diff --git a/compiler/optimizer.go b/compiler/optimizer.go
index 669e089c..22153486 100644
--- a/compiler/optimizer.go
+++ b/compiler/optimizer.go
@@ -50,7 +50,7 @@ func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) erro
 
 		// Run Go-specific optimization passes.
 		transform.OptimizeMaps(c.mod)
-		c.OptimizeStringToBytes()
+		transform.OptimizeStringToBytes(c.mod)
 		transform.OptimizeAllocs(c.mod)
 		c.LowerInterfaces()
 		c.LowerFuncValues()
@@ -62,7 +62,7 @@ func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) erro
 
 		// Run TinyGo-specific interprocedural optimizations.
 		transform.OptimizeAllocs(c.mod)
-		c.OptimizeStringToBytes()
+		transform.OptimizeStringToBytes(c.mod)
 
 		// Lower runtime.isnil calls to regular nil comparisons.
 		isnil := c.mod.NamedFunction("runtime.isnil")
@@ -158,103 +158,3 @@ func (c *Compiler) replacePanicsWithTrap() {
 		}
 	}
 }
-
-// Transform runtime.stringToBytes(...) calls into const []byte slices whenever
-// possible. This optimizes the following pattern:
-//     w.Write([]byte("foo"))
-// where Write does not store to the slice.
-func (c *Compiler) OptimizeStringToBytes() {
-	stringToBytes := c.mod.NamedFunction("runtime.stringToBytes")
-	if stringToBytes.IsNil() {
-		// nothing to optimize
-		return
-	}
-
-	for _, call := range getUses(stringToBytes) {
-		strptr := call.Operand(0)
-		strlen := call.Operand(1)
-
-		// strptr is always constant because strings are always constant.
-
-		convertedAllUses := true
-		for _, use := range getUses(call) {
-			nilValue := llvm.Value{}
-			if use.IsAExtractValueInst() == nilValue {
-				convertedAllUses = false
-				continue
-			}
-			switch use.Type().TypeKind() {
-			case llvm.IntegerTypeKind:
-				// A length (len or cap). Propagate the length value.
-				use.ReplaceAllUsesWith(strlen)
-				use.EraseFromParentAsInstruction()
-			case llvm.PointerTypeKind:
-				// The string pointer itself.
-				if !c.isReadOnly(use) {
-					convertedAllUses = false
-					continue
-				}
-				use.ReplaceAllUsesWith(strptr)
-				use.EraseFromParentAsInstruction()
-			default:
-				// should not happen
-				panic("unknown return type of runtime.stringToBytes: " + use.Type().String())
-			}
-		}
-		if convertedAllUses {
-			// Call to runtime.stringToBytes can be eliminated: both the input
-			// and the output is constant.
-			call.EraseFromParentAsInstruction()
-		}
-	}
-}
-
-// Check whether the given value (which is of pointer type) is never stored to.
-func (c *Compiler) isReadOnly(value llvm.Value) bool {
-	uses := getUses(value)
-	for _, use := range uses {
-		nilValue := llvm.Value{}
-		if use.IsAGetElementPtrInst() != nilValue {
-			if !c.isReadOnly(use) {
-				return false
-			}
-		} else if use.IsACallInst() != nilValue {
-			if !c.hasFlag(use, value, "readonly") {
-				return false
-			}
-		} else {
-			// Unknown instruction, might not be readonly.
-			return false
-		}
-	}
-	return true
-}
-
-// Check whether all uses of this param as parameter to the call have the given
-// flag. In most cases, there will only be one use but a function could take the
-// same parameter twice, in which case both must have the flag.
-// A flag can be any enum flag, like "readonly".
-func (c *Compiler) hasFlag(call, param llvm.Value, kind string) bool {
-	fn := call.CalledValue()
-	nilValue := llvm.Value{}
-	if fn.IsAFunction() == nilValue {
-		// This is not a function but something else, like a function pointer.
-		return false
-	}
-	kindID := llvm.AttributeKindID(kind)
-	for i := 0; i < fn.ParamsCount(); i++ {
-		if call.Operand(i) != param {
-			// This is not the parameter we're checking.
-			continue
-		}
-		index := i + 1 // param attributes start at 1
-		attr := fn.GetEnumAttributeAtIndex(index, kindID)
-		nilAttribute := llvm.Attribute{}
-		if attr == nilAttribute {
-			// At least one parameter doesn't have the flag (there may be
-			// multiple).
-			return false
-		}
-	}
-	return true
-}
diff --git a/transform/stringtobytes.go b/transform/stringtobytes.go
new file mode 100644
index 00000000..b3cfe115
--- /dev/null
+++ b/transform/stringtobytes.go
@@ -0,0 +1,57 @@
+package transform
+
+import (
+	"tinygo.org/x/go-llvm"
+)
+
+// OptimizeStringToBytes transforms runtime.stringToBytes(...) calls into const
+// []byte slices whenever possible. This optimizes the following pattern:
+//
+//     w.Write([]byte("foo"))
+//
+// where Write does not store to the slice.
+func OptimizeStringToBytes(mod llvm.Module) {
+	stringToBytes := mod.NamedFunction("runtime.stringToBytes")
+	if stringToBytes.IsNil() {
+		// nothing to optimize
+		return
+	}
+
+	for _, call := range getUses(stringToBytes) {
+		strptr := call.Operand(0)
