all: add support for GOARM

This environment variable can be set to 5, 6, or 7 and controls which
ARM version (ARMv5, ARMv6, ARMv7) is used when compiling for GOARCH=arm.

I have picked the default value ARMv6, which I believe is supported on
most common single board computers including all Raspberry Pis. The
difference in code size is pretty big.

We could even go further and support ARMv4 if anybody is interested. It
should be pretty simple to add this if needed.

compileopts/config.go

@@ -60,6