diff --git a/.gitmodules b/.gitmodules
index c9e2db4e..0e29f9f1 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -10,3 +10,7 @@
 [submodule "lib/cmsis-svd"]
 	path = lib/cmsis-svd
 	url = https://github.com/posborne/cmsis-svd
+[submodule "lib/compiler-rt"]
+	path = lib/compiler-rt
+	url = https://github.com/llvm-mirror/compiler-rt.git
+	branch = release_70
diff --git a/.travis.yml b/.travis.yml
index 3c472f2b..8cf56033 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,8 +8,8 @@ before_install:
   - echo "deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-7 main"            | sudo tee -a /etc/apt/sources.list
   - echo "deb http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu trusty main" | sudo tee -a /etc/apt/sources.list
   - sudo apt-get update -qq
-  - sudo apt-get install llvm-7-dev gcc-8 gcc-arm-none-eabi --allow-unauthenticated -y
-  - sudo ln -s /usr/bin/gcc-8 /usr/local/bin/cc # work around missing -no-pie in old GCC version
+  - sudo apt-get install llvm-7-dev clang-7 gcc-arm-none-eabi --allow-unauthenticated -y
+  - sudo ln -s /usr/bin/clang-7 /usr/local/bin/cc # work around missing -no-pie in old GCC version
 
 install:
   - go get github.com/aykevl/go-llvm
diff --git a/buildcache.go b/buildcache.go
new file mode 100644
index 00000000..3c2ea4b3
--- /dev/null
+++ b/buildcache.go
@@ -0,0 +1,108 @@
+package main
+
+import (
+	"io"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+// Get the cache directory, usually ~/.cache/tinygo
+func cacheDir() string {
+	home := getHomeDir()
+	dir := filepath.Join(home, ".cache", "tinygo")
+	return dir
+}
+
+// Return the newest timestamp of all the file paths passed in. Used to check
+// for stale caches.
+func cacheTimestamp(paths []string) (time.Time, error) {
+	var timestamp time.Time
+	for _, path := range paths {
+		st, err := os.Stat(path)
+		if err != nil {
+			return time.Time{}, err
+		}
+		if timestamp.IsZero() {
+			timestamp = st.ModTime()
+		} else if timestamp.Before(st.ModTime()) {
+			timestamp = st.ModTime()
+		}
+	}
+	return timestamp, nil
+}
+
+// Try to load a given file from the cache. Return "", nil if no cached file can
+// be found (or the file is stale), return the absolute path if there is a cache
+// and return an error on I/O errors.
+//
+// TODO: the configKey is currently ignored. It is supposed to be used as extra
+// data for the cache key, like the compiler version and arguments.
+func cacheLoad(name, configKey string, sourceFiles []string) (string, error) {
+	dir := cacheDir()
+	cachepath := filepath.Join(dir, name)
+	cacheStat, err := os.Stat(cachepath)
+	if os.IsNotExist(err) {
+		return "", nil // does not exist
+	} else if err != nil {
+		return "", err // cannot stat cache file
+	}
+
+	sourceTimestamp, err := cacheTimestamp(sourceFiles)
+	if err != nil {
+		return "", err // cannot stat source files
+	}
+
+	if cacheStat.ModTime().After(sourceTimestamp) {
+		return cachepath, nil
+	} else {
+		os.Remove(cachepath)
+		// stale cache
+		return "", nil
+	}
+}
+
+// Store the file located at tmppath in the cache with the given name. The
+// tmppath may or may not be gone afterwards.
+//
+// Note: the configKey is ignored, see cacheLoad.
