diff --git a/builder/build.go b/builder/build.go
index e7029afd..3ce7f644 100644
--- a/builder/build.go
+++ b/builder/build.go
@@ -72,16 +72,16 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(stri
 	// exactly.
 	errs = nil
 	switch config.Options.Opt {
-	case "none:", "0":
-		errs = c.Optimize(0, 0, 0) // -O0
+	case "none", "0":
+		errs = transform.Optimize(c.Module(), config, 0, 0, 0) // -O0
 	case "1":
-		errs = c.Optimize(1, 0, 0) // -O1
+		errs = transform.Optimize(c.Module(), config, 1, 0, 0) // -O1
 	case "2":
-		errs = c.Optimize(2, 0, 225) // -O2
+		errs = transform.Optimize(c.Module(), config, 2, 0, 225) // -O2
 	case "s":
-		errs = c.Optimize(2, 1, 225) // -Os
+		errs = transform.Optimize(c.Module(), config, 2, 1, 225) // -Os
 	case "z":
-		errs = c.Optimize(2, 2, 5) // -Oz, default
+		errs = transform.Optimize(c.Module(), config, 2, 2, 5) // -Oz, default
 	default:
 		errs = []error{errors.New("unknown optimization level: -opt=" + config.Options.Opt)}
 	}
diff --git a/compiler/compiler.go b/compiler/compiler.go
index 609d57f7..63b1dc92 100644
--- a/compiler/compiler.go
+++ b/compiler/compiler.go
@@ -34,30 +34,6 @@ func init() {
 // The TinyGo import path.
 const tinygoPath = "github.com/tinygo-org/tinygo"
 
-// functionsUsedInTransform is a list of function symbols that may be used
-// during TinyGo optimization passes so they have to be marked as external
-// linkage until all TinyGo passes have finished.
-var functionsUsedInTransforms = []string{
-	"runtime.alloc",
-	"runtime.free",
-	"runtime.nilPanic",
-}
-
-var taskFunctionsUsedInTransforms = []string{}
-
-var coroFunctionsUsedInTransforms = []string{
-	"internal/task.start",
-	"internal/task.Pause",
-	"internal/task.fake",
-	"internal/task.Current",
-	"internal/task.createTask",
-	"(*internal/task.Task).setState",
-	"(*internal/task.Task).returnTo",
-	"(*internal/task.Task).returnCurrent",
-	"(*internal/task.Task).setReturnPtr",
-	"(*internal/task.Task).getReturnPtr",
-}
-
 type Compiler struct {
 	*compileopts.Config
 	mod                     llvm.Module
@@ -156,21 +132,6 @@ func (c *Compiler) Module() llvm.Module {
 	return c.mod
 }
 
-// getFunctionsUsedInTransforms gets a list of all special functions that should be preserved during transforms and optimization.
-func (c *Compiler) getFunctionsUsedInTransforms() []string {
-	fnused := functionsUsedInTransforms
-	switch c.Scheduler() {
-	case "none":
-	case "coroutines":
-		fnused = append(append([]string{}, fnused...), coroFunctionsUsedInTransforms...)
-	case "tasks":
-		fnused = append(append([]string{}, fnused...), taskFunctionsUsedInTransforms...)
-	default:
-		panic(fmt.Errorf("invalid scheduler %q", c.Scheduler()))
-	}
-	return fnused
-}
-
 // Compile the given package path or .go file path. Return an error when this
 // fails (in any stage).
 func (c *Compiler) Compile(mainPath string) []error {
@@ -338,14 +299,8 @@ func (c *Compiler) Compile(mainPath string) []error {
 	realMain := c.mod.NamedFunction(c.ir.MainPkg().Pkg.Path() + ".main")
 	realMain.SetLinkage(llvm.ExternalLinkage) // keep alive until goroutine lowering
 
-	// Make sure these functions are kept in tact during TinyGo transformation passes.
-	for _, name := range c.getFunctionsUsedInTransforms() {
-		fn := c.mod.NamedFunction(name)
-		if fn.IsNil() {
-			panic(fmt.Errorf("missing core function %q", name))
-		}
-		fn.SetLinkage(llvm.ExternalLinkage)
-	}
+	// Replace callMain placeholder with actual main function.
