all: make targets configurable with a JSON file

This is intentionally similar to the target specifications in Rust:
https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/spec/struct.TargetOptions.html

main.go

@@ -12,21 +12,9 @@ import (
 
 // Helper function for Compiler object.
 func Compile(pkgName, runtimePath, outpath, target string, printIR, dumpSSA bool) error {
-	var buildTags []string
+	spec, err := LoadTarget(target)
-	// TODO: put this somewhere else
-	if target == "pca10040" {
-		// Pretend to be a WASM target, not ARM (for standard library support).
-		buildTags = append(buildTags, "nrf", "nrf52", "nrf52832", "js", "wasm")
-		target = "armv7m-none-eabi"
-	} else if target == "arduino" {
-		// Pretend to be a WASM target, not AVR (for standard library support).
-		buildTags = append(buildTags, "avr", "avr8", "atmega", "atmega328p", "js", "wasm")
-		target = "avr--"
-	} else {
-		buildTags = append(buildTags, runtime.GOOS, runtime.GOARCH)
-	}
 
-	c, err := NewCompiler(pkgName, target, dumpSSA)
+	c, err := NewCompiler(pkgName, spec.Triple, dumpSSA)
 	if err != nil {
 		return err
 	}
@@ -52,7 +40,7 @@ func Compile(pkgName, runtimePath, outpath, target string, printIR, dumpSSA bool
 				fmt.Println(c.IR())
 			}()
 		}
-		return c.Parse(pkgName, buildTags)
+		return c.Parse(pkgName, spec.BuildTags)
 	}()
 	if parseErr != nil {
 		return parseErr

target.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+)
+
+// Target specification for a given target. Used for bare metal targets.
+//
+// The target specification is mostly inspired by Rust:
+// https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/spec/struct.TargetOptions.html
+// https://github.com/shepmaster/rust-arduino-blink-led-no-core-with-cargo/blob/master/blink/arduino.json
+type TargetSpec struct {
+	Triple    string   `json:"llvm-target"`
+	BuildTags []string `json:"build-tags"`
+}
+
+// Load a target specification
+func LoadTarget(target string) (*TargetSpec, error) {
+	spec := &TargetSpec{
+		Triple:    target,
+		BuildTags: []string{runtime.GOOS, runtime.GOARCH},
+	}
+
+	// See whether there is a target specification for this target (e.g.
+	// Arduino).
+	path := filepath.Join("targets", strings.ToLower(target)+".json")
+	if fp, err := os.Open(path); err == nil {
+		defer fp.Close()
+		err := json.NewDecoder(fp).Decode(spec)
+		if err != nil {
+			return nil, err
+		}
+	} else if !os.IsNotExist(err) {
+		// Expected a 'file not found' error, got something else.
+		return nil, err
+	} else {
+		// No target spec available. This is fine.
+	}
+
+	return spec, nil
+}

targets/arduino.json

@@ -0,0 +1,4 @@
+{
+	"llvm-target": "avr-atmel-none",
+	"build-tags": ["avr", "avr8", "atmega", "atmega328p", "js", "wasm"]
+}

targets/pca10040.json

@@ -0,0 +1,4 @@
+{
+	"llvm-target": "armv7m-none-eabi",
+	"build-tags": ["nrf", "nrf52", "nrf52832", "js", "wasm"]
+}