+func cacheStore(tmppath, name, configKey string, sourceFiles []string) (string, error) {
+	// get the last modified time
+	if len(sourceFiles) == 0 {
+		panic("cache: no source files")
+	}
+
+	// TODO: check the config key
+
+	dir := cacheDir()
+	err := os.MkdirAll(dir, 0777)
+	if err != nil {
+		return "", err
+	}
+	cachepath := filepath.Join(dir, name)
+	err = os.Rename(tmppath, cachepath)
+	if err != nil {
+		inf, err := os.Open(tmppath)
+		if err != nil {
+			return "", err
+		}
+		defer inf.Close()
+		outf, err := os.Create(cachepath + ".tmp")
+		if err != nil {
+			return "", err
+		}
+
+		_, err = io.Copy(outf, inf)
+		if err != nil {
+			return "", err
+		}
+
+		err = os.Rename(cachepath+".tmp", cachepath)
+		if err != nil {
+			return "", err
+		}
+
+		return cachepath, outf.Close()
+	}
+	return cachepath, nil
+}
diff --git a/builtins.go b/builtins.go
new file mode 100644
index 00000000..8c1817ed
--- /dev/null
+++ b/builtins.go
@@ -0,0 +1,186 @@
+package main
+
+import (
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+)
+
+// These are the GENERIC_SOURCES according to CMakeList.txt.
+var genericBuiltins = []string{
+	"absvdi2",
+	"absvsi2",
+	"absvti2",
+	"adddf3",
+	"addsf3",
+	"addtf3",
+	"addvdi3",
+	"addvsi3",
+	"addvti3",
+	"apple_versioning",
+	"ashldi3",
+	"ashlti3",
+	"ashrdi3",
+	"ashrti3",
+	"bswapdi2",
+	"bswapsi2",
+	"clzdi2",
+	"clzsi2",
+	"clzti2",
+	"cmpdi2",
+	"cmpti2",
+	"comparedf2",
+	"comparesf2",
+	"ctzdi2",
+	"ctzsi2",
+	"ctzti2",
+	"divdc3",
+	"divdf3",
+	"divdi3",
+	"divmoddi4",
+	"divmodsi4",
+	"divsc3",
+	"divsf3",
+	"divsi3",
+	"divtc3",
+	"divti3",
+	"divtf3",
+	"extendsfdf2",
+	"extendhfsf2",
+	"ffsdi2",
+	"ffssi2",
+	"ffsti2",
+	"fixdfdi",
+	"fixdfsi",
+	"fixdfti",
+	"fixsfdi",
+	"fixsfsi",
+	"fixsfti",
+	"fixunsdfdi",
+	"fixunsdfsi",
+	"fixunsdfti",
+	"fixunssfdi",
+	"fixunssfsi",
+	"fixunssfti",
+	"floatdidf",
+	"floatdisf",
+	"floatsidf",
+	"floatsisf",
+	"floattidf",
+	"floattisf",
+	"floatundidf",
+	"floatundisf",
+	"floatunsidf",
+	"floatunsisf",
+	"floatuntidf",
+	"floatuntisf",
+	//"int_util",
+	"lshrdi3",
+	"lshrti3",
+	"moddi3",
+	"modsi3",
+	"modti3",
+	"muldc3",
+	"muldf3",
+	"muldi3",
+	"mulodi4",
+	"mulosi4",
+	"muloti4",
+	"mulsc3",
+	"mulsf3",
+	"multi3",
+	"multf3",
+	"mulvdi3",
+	"mulvsi3",
+	"mulvti3",
+	"negdf2",
+	"negdi2",
+	"negsf2",
+	"negti2",
+	"negvdi2",
+	"negvsi2",
+	"negvti2",
+	"os_version_check",
+	"paritydi2",
+	"paritysi2",
+	"parityti2",
+	"popcountdi2",
+	"popcountsi2",
+	"popcountti2",
+	"powidf2",
+	"powisf2",
+	"powitf2",
+	"subdf3",
+	"subsf3",
+	"subvdi3",
+	"subvsi3",
+	"subvti3",
+	"subtf3",
+	"trampoline_setup",
+	"truncdfhf2",
+	"truncdfsf2",
+	"truncsfhf2",
+	"ucmpdi2",
+	"ucmpti2",
+	"udivdi3",
+	"udivmoddi4",
+	"udivmodsi4",
+	"udivmodti4",
+	"udivsi3",
+	"udivti3",
+	"umoddi3",
+	"umodsi3",
+	"umodti3",
+}
+
+// Get the builtins archive, possibly generating it as needed.