+	c.mod.NamedFunction("runtime.callMain").ReplaceAllUsesWith(realMain)
 
 	// Load some attributes
 	getAttr := func(attrName string) llvm.Attribute {
diff --git a/compiler/optimizer.go b/compiler/optimizer.go
deleted file mode 100644
index 4b08eb44..00000000
--- a/compiler/optimizer.go
+++ /dev/null
@@ -1,185 +0,0 @@
-package compiler
-
-import (
-	"errors"
-
-	"github.com/tinygo-org/tinygo/compileopts"
-	"github.com/tinygo-org/tinygo/compiler/ircheck"
-	"github.com/tinygo-org/tinygo/transform"
-	"tinygo.org/x/go-llvm"
-)
-
-// Run the LLVM optimizer over the module.
-// The inliner can be disabled (if necessary) by passing 0 to the inlinerThreshold.
-func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) []error {
-	builder := llvm.NewPassManagerBuilder()
-	defer builder.Dispose()
-	builder.SetOptLevel(optLevel)
-	builder.SetSizeLevel(sizeLevel)
-	if inlinerThreshold != 0 {
-		builder.UseInlinerWithThreshold(inlinerThreshold)
-	}
-	builder.AddCoroutinePassesToExtensionPoints()
-
-	if c.PanicStrategy() == "trap" {
-		transform.ReplacePanicsWithTrap(c.mod) // -panic=trap
-	}
-
-	// run a check of all of our code
-	if c.VerifyIR() {
-		errs := ircheck.Module(c.mod)
-		if errs != nil {
-			return errs
-		}
-	}
-
-	// Replace callMain placeholder with actual main function.
-	c.mod.NamedFunction("runtime.callMain").ReplaceAllUsesWith(c.mod.NamedFunction(c.ir.MainPkg().Pkg.Path() + ".main"))
-
-	// Run function passes for each function.
-	funcPasses := llvm.NewFunctionPassManagerForModule(c.mod)
-	defer funcPasses.Dispose()
-	builder.PopulateFunc(funcPasses)
-	funcPasses.InitializeFunc()
-	for fn := c.mod.FirstFunction(); !fn.IsNil(); fn = llvm.NextFunction(fn) {
-		funcPasses.RunFunc(fn)
-	}
-	funcPasses.FinalizeFunc()
-
-	if optLevel > 0 {
-		// Run some preparatory passes for the Go optimizer.
-		goPasses := llvm.NewPassManager()
-		defer goPasses.Dispose()
-		goPasses.AddGlobalDCEPass()
-		goPasses.AddGlobalOptimizerPass()
-		goPasses.AddConstantPropagationPass()
-		goPasses.AddAggressiveDCEPass()
-		goPasses.AddFunctionAttrsPass()
-		goPasses.Run(c.mod)
-
-		// Run Go-specific optimization passes.
-		transform.OptimizeMaps(c.mod)
-		transform.OptimizeStringToBytes(c.mod)
-		transform.OptimizeAllocs(c.mod)
-		transform.LowerInterfaces(c.mod)
-
-		errs := transform.LowerInterrupts(c.mod)
-		if len(errs) > 0 {
-			return errs
-		}
-
-		if c.FuncImplementation() == compileopts.FuncValueSwitch {
-			transform.LowerFuncValues(c.mod)
-		}
-
-		// After interfaces are lowered, there are many more opportunities for
-		// interprocedural optimizations. To get them to work, function
-		// attributes have to be updated first.
-		goPasses.Run(c.mod)
-
-		// Run TinyGo-specific interprocedural optimizations.
-		transform.OptimizeAllocs(c.mod)
-		transform.OptimizeStringToBytes(c.mod)
-
-		// Lower runtime.isnil calls to regular nil comparisons.
-		isnil := c.mod.NamedFunction("runtime.isnil")
-		if !isnil.IsNil() {
-			for _, use := range getUses(isnil) {
-				c.builder.SetInsertPointBefore(use)
-				ptr := use.Operand(0)
-				if !ptr.IsABitCastInst().IsNil() {
-					ptr = ptr.Operand(0)
-				}
-				nilptr := llvm.ConstPointerNull(ptr.Type())
-				icmp := c.builder.CreateICmp(llvm.IntEQ, ptr, nilptr, "")
-				use.ReplaceAllUsesWith(icmp)
-				use.EraseFromParentAsInstruction()
-			}
-		}
-
-	} else {
-		// Must be run at any optimization level.
