wasip1: add //go:wasmexport support

This adds support for the `//go:wasmexport` pragma as proposed here:
golang/go#65199

It is currently implemented only for wasip1, but it is certainly
possible to extend it to other targets like GOOS=js, wasm-unknown, or
wasip2. It is also currently limited to -buildmode=c-shared, this is a
limitation that could easily be lifted in the future.

builder/build.go

@@ -197,6 +197,7 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 		ABI:             config.ABI(),
 		GOOS:            config.GOOS(),
 		GOARCH:          config.GOARCH(),
+		BuildMode:       config.BuildMode(),
 		CodeModel:       config.CodeModel(),
 		RelocationModel: config.RelocationModel(),
 		SizeLevel:       sizeLevel,
@@ -649,6 +650,13 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 	result.Binary = result.Executable // final file
 	ldflags := append(config.LDFlags(), "-o", result.Executable)
 
+	if config.Options.BuildMode == "c-shared" {
+		if config.GOOS() != "wasip1" {
+			return result, fmt.Errorf("buildmode c-shared is only supported on wasip1 at the moment")
+		}
+		ldflags = append(ldflags, "--no-entry")
+	}
+
 	// Add compiler-rt dependency if needed. Usually this is a simple load from
 	// a cache.
 	if config.Target.RTLib == "compiler-rt" {

compileopts/config.go

@@ -33,6 +33,14 @@ func (c *Config) CPU() string {
 	return c.Target.CPU
 }
 
+// The current build mode (like the `-buildmode` command line flag).
+func (c *Config) BuildMode() string {
+	if c.Options.BuildMode != "" {
+		return c.Options.BuildMode
+	}
+	return "default"
+}
+
 // Features returns a list of features this CPU supports. For example, for a
 // RISC-V processor, that could be "+a,+c,+m". For many targets, an empty list
 // will be returned.
@@ -91,6 +99,12 @@ func (c *Config) BuildTags() []string {
 	for i := 1; i <= c.GoMinorVersion; i++ {
 		tags = append(tags, fmt.Sprintf("go1.%d", i))
 	}
+	if c.BuildMode() == "c-shared" && strings.HasPrefix(c.Triple(), "wasm") {
+		// Use reactor mode, which means the application is initialized in
+		// _initialize and main.main is not called by default. See for example:
+		// https://github.com/WebAssembly/WASI/blob/main/legacy/application-abi.md
+		tags = append(tags, "tinygo_wasm_reactor")
+	}
 	tags = append(tags, c.Options.Tags...)
 	return tags
 }

compileopts/options.go

@@ -8,6 +8,7 @@ import (
 )
 
 var (
+	validBuildModeOptions     = []string{"default", "c-shared"}
 	validGCOptions            = []string{"none", "leaking", "conservative", "custom", "precise"}
 	validSchedulerOptions     = []string{"none", "tasks", "asyncify"}
 	validSerialOptions        = []string{"none", "uart", "usb", "rtt"}
@@ -26,6 +27,7 @@ type Options struct {
 	GOMIPS          string // environment variable (only used with GOARCH=mips and GOARCH=mipsle)
 	Directory       string // working dir, leave it unset to use the current working dir
 	Target          string
+	BuildMode       string // -buildmode flag
 	Opt             string
 	GC              string
 	PanicStrategy   string
@@ -60,6 +62,14 @@ type Options struct {
 
 // Verify performs a validation on the given options, raising an error if options are not valid.
 func (o *Options) Verify() error {
+	if o.BuildMode != "" {
+		valid := isInArray(validBuildModeOptions, o.BuildMode)
+		if !valid {
+			return fmt.Errorf(`invalid buildmode option '%s': valid values are %s`,
+				o.BuildMode,
+				strings.Join(validBuildModeOptions, ", "))
+		}
+	}
 	if o.GC != "" {
 		valid := isInArray(validGCOptions, o.GC)
 		if !valid {

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