build: mark need runtime

internal/build/build.go

@@ -132,6 +132,14 @@ func Do(args []string, conf *Config) {
 	}
 }
 
+func setNeedRuntime(pkg *packages.Package) {
+	pkg.Module = nil // just use pkg.Module to mark it needs runtime
+}
+
+func isNeedRuntime(pkg *packages.Package) bool {
+	return pkg.Module == nil
+}
+
 func buildAllPkgs(prog llssa.Program, initial []*packages.Package, mode Mode, verbose bool) {
 	// Create SSA-form program representation.
 	ssaProg, pkgs, errPkgs := allPkgs(initial, ssa.SanityCheckFunctions)
@@ -141,6 +149,9 @@ func buildAllPkgs(prog llssa.Program, initial []*packages.Package, mode Mode, ve
 	}
 	for _, pkg := range pkgs {
 		buildPkg(prog, pkg, mode, verbose)
+		if prog.NeedRuntime() {
+			setNeedRuntime(pkg.Package)
+		}
 	}
 }
 
@@ -156,14 +167,18 @@ func linkMainPkg(pkg *packages.Package, runtimeFiles []string, conf *Config, mod
 	args[0] = "-o"
 	args[1] = app
 	args[2] = "-Wno-override-module"
-	if runtimeFiles != nil {
-		args = append(args, runtimeFiles...)
-	}
+	needRuntime := false
 	packages.Visit([]*packages.Package{pkg}, nil, func(p *packages.Package) {
 		if p.PkgPath != "unsafe" { // TODO(xsw): maybe can remove this special case
 			args = append(args, p.ExportFile+".ll")
+			if !needRuntime {
+				needRuntime = isNeedRuntime(p)
+			}
 		}
 	})
+	if needRuntime && runtimeFiles != nil {
+		args = append(args, runtimeFiles...)
+	}
 
 	// TODO(xsw): show work
 	// fmt.Fprintln(os.Stderr, "clang", args)

ssa/package.go

@@ -126,6 +126,8 @@ type aProgram struct {
 	uintptrTy Type
 	intTy     Type
 	f64Ty     Type
+
+	needRuntime bool
 }
 
 // A Program presents a program.
@@ -154,10 +156,16 @@ func (p Program) SetRuntime(runtime any) {
 	}
 }
 
+// NeedRuntime returns if the current package needs runtime.
+func (p Program) NeedRuntime() bool {
+	return p.needRuntime
+}
+
 func (p Program) runtime() *types.Package {
 	if p.rt == nil {
 		p.rt = p.rtget()
 	}
+	p.needRuntime = true
 	return p.rt
 }
 
@@ -197,6 +205,7 @@ func (p Program) NewPackage(name, pkgPath string) Package {
 	// mod.Finalize()
 	fns := make(map[string]Function)
 	gbls := make(map[string]Global)
+	p.needRuntime = false
 	return &aPackage{mod, fns, gbls, p}
 }