wasm: add Boehm GC support

This adds support for `-gc=boehm` on `-target=wasip1` and `-target=wasm`
(in a browser or NodeJS). Notably it does *not* add Boehm GC support for
`-target=wasip2`, since that target doesn't have a real libc.

compileopts/config.go

@@ -22,7 +22,8 @@ import (
 // builder.Library struct but that's hard to do since we want to know the
 // library path in advance in several places).
 var libVersions = map[string]int{
-	"musl": 3,
+	"musl":  3,
+	"bdwgc": 2,
 }
 
 // Config keeps all configuration affecting the build in a single struct.
@@ -138,7 +139,7 @@ func (c *Config) GC() string {
 // that can be traced by the garbage collector.
 func (c *Config) NeedsStackObjects() bool {
 	switch c.GC() {
-	case "conservative", "custom", "precise":
+	case "conservative", "custom", "precise", "boehm":
 		for _, tag := range c.BuildTags() {
 			if tag == "tinygo.wasm" {
 				return true
@@ -263,6 +264,15 @@ func MuslArchitecture(triple string) string {
 	return CanonicalArchName(triple)
 }
 
+// Returns true if the libc needs to include malloc, for the libcs where this
+// matters.
+func (c *Config) LibcNeedsMalloc() bool {
+	if c.GC() == "boehm" && c.Target.Libc == "wasi-libc" {
+		return true
+	}
+	return false
+}
+
 // LibraryPath returns the path to the library build directory. The path will be
 // a library path in the cache directory (which might not yet be built).
 func (c *Config) LibraryPath(name string) string {
@@ -286,9 +296,14 @@ func (c *Config) LibraryPath(name string) string {
 		archname += "-v" + strconv.Itoa(v)
 	}
 
+	options := ""
+	if c.LibcNeedsMalloc() {
+		options += "+malloc"
+	}
+
 	// No precompiled library found. Determine the path name that will be used
 	// in the build cache.
-	return filepath.Join(goenv.Get("GOCACHE"), name+"-"+archname)
+	return filepath.Join(goenv.Get("GOCACHE"), name+options+"-"+archname)
 }
 
 // DefaultBinaryExtension returns the default extension for binaries, such as

compileopts/target.go

@@ -470,6 +470,12 @@ func defaultTarget(options *Options) (*TargetSpec, error) {
 		return nil, fmt.Errorf("unknown GOOS=%s", options.GOOS)
 	}
 
+	if spec.GC == "boehm" {
+		// Add this file only when needed. This fixes a build failure on
+		// Windows.
+		spec.ExtraFiles = append(spec.ExtraFiles, "src/runtime/gc_boehm.c")
+	}
+
 	// Target triples (which actually have four components, but are called
 	// triples for historical reasons) have the form:
 	//   arch-vendor-os-environment