loader: load packages using Go modules

This commit replaces the existing ad-hoc package loader with a package
loader that uses the x/tools/go/packages package to find all
to-be-loaded packages.

ir/ir.go

@@ -63,73 +63,14 @@ const (
 )
 
 // Create and initialize a new *Program from a *ssa.Program.
-func NewProgram(lprogram *loader.Program, mainPath string) *Program {
+func NewProgram(lprogram *loader.Program) *Program {
 	program := lprogram.LoadSSA()
 	program.Build()
 
-	// Find the main package, which is a bit difficult when running a .go file
-	// directly.
-	mainPkg := program.ImportedPackage(mainPath)
-	if mainPkg == nil {
-		for _, pkgInfo := range program.AllPackages() {
-			if pkgInfo.Pkg.Name() == "main" {
-				if mainPkg != nil {
-					panic("more than one main package found")
-				}
-				mainPkg = pkgInfo
-			}
-		}
-	}
+	mainPkg := program.ImportedPackage(lprogram.MainPkg.PkgPath)
 	if mainPkg == nil {
 		panic("could not find main package")
 	}
-
-	// Make a list of packages in import order.
-	packageList := []*ssa.Package{}
-	packageSet := map[string]struct{}{}
-	worklist := []string{"runtime", mainPath}
-	for len(worklist) != 0 {
-		pkgPath := worklist[0]
-		var pkg *ssa.Package
-		if pkgPath == mainPath {
-			pkg = mainPkg // necessary for compiling individual .go files
-		} else {
-			pkg = program.ImportedPackage(pkgPath)
-		}
-		if pkg == nil {
-			// Non-SSA package (e.g. cgo).
-			packageSet[pkgPath] = struct{}{}
-			worklist = worklist[1:]
-			continue
-		}
-		if _, ok := packageSet[pkgPath]; ok {
-			// Package already in the final package list.
-			worklist = worklist[1:]
-			continue
-		}
-
-		unsatisfiedImports := make([]string, 0)
-		imports := pkg.Pkg.Imports()
-		for _, pkg := range imports {
-			if _, ok := packageSet[pkg.Path()]; ok {
-				continue
-			}
-			unsatisfiedImports = append(unsatisfiedImports, pkg.Path())
-		}
-		if len(unsatisfiedImports) == 0 {
-			// All dependencies of this package are satisfied, so add this
-			// package to the list.
-			packageList = append(packageList, pkg)
-			packageSet[pkgPath] = struct{}{}
-			worklist = worklist[1:]
-		} else {
-			// Prepend all dependencies to the worklist and reconsider this
-			// package (by not removing it from the worklist). At that point, it
-			// must be possible to add it to packageList.
-			worklist = append(unsatisfiedImports, worklist...)
-		}
-	}
-
 	p := &Program{
 		Program:       program,
 		LoaderProgram: lprogram,
@@ -137,8 +78,8 @@ func NewProgram(lprogram *loader.Program, mainPath string) *Program {
 		functionMap:   make(map[*ssa.Function]*Function),
 	}
 
-	for _, pkg := range packageList {
-		p.AddPackage(pkg)
+	for _, pkg := range lprogram.Sorted() {
+		p.AddPackage(program.ImportedPackage(pkg.PkgPath))
 	}
 
 	return p