+		strlen := call.Operand(1)
+
+		// strptr is always constant because strings are always constant.
+
+		convertedAllUses := true
+		for _, use := range getUses(call) {
+			if use.IsAExtractValueInst().IsNil() {
+				// Expected an extractvalue, but this is something else.
+				convertedAllUses = false
+				continue
+			}
+			switch use.Type().TypeKind() {
+			case llvm.IntegerTypeKind:
+				// A length (len or cap). Propagate the length value.
+				use.ReplaceAllUsesWith(strlen)
+				use.EraseFromParentAsInstruction()
+			case llvm.PointerTypeKind:
+				// The string pointer itself.
+				if !isReadOnly(use) {
+					convertedAllUses = false
+					continue
+				}
+				use.ReplaceAllUsesWith(strptr)
+				use.EraseFromParentAsInstruction()
+			default:
+				// should not happen
+				panic("unknown return type of runtime.stringToBytes: " + use.Type().String())
+			}
+		}
+		if convertedAllUses {
+			// Call to runtime.stringToBytes can be eliminated: both the input
+			// and the output is constant.
+			call.EraseFromParentAsInstruction()
+		}
+	}
+}
diff --git a/transform/stringtobytes_test.go b/transform/stringtobytes_test.go
new file mode 100644
index 00000000..91553840
--- /dev/null
+++ b/transform/stringtobytes_test.go
@@ -0,0 +1,15 @@
+package transform
+
+import (
+	"testing"
+
+	"tinygo.org/x/go-llvm"
+)
+
+func TestOptimizeStringToBytes(t *testing.T) {
+	t.Parallel()
+	testTransform(t, "testdata/stringtobytes", func(mod llvm.Module) {
+		// Run optimization pass.
+		OptimizeStringToBytes(mod)
+	})
+}
diff --git a/transform/testdata/stringtobytes.ll b/transform/testdata/stringtobytes.ll
new file mode 100644
index 00000000..f3cec823
--- /dev/null
+++ b/transform/testdata/stringtobytes.ll
@@ -0,0 +1,32 @@
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64--linux"
+
+@str = constant [6 x i8] c"foobar"
+
+declare { i8*, i64, i64 } @runtime.stringToBytes(i8*, i64)
+
+declare void @printSlice(i8* nocapture readonly, i64, i64)
+
+declare void @writeToSlice(i8* nocapture, i64, i64)
+
+; Test that runtime.stringToBytes can be fully optimized away.
+define void @testReadOnly() {
+entry:
+  %0 = call fastcc { i8*, i64, i64 } @runtime.stringToBytes(i8* getelementptr inbounds ([6 x i8], [6 x i8]* @str, i32 0, i32 0), i64 6)
+  %1 = extractvalue { i8*, i64, i64 } %0, 0
+  %2 = extractvalue { i8*, i64, i64 } %0, 1
+  %3 = extractvalue { i8*, i64, i64 } %0, 2
+  call fastcc void @printSlice(i8* %1, i64 %2, i64 %3)
+  ret void
+}
+
+; Test that even though the slice is written to, some values can be propagated.
+define void @testReadWrite() {
+entry:
+  %0 = call fastcc { i8*, i64, i64 } @runtime.stringToBytes(i8* getelementptr inbounds ([6 x i8], [6 x i8]* @str, i32 0, i32 0), i64 6)
+  %1 = extractvalue { i8*, i64, i64 } %0, 0
+  %2 = extractvalue { i8*, i64, i64 } %0, 1
+  %3 = extractvalue { i8*, i64, i64 } %0, 2
+  call fastcc void @writeToSlice(i8* %1, i64 %2, i64 %3)
+  ret void
+}
diff --git a/transform/testdata/stringtobytes.out.ll b/transform/testdata/stringtobytes.out.ll
new file mode 100644
index 00000000..49b06581
--- /dev/null
+++ b/transform/testdata/stringtobytes.out.ll
@@ -0,0 +1,24 @@
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64--linux"
+
+@str = constant [6 x i8] c"foobar"
+
+declare { i8*, i64, i64 } @runtime.stringToBytes(i8*, i64)
+
+declare void @printSlice(i8* nocapture readonly, i64, i64)
+
+declare void @writeToSlice(i8* nocapture, i64, i64)
+
+define void @testReadOnly() {
+entry:
+  call fastcc void @printSlice(i8* getelementptr inbounds ([6 x i8], [6 x i8]* @str, i32 0, i32 0), i64 6, i64 6)
+  ret void
+}
+
+define void @testReadWrite() {
+entry:
+  %0 = call fastcc { i8*, i64, i64 } @runtime.stringToBytes(i8* getelementptr inbounds ([6 x i8], [6 x i8]* @str, i32 0, i32 0), i64 6)
+  %1 = extractvalue { i8*, i64, i64 } %0, 0
+  call fastcc void @writeToSlice(i8* %1, i64 6, i64 6)
+  ret void
+}
diff --git a/transform/util.go b/transform/util.go
index a27bf9ec..9923669d 100644
--- a/transform/util.go
+++ b/transform/util.go
@@ -32,3 +32,24 @@ func hasFlag(call, param llvm.Value, kind string) bool {
 	}
 	return true
 }
+
+// isReadOnly returns true if the given value (which must be of pointer type) is
+// never stored to, and false if this cannot be proven.
+func isReadOnly(value llvm.Value) bool {
+	uses := getUses(value)
+	for _, use := range uses {
+		if !use.IsAGetElementPtrInst().IsNil() {
+			if !isReadOnly(use) {
+				return false
+			}
+		} else if !use.IsACallInst().IsNil() {
+			if !hasFlag(use, value, "readonly") {
+				return false
+			}
+		} else {
+			// Unknown instruction, might not be readonly.
+			return false
+		}
+	}
+	return true
+}