-		transform.LowerInterfaces(c.mod)
-		if c.FuncImplementation() == compileopts.FuncValueSwitch {
-			transform.LowerFuncValues(c.mod)
-		}
-		errs := transform.LowerInterrupts(c.mod)
-		if len(errs) > 0 {
-			return errs
-		}
-	}
-
-	// Lower async implementations.
-	switch c.Scheduler() {
-	case "coroutines":
-		// Lower async as coroutines.
-		err := transform.LowerCoroutines(c.mod, c.NeedsStackObjects())
-		if err != nil {
-			return []error{err}
-		}
-	case "tasks":
-		// No transformations necessary.
-	case "none":
-		// Check for any goroutine starts.
-		if start := c.mod.NamedFunction("internal/task.start"); !start.IsNil() && len(getUses(start)) > 0 {
-			errs := []error{}
-			for _, call := range getUses(start) {
-				errs = append(errs, errorAt(call, "attempted to start a goroutine without a scheduler"))
-			}
-			return errs
-		}
-	default:
-		return []error{errors.New("invalid scheduler")}
-	}
-
-	if c.VerifyIR() {
-		if errs := ircheck.Module(c.mod); errs != nil {
-			return errs
-		}
-	}
-	if err := c.Verify(); err != nil {
-		return []error{errors.New("optimizations caused a verification failure")}
-	}
-
-	if sizeLevel >= 2 {
-		// Set the "optsize" attribute to make slightly smaller binaries at the
-		// cost of some performance.
-		kind := llvm.AttributeKindID("optsize")
-		attr := c.ctx.CreateEnumAttribute(kind, 0)
-		for fn := c.mod.FirstFunction(); !fn.IsNil(); fn = llvm.NextFunction(fn) {
-			fn.AddFunctionAttr(attr)
-		}
-	}
-
-	// After TinyGo-specific transforms have finished, undo exporting these functions.
-	for _, name := range c.getFunctionsUsedInTransforms() {
-		fn := c.mod.NamedFunction(name)
-		if fn.IsNil() {
-			continue
-		}
-		fn.SetLinkage(llvm.InternalLinkage)
-	}
-
-	// Run function passes again, because without it, llvm.coro.size.i32()
-	// doesn't get lowered.
-	for fn := c.mod.FirstFunction(); !fn.IsNil(); fn = llvm.NextFunction(fn) {
-		funcPasses.RunFunc(fn)
-	}
-	funcPasses.FinalizeFunc()
-
-	// Run module passes.
-	modPasses := llvm.NewPassManager()
-	defer modPasses.Dispose()
-	builder.Populate(modPasses)
-	modPasses.Run(c.mod)
-
-	hasGCPass := transform.AddGlobalsBitmap(c.mod)
-	hasGCPass = transform.MakeGCStackSlots(c.mod) || hasGCPass
-	if hasGCPass {
-		if err := c.Verify(); err != nil {
-			return []error{errors.New("GC pass caused a verification failure")}
-		}
-	}
-
-	return nil
-}
diff --git a/transform/coroutines.go b/transform/coroutines.go
index 5df2d746..d1609b65 100644
--- a/transform/coroutines.go
+++ b/transform/coroutines.go
@@ -5,8 +5,9 @@ package transform
 
 import (
 	"errors"
-	"github.com/tinygo-org/tinygo/compiler/llvmutil"
 	"strconv"
+
+	"github.com/tinygo-org/tinygo/compiler/llvmutil"
 	"tinygo.org/x/go-llvm"
 )
 
diff --git a/transform/optimizer.go b/transform/optimizer.go
new file mode 100644
index 00000000..71bf4d61
--- /dev/null
+++ b/transform/optimizer.go
@@ -0,0 +1,242 @@
+package transform
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/tinygo-org/tinygo/compileopts"
+	"github.com/tinygo-org/tinygo/compiler/ircheck"
+	"tinygo.org/x/go-llvm"
+)
+
+// Optimize runs a number of optimization and transformation passes over the
+// given module. Some passes are specific to TinyGo, others are generic LLVM
+// passes. You can set a preferred performance (0-3) and size (0-2) level and
+// control the limits of the inliner (higher numbers mean more inlining, set it
+// to 0 to disable entirely).
