diff --git a/builder/build.go b/builder/build.go
index 104e0f386..974ddc2a3 100644
--- a/builder/build.go
+++ b/builder/build.go
@@ -24,6 +24,7 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	"sync"
 
 	"github.com/gofrs/flock"
 	"github.com/tinygo-org/tinygo/compileopts"
@@ -269,6 +270,7 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 	// Create the *ssa.Program. This does not yet build the entire SSA of the
 	// program so it's pretty fast and doesn't need to be parallelized.
 	program := lprogram.LoadSSA()
+	buildProgram := sync.OnceFunc(program.Build)
 
 	// Add jobs to compile each package.
 	// Packages that have a cache hit will not be compiled again.
@@ -398,8 +400,13 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 					return nil
 				}
 
-				// Compile AST to IR. The compiler.CompilePackage function will
-				// build the SSA as needed.
+				// SSA package builds may run concurrently, but the resulting
+				// functions cannot be inspected until all builds have finished:
+				// generic instances and wrappers can be shared across packages.
+				// Build the whole program once before compiling any package.
+				buildProgram()
+
+				// Compile AST to IR.
 				mod, errs := compiler.CompilePackage(pkg.ImportPath, pkg, program.Package(pkg.Pkg), machine, compilerConfig, config.DumpSSA())
 				defer mod.Context().Dispose()
 				defer mod.Dispose()
diff --git a/compiler/compiler.go b/compiler/compiler.go
index 19410d49e..51197bac3 100644
--- a/compiler/compiler.go
+++ b/compiler/compiler.go
@@ -296,6 +296,9 @@ func Sizes(machine llvm.TargetMachine) types.Sizes {
 }
 
 // CompilePackage compiles a single package to a LLVM module.
+//
+// The SSA package must already be built. When packages are compiled
+// concurrently, the entire SSA program must be built before compilation starts.
 func CompilePackage(moduleName string, pkg *loader.Package, ssaPkg *ssa.Package, machine llvm.TargetMachine, config *Config, dumpSSA bool) (llvm.Module, []error) {
 	c := newCompilerContext(moduleName, machine, config, dumpSSA)
 	defer c.dispose()
@@ -306,9 +309,6 @@ func CompilePackage(moduleName string, pkg *loader.Package, ssaPkg *ssa.Package,
 	c.runtimePkg = ssaPkg.Prog.ImportedPackage("runtime").Pkg
 	c.program = ssaPkg.Prog
 
-	// Convert AST to SSA.
-	ssaPkg.Build()
-
 	// Assign names to function-local named types before compiling the
 	// package, so that types declared in different functions (or in
 	// different instantiations of a generic function) do not collide.
diff --git a/compiler/compiler_test.go b/compiler/compiler_test.go
index 99de5bc62..d9188790c 100644
--- a/compiler/compiler_test.go
+++ b/compiler/compiler_test.go
@@ -413,5 +413,7 @@ func testCompilePackage(t *testing.T, options *compileopts.Options, file string)
 	// Compile AST to IR.
 	program := lprogram.LoadSSA()
 	pkg := lprogram.MainPkg()
-	return CompilePackage(file, pkg, program.Package(pkg.Pkg), machine, compilerConfig, false)
+	ssaPkg := program.Package(pkg.Pkg)
+	ssaPkg.Build()
+	return CompilePackage(file, pkg, ssaPkg, machine, compilerConfig, false)
 }
diff --git a/transform/transform_test.go b/transform/transform_test.go
index 4af128387..89fda55e2 100644
--- a/transform/transform_test.go
+++ b/transform/transform_test.go
@@ -224,7 +224,9 @@ func compileGoFileForTesting(t *testing.T, filename string) llvm.Module {
 	// Compile AST to IR.
 	program := lprogram.LoadSSA()
 	pkg := lprogram.MainPkg()
-	mod, errs := compiler.CompilePackage(filename, pkg, program.Package(pkg.Pkg), machine, compilerConfig, false)
+	ssaPkg := program.Package(pkg.Pkg)
+	ssaPkg.Build()
+	mod, errs := compiler.CompilePackage(filename, pkg, ssaPkg, machine, compilerConfig, false)
 	if errs != nil {
 		for _, err := range errs {
 			t.Error(err)