+func loadBuiltins(target string) (path string, err error) {
+	outfile := "librt-" + target + ".a"
+	builtinsDir := filepath.Join(sourceDir(), "lib", "compiler-rt", "lib", "builtins")
+
+	srcs := make([]string, len(genericBuiltins))
+	for i, name := range genericBuiltins {
+		srcs[i] = filepath.Join(builtinsDir, name+".c")
+	}
+
+	if path, err := cacheLoad(outfile, commands["clang"], srcs); path != "" || err != nil {
+		return path, err
+	}
+
+	dir, err := ioutil.TempDir("", "tinygo-builtins")
+	if err != nil {
+		return "", err
+	}
+	defer os.RemoveAll(dir)
+
+	// Compile all builtins.
+	// TODO: use builtins optimized for a given target if available.
+	objs := make([]string, 0, len(genericBuiltins))
+	for _, name := range genericBuiltins {
+		objpath := filepath.Join(dir, name+".o")
+		objs = append(objs, objpath)
+		srcpath := filepath.Join(builtinsDir, name+".c")
+		cmd := exec.Command(commands["clang"], "-c", "-Oz", "-g", "-Werror", "-Wall", "-std=c11", "--target="+target, "-o", objpath, srcpath)
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		cmd.Dir = dir
+		err = cmd.Run()
+		if err != nil {
+			return "", err
+		}
+	}
+
+	// Put all builtins in an archive to link as a static library.
+	arpath := filepath.Join(dir, "librt.a")
+	cmd := exec.Command(commands["ar"], append([]string{"cr", arpath}, objs...)...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.Dir = dir
+	err = cmd.Run()
+	if err != nil {
+		return "", err
+	}
+
+	return cacheStore(arpath, outfile, commands["clang"], srcs)
+}
diff --git a/docs/installation.rst b/docs/installation.rst
index 55e2ca20..455fdc5e 100644
--- a/docs/installation.rst
+++ b/docs/installation.rst
@@ -27,6 +27,8 @@ final binary and flashing it needs some extra tools.
     * binutils (``arm-none-eabi-objcopy``) for producing .hex files for
       flashing.
     * GCC (``arm-none-eabi-gcc``) for linking object files.
+    * Clang 7 (``clang-7``) for building the `compiler runtime library
+      <https://compiler-rt.llvm.org/>`_.
     * The flashing tool for the particular chip, like ``openocd`` or
       ``nrfjprog``.
 
diff --git a/lib/compiler-rt b/lib/compiler-rt
new file mode 160000
index 00000000..a4cbb02b
--- /dev/null
+++ b/lib/compiler-rt
@@ -0,0 +1 @@
+Subproject commit a4cbb02bca3b952117e9ccfbad8a485857f25935
diff --git a/main.go b/main.go
index 1abaef55..529b19b3 100644
--- a/main.go
+++ b/main.go
@@ -17,6 +17,11 @@ import (
 	"github.com/aykevl/tinygo/compiler"
 )
 
+var commands = map[string]string{
+	"ar":    "ar",
+	"clang": "clang-7",
+}
+
 // Helper function for Compiler object.
 func Compile(pkgName, outpath string, spec *TargetSpec, printIR, dumpSSA, debug bool, printSizes string, action func(string) error) error {
 	config := compiler.Config{
@@ -95,10 +100,24 @@ func Compile(pkgName, outpath string, spec *TargetSpec, printIR, dumpSSA, debug
 			return err
 		}
 
+		// Load builtins library from the cache, possibly compiling it on the
+		// fly.
+		var cachePath string
+		if spec.CompilerRT {
+			librt, err := loadBuiltins(spec.Triple)
+			if err != nil {
+				return err
+			}
+			cachePath, _ = filepath.Split(librt)
+		}
+
 		// Link the object file with the system compiler.
 		executable := filepath.Join(dir, "main")
 		tmppath := executable // final file
 		args := append(spec.PreLinkArgs, "-o", executable, objfile)
+		if spec.CompilerRT {
+			args = append(args, "-L", cachePath, "-lrt-"+spec.Triple)
+		}
 		cmd := exec.Command(spec.Linker, args...)
