContext) getFunctionInfo(f *ssa.Function) functionInfo {
 		// Pick the default linkName.
 		linkName: f.RelString(nil),
 	}
+
+	// Check for a few runtime functions that are treated specially.
+	if info.linkName == "runtime.wasmEntryReactor" && c.BuildMode == "c-shared" {
+		info.linkName = "_initialize"
+		info.wasmName = "_initialize"
+		info.exported = true
+	}
+	if info.linkName == "runtime.wasmEntryCommand" && c.BuildMode == "default" {
+		info.linkName = "_start"
+		info.wasmName = "_start"
+		info.exported = true
+	}
+
 	// Check for //go: pragmas, which may change the link name (among others).
 	c.parsePragmas(&info, f)
+
 	c.functionInfos[f] = info
 	return info
 }
@@ -296,10 +312,39 @@ func (c *compilerContext) parsePragmas(info *functionInfo, f *ssa.Function) {
 				if len(parts) != 3 {
 					continue
 				}
-				c.checkWasmImport(f, comment.Text)
+				if f.Blocks != nil {
+					// Defined functions cannot be exported.
+					c.addError(f.Pos(), "can only use //go:wasmimport on declarations")
+					continue
+				}
+				c.checkWasmImportExport(f, comment.Text)
 				info.exported = true
 				info.wasmModule = parts[1]
 				info.wasmName = parts[2]
+			case "//go:wasmexport":
+				if f.Blocks == nil {
+					c.addError(f.Pos(), "can only use //go:wasmexport on definitions")
+					continue
+				}
+				if len(parts) != 2 {
+					c.addError(f.Pos(), fmt.Sprintf("expected one parameter to //go:wasmimport, not %d", len(parts)-1))
+					continue
+				}
+				name := parts[1]
+				if name == "_start" || name == "_initialize" {
+					c.addError(f.Pos(), fmt.Sprintf("//go:wasmexport does not allow %#v", name))
+					continue
+				}
+				if c.BuildMode != "c-shared" && f.RelString(nil) == "main.main" {
+					c.addError(f.Pos(), fmt.Sprintf("//go:wasmexport does not allow main.main to be exported with -buildmode=%s", c.BuildMode))
+					continue
+				}
+				if c.archFamily() != "wasm32" {
+					c.addError(f.Pos(), "//go:wasmexport is only supported on wasm")
+				}
+				c.checkWasmImportExport(f, comment.Text)
+				info.wasmExport = name
+				info.wasmExportPos = comment.Slash
 			case "//go:inline":
 				info.inline = inlineHint
 			case "//go:noinline":
@@ -346,22 +391,17 @@ func (c *compilerContext) parsePragmas(info *functionInfo, f *ssa.Function) {
 	}
 }
 
-// Check whether this function cannot be used in //go:wasmimport. It will add an
-// error if this is the case.
+// Check whether this function can be used in //go:wasmimport or
+// //go:wasmexport. It will add an error if this is not the case.
 //
 // The list of allowed types is based on this proposal:
 // https://github.com/golang/go/issues/59149
-func (c *compilerContext) checkWasmImport(f *ssa.Function, pragma string) {
+func (c *compilerContext) checkWasmImportExport(f *ssa.Function, pragma string) {
 	if c.pkg.Path() == "runtime" || c.pkg.Path() == "syscall/js" || c.pkg.Path() == "syscall" {
 		// The runtime is a special case. Allow all kinds of parameters
 		// (importantly, including pointers).
 		return
 	}
-	if f.Blocks != nil {
-		// Defined functions cannot be exported.
-		c.addError(f.Pos(), "can only use //go:wasmimport on declarations")
-		return
-	}
 	if f.Signature.Results().Len() > 1 {
 		c.addError(f.Signature.Results().At(1).Pos(), fmt.Sprintf("%s: too many return values", pragma))
 	} else if f.Signature.Results().Len() == 1 {