Reduce code size for AVR (#116)

* all: add support for specifying target CPU in target config
* avr: specify the chip name in the target CPU

This reduces code size by a large margin. For examples/blinky, it
reduces code size from 1360 to 1266 when compiling for the Arduino Uno
(94 bytes, or ~7%).

compiler/compiler.go

@@ -31,6 +31,7 @@ func init() {
 // Configure the compiler.
 type Config struct {
 	Triple     string   // LLVM target triple, e.g. x86_64-unknown-linux-gnu (empty string means default)
+	CPU        string   // LLVM CPU name, e.g. atmega328p (empty string means default)
 	GC         string   // garbage collection strategy
 	CFlags     []string // cflags to pass to cgo
 	LDFlags    []string // ldflags to pass to cgo
@@ -108,7 +109,7 @@ func NewCompiler(pkgName string, config Config) (*Compiler, error) {
 	if err != nil {
 		return nil, err
 	}
-	c.machine = target.CreateTargetMachine(config.Triple, "", "", llvm.CodeGenLevelDefault, llvm.RelocStatic, llvm.CodeModelDefault)
+	c.machine = target.CreateTargetMachine(config.Triple, config.CPU, "", llvm.CodeGenLevelDefault, llvm.RelocStatic, llvm.CodeModelDefault)
 	c.targetData = c.machine.CreateTargetData()
 
 	c.ctx = llvm.NewContext()

main.go

@@ -51,6 +51,7 @@ func Compile(pkgName, outpath string, spec *TargetSpec, config *BuildConfig, act
 
 	compilerConfig := compiler.Config{
 		Triple:     spec.Triple,
+		CPU:        spec.CPU,
 		GC:         config.gc,
 		CFlags:     spec.CFlags,
 		LDFlags:    spec.LDFlags,

target.go

@@ -20,6 +20,7 @@ import (
 type TargetSpec struct {
 	Inherits   []string `json:"inherits"`
 	Triple     string   `json:"llvm-target"`
+	CPU        string   `json:"cpu"`
 	BuildTags  []string `json:"build-tags"`
 	GC         string   `json:"gc"`
 	Compiler   string   `json:"compiler"`
@@ -44,6 +45,9 @@ func (spec *TargetSpec) copyProperties(spec2 *TargetSpec) {
 	if spec2.Triple != "" {
 		spec.Triple = spec2.Triple
 	}
+	if spec2.CPU != "" {
+		spec.CPU = spec2.CPU
+	}
 	spec.BuildTags = append(spec.BuildTags, spec2.BuildTags...)
 	if spec2.GC != "" {
 		spec.GC = spec2.GC

targets/arduino.json

@@ -1,6 +1,7 @@
 {
 	"inherits": ["avr"],
 	"llvm-target": "avr-atmel-none",
+	"cpu": "atmega328p",
 	"build-tags": ["arduino", "atmega328p", "atmega", "avr5"],
 	"cflags": [
 		"-mmcu=atmega328p"

targets/digispark.json

@@ -1,6 +1,7 @@
 {
 	"inherits": ["avr"],
 	"llvm-target": "avr-atmel-none",
+	"cpu": "attiny85",
 	"build-tags": ["digispark", "attiny85", "attiny", "avr2", "avr25"],
 	"cflags": [
 		"-mmcu=attiny85"