 		cmd.Stdout = os.Stdout
 		cmd.Stderr = os.Stderr
@@ -371,6 +390,14 @@ func main() {
 			fmt.Fprintln(os.Stderr, "error:", err)
 			os.Exit(1)
 		}
+	case "clean":
+		// remove cache directory
+		dir := cacheDir()
+		err := os.RemoveAll(dir)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, "cannot clean cache:", err)
+			os.Exit(1)
+		}
 	case "help":
 		usage()
 	case "run":
diff --git a/target.go b/target.go
index eb4f48b7..87678e50 100644
--- a/target.go
+++ b/target.go
@@ -3,6 +3,7 @@ package main
 import (
 	"encoding/json"
 	"os"
+	"os/user"
 	"path/filepath"
 	"runtime"
 	"strings"
@@ -17,6 +18,7 @@ type TargetSpec struct {
 	Triple      string   `json:"llvm-target"`
 	BuildTags   []string `json:"build-tags"`
 	Linker      string   `json:"linker"`
+	CompilerRT  bool     `json:"compiler-rt"`
 	PreLinkArgs []string `json:"pre-link-args"`
 	Objcopy     string   `json:"objcopy"`
 	Flasher     string   `json:"flash"`
@@ -72,16 +74,19 @@ func getGopath() string {
 	}
 
 	// fallback
-	var home string
-	if runtime.GOOS == "windows" {
-		home = os.Getenv("USERPROFILE")
-	} else {
-		home = os.Getenv("HOME")
-	}
-	if home == "" {
-		// This is very unlikely, so panic here.
-		// Not the nicest solution, however.
-		panic("no $HOME or %USERPROFILE% found")
-	}
+	home := getHomeDir()
 	return filepath.Join(home, "go")
 }
+
+func getHomeDir() string {
+	u, err := user.Current()
+	if err != nil {
+		panic("cannot get current user: " + err.Error())
+	}
+	if u.HomeDir == "" {
+		// This is very unlikely, so panic here.
+		// Not the nicest solution, however.
+		panic("could not find home directory")
+	}
+	return u.HomeDir
+}
diff --git a/targets/bluepill.json b/targets/bluepill.json
index 79c9a48e..ed3079b5 100644
--- a/targets/bluepill.json
+++ b/targets/bluepill.json
@@ -2,6 +2,7 @@
 	"llvm-target": "armv7m-none-eabi",
 	"build-tags": ["bluepill", "stm32f103xx", "stm32", "arm", "js", "wasm"],
 	"linker": "arm-none-eabi-gcc",
+	"compiler-rt": true,
 	"pre-link-args": ["-nostdlib", "-nostartfiles", "-mcpu=cortex-m3", "-mthumb", "-T", "targets/stm32.ld", "-Wl,--gc-sections", "-fno-exceptions", "-fno-unwind-tables", "-ffunction-sections", "-fdata-sections", "-Os", "src/device/stm32/stm32f103xx.s"],
 	"objcopy": "arm-none-eabi-objcopy",
 	"flash": "openocd -f interface/stlink-v2.cfg -f target/stm32f1x.cfg -c 'program {hex} reset exit'"
diff --git a/targets/microbit.json b/targets/microbit.json
index 02f4fc26..03bbe721 100644
--- a/targets/microbit.json
+++ b/targets/microbit.json
@@ -2,6 +2,7 @@
 	"llvm-target": "armv6m-none-eabi",
 	"build-tags": ["microbit", "nrf51822", "nrf51", "nrf", "arm", "js", "wasm"],
 	"linker": "arm-none-eabi-gcc",
+	"compiler-rt": true,
 	"pre-link-args": [
 		"-nostdlib",
 		"-nostartfiles",
@@ -11,6 +12,7 @@
 		"-Wl,--gc-sections",
 		"-fno-exceptions", "-fno-unwind-tables",
 		"-ffunction-sections", "-fdata-sections",
+		"-fno-short-enums",
 		"-Os",
 		"-DNRF51",
 		"-Ilib/CMSIS/CMSIS/Include",
diff --git a/targets/pca10040.json b/targets/pca10040.json
index 0f3ff15c..dd9ba375 100644
--- a/targets/pca10040.json
+++ b/targets/pca10040.json
@@ -2,6 +2,7 @@
 	"llvm-target": "armv7em-none-eabi",
 	"build-tags": ["pca10040", "nrf52832", "nrf52", "nrf", "arm", "js", "wasm"],
 	"linker": "arm-none-eabi-gcc",
+	"compiler-rt": true,
 	"pre-link-args": [
 		"-nostdlib",
 		"-nostartfiles",