windows: use ThinLTO

Switch Windows builds to use ThinLTO. This gets us closer to using
ThinLTO everywhere.

builder/build.go

@@ -694,15 +694,23 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
 				config.Options.PrintCommands(config.Target.Linker, ldflags...)
 			}
 			if config.UseThinLTO() {
-				ldflags = append(ldflags,
-					"-mllvm", "-mcpu="+config.CPU(),
-					"--lto-O"+strconv.Itoa(optLevel))
-				if config.GOOS() == "darwin" {
+				ldflags = append(ldflags, "-mllvm", "-mcpu="+config.CPU())
+				if config.GOOS() == "windows" {
+					// Options for the MinGW wrapper for the lld COFF linker.
+					ldflags = append(ldflags,
+						"-Xlink=/opt:lldlto="+strconv.Itoa(optLevel),
+						"--thinlto-cache-dir="+filepath.Join(cacheDir, "thinlto"))
+				} else if config.GOOS() == "darwin" {
 					// Options for the ld64-compatible lld linker.
-					ldflags = append(ldflags, "-cache_path_lto", filepath.Join(cacheDir, "thinlto"))
+					ldflags = append(ldflags,
+						"--lto-O"+strconv.Itoa(optLevel),
+						"-cache_path_lto", filepath.Join(cacheDir, "thinlto"))
 				} else {
 					// Options for the ELF linker.
-					ldflags = append(ldflags, "--thinlto-cache-dir="+filepath.Join(cacheDir, "thinlto"))
+					ldflags = append(ldflags,
+						"--lto-O"+strconv.Itoa(optLevel),
+						"--thinlto-cache-dir="+filepath.Join(cacheDir, "thinlto"),
+					)
 				}
 				if config.CodeModel() != "default" {
 					ldflags = append(ldflags,

compileopts/config.go

@@ -192,11 +192,6 @@ func (c *Config) UseThinLTO() bool {
 		// through a plugin, but it's too much hassle to set up.
 		return false
 	}
-	if len(parts) >= 2 && parts[2] == "windows" {
-		// Linker error (undefined runtime.trackedGlobalsBitmap) when linking
-		// for Windows. Disable it for now until that's figured out and fixed.
-		return false
-	}
 	// Other architectures support ThinLTO.
 	return true
 }