windows: add support for the Boehm-Demers-Weiser GC

A few small changes were needed to make this work. In particular, I
found a critical bug (see the previous commit) that needed to be fixed
to make this work on Windows.

builder/bdwgc.go

@@ -5,6 +5,7 @@ package builder
 
 import (
 	"path/filepath"
+	"strings"
 
 	"github.com/tinygo-org/tinygo/goenv"
 )
@@ -25,6 +26,10 @@ var BoehmGC = Library{
 			"-DIGNORE_DYNAMIC_LOADING", // we don't support dynamic loading at the moment
 			"-DNO_GETCONTEXT",          // musl doesn't support getcontext()
 
+			// Use a minimal environment.
+			"-DNO_MSGBOX_ON_ERROR", // don't call MessageBoxA on Windows
+			"-DDONT_USE_ATEXIT",
+
 			// Special flag to work around the lack of __data_start in ld.lld.
 			// TODO: try to fix this in LLVM/lld directly so we don't have to
 			// work around it anymore.
@@ -47,7 +52,7 @@ var BoehmGC = Library{
 		return filepath.Join(goenv.Get("TINYGOROOT"), "lib/bdwgc")
 	},
 	librarySources: func(target string) ([]string, error) {
-		return []string{
+		sources := []string{
 			"allchblk.c",
 			"alloc.c",
 			"blacklst.c",
@@ -66,6 +71,15 @@ var BoehmGC = Library{
 			"pthread_stop_world.c",
 			"pthread_support.c",
 			"reclaim.c",
-		}, nil
+		}
+		if strings.Split(target, "-")[2] == "windows" {
+			// Due to how the linker on Windows works (that doesn't allow
+			// undefined functions), we need to include these extra files.
+			sources = append(sources,
+				"mallocx.c",
+				"ptr_chck.c",
+			)
+		}
+		return sources, nil
 	},
 }

builder/build.go

@@ -182,12 +182,13 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 		defer unlock()
 		libcDependencies = append(libcDependencies, libcJob)
 	case "mingw-w64":
-		job, unlock, err := libMinGW.load(config, tmpdir, nil)
+		var unlock func()
+		libcJob, unlock, err = libMinGW.load(config, tmpdir, nil)
 		if err != nil {
 			return BuildResult{}, err
 		}
 		defer unlock()
-		libcDependencies = append(libcDependencies, job)
+		libcDependencies = append(libcDependencies, libcJob)
 		libcDependencies = append(libcDependencies, makeMinGWExtraLibs(tmpdir, config.GOARCH())...)
 	case "":
 		// no library specified, so nothing to do