+//
+// Please note that some optimizations are not optional, thus Optimize must
+// alwasy be run before emitting machine code. Set all controls (optLevel,
+// sizeLevel, inlinerThreshold) to 0 to reduce the number of optimizations to a
+// minimum.
+func Optimize(mod llvm.Module, config *compileopts.Config, optLevel, sizeLevel int, inlinerThreshold uint) []error {
+	builder := llvm.NewPassManagerBuilder()
+	defer builder.Dispose()
+	builder.SetOptLevel(optLevel)
+	builder.SetSizeLevel(sizeLevel)
+	if inlinerThreshold != 0 {
+		builder.UseInlinerWithThreshold(inlinerThreshold)
+	}
+	builder.AddCoroutinePassesToExtensionPoints()
+
+	// Make sure these functions are kept in tact during TinyGo transformation passes.
+	for _, name := range getFunctionsUsedInTransforms(config) {
+		fn := mod.NamedFunction(name)
+		if fn.IsNil() {
+			panic(fmt.Errorf("missing core function %q", name))
+		}
+		fn.SetLinkage(llvm.ExternalLinkage)
+	}
+
+	if config.PanicStrategy() == "trap" {
+		ReplacePanicsWithTrap(mod) // -panic=trap
+	}
+
+	// run a check of all of our code
+	if config.VerifyIR() {
+		errs := ircheck.Module(mod)
+		if errs != nil {
+			return errs
+		}
+	}
+
+	// Run function passes for each function.
+	funcPasses := llvm.NewFunctionPassManagerForModule(mod)
+	defer funcPasses.Dispose()
+	builder.PopulateFunc(funcPasses)
+	funcPasses.InitializeFunc()
+	for fn := mod.FirstFunction(); !fn.IsNil(); fn = llvm.NextFunction(fn) {
+		funcPasses.RunFunc(fn)
+	}
+	funcPasses.FinalizeFunc()
+
+	if optLevel > 0 {
+		// Run some preparatory passes for the Go optimizer.
+		goPasses := llvm.NewPassManager()
+		defer goPasses.Dispose()
+		goPasses.AddGlobalDCEPass()
+		goPasses.AddGlobalOptimizerPass()
+		goPasses.AddConstantPropagationPass()
+		goPasses.AddAggressiveDCEPass()
+		goPasses.AddFunctionAttrsPass()
+		goPasses.Run(mod)
+
+		// Run Go-specific optimization passes.
+		OptimizeMaps(mod)
+		OptimizeStringToBytes(mod)
+		OptimizeAllocs(mod)
+		LowerInterfaces(mod)
+
+		errs := LowerInterrupts(mod)
+		if len(errs) > 0 {
+			return errs
+		}
+
+		if config.FuncImplementation() == compileopts.FuncValueSwitch {
+			LowerFuncValues(mod)
+		}
+
+		// After interfaces are lowered, there are many more opportunities for
+		// interprocedural optimizations. To get them to work, function
+		// attributes have to be updated first.
+		goPasses.Run(mod)
+
+		// Run TinyGo-specific interprocedural optimizations.
+		OptimizeAllocs(mod)
+		OptimizeStringToBytes(mod)
+
+		// Lower runtime.isnil calls to regular nil comparisons.
+		isnil := mod.NamedFunction("runtime.isnil")
+		if !isnil.IsNil() {
+			builder := mod.Context().NewBuilder()
+			defer builder.Dispose()
+			for _, use := range getUses(isnil) {
+				builder.SetInsertPointBefore(use)
+				ptr := use.Operand(0)
+				if !ptr.IsABitCastInst().IsNil() {
+					ptr = ptr.Operand(0)
+				}
+				nilptr := llvm.ConstPointerNull(ptr.Type())
+				icmp := builder.CreateICmp(llvm.IntEQ, ptr, nilptr, "")
+				use.ReplaceAllUsesWith(icmp)
+				use.EraseFromParentAsInstruction()
+			}
+		}
+
+	} else {
+		// Must be run at any optimization level.
+		LowerInterfaces(mod)
+		if config.FuncImplementation() == compileopts.FuncValueSwitch {
+			LowerFuncValues(mod)
+		}
+		errs := LowerInterrupts(mod)
+		if len(errs) > 0 {
+			return errs
+		}
+	}
+
+	// Lower async implementations.
+	switch config.Scheduler() {
+	case "coroutines":
+		// Lower async as coroutines.
+		err := LowerCoroutines(mod, config.NeedsStackObjects())
+		if err != nil {
+			return []error{err}
+		}
+	case "tasks":
+		// No transformations necessary.
+	case "none":
+		// Check for any goroutine starts.
+		if start := mod.NamedFunction("internal/task.start"); !start.IsNil() && len(getUses(start)) > 0 {
+			errs := []error{}
+			for _, call := range getUses(start) {
+				errs = append(errs, errorAt(call, "attempted to start a goroutine without a scheduler"))
+			}
+			return errs
+		}
+	default:
+		return []error{errors.New("invalid scheduler")}
+	}
+
+	if config.VerifyIR() {
+		if errs := ircheck.Module(mod); errs != nil {
+			return errs
+		}
+	}
+	if err := llvm.VerifyModule(mod, llvm.PrintMessageAction); err != nil {
+		return []error{errors.New("optimizations caused a verification failure")}
+	}
+
+	if sizeLevel >= 2 {
+		// Set the "optsize" attribute to make slightly smaller binaries at the
+		// cost of some performance.
+		kind := llvm.AttributeKindID("optsize")
+		attr := mod.Context().CreateEnumAttribute(kind, 0)
+		for fn := mod.FirstFunction(); !fn.IsNil(); fn = llvm.NextFunction(fn) {
+			fn.AddFunctionAttr(attr)
+		}
+	}
+
+	// After TinyGo-specific transforms have finished, undo exporting these functions.
+	for _, name := range getFunctionsUsedInTransforms(config) {
+		fn := mod.NamedFunction(name)
+		if fn.IsNil() {
+			continue
+		}
+		fn.SetLinkage(llvm.InternalLinkage)
+	}
+
+	// Run function passes again, because without it, llvm.coro.size.i32()
+	// doesn't get lowered.
+	for fn := mod.FirstFunction(); !fn.IsNil(); fn = llvm.NextFunction(fn) {
+		funcPasses.RunFunc(fn)
+	}
+	funcPasses.FinalizeFunc()
+
+	// Run module passes.
+	modPasses := llvm.NewPassManager()
+	defer modPasses.Dispose()
+	builder.Populate(modPasses)
+	modPasses.Run(mod)
+
+	hasGCPass := AddGlobalsBitmap(mod)
+	hasGCPass = MakeGCStackSlots(mod) || hasGCPass
+	if hasGCPass {
+		if err := llvm.VerifyModule(mod, llvm.PrintMessageAction); err != nil {
+			return []error{errors.New("GC pass caused a verification failure")}
+		}
+	}
+
+	return nil
+}
+
+// functionsUsedInTransform is a list of function symbols that may be used
+// during TinyGo optimization passes so they have to be marked as external
+// linkage until all TinyGo passes have finished.
+var functionsUsedInTransforms = []string{
+	"runtime.alloc",
+	"runtime.free",
+	"runtime.nilPanic",
+}
+
+var taskFunctionsUsedInTransforms = []string{}
+
+// These functions need to be preserved in the IR until after the coroutines
+// pass has run.
+var coroFunctionsUsedInTransforms = []string{
+	"internal/task.start",
+	"internal/task.Pause",
+	"internal/task.fake",
+	"internal/task.Current",
+	"internal/task.createTask",
+	"(*internal/task.Task).setState",
+	"(*internal/task.Task).returnTo",
+	"(*internal/task.Task).returnCurrent",
+	"(*internal/task.Task).setReturnPtr",
+	"(*internal/task.Task).getReturnPtr",
+}
+
+// getFunctionsUsedInTransforms gets a list of all special functions that should be preserved during transforms and optimization.
+func getFunctionsUsedInTransforms(config *compileopts.Config) []string {
+	fnused := functionsUsedInTransforms
+	switch config.Scheduler() {
+	case "none":
+	case "coroutines":
+		fnused = append(append([]string{}, fnused...), coroFunctionsUsedInTransforms...)
+	case "tasks":
+		fnused = append(append([]string{}, fnused...), taskFunctionsUsedInTransforms...)
+	default:
+		panic(fmt.Errorf("invalid scheduler %q", config.Scheduler()))
+	}
+	return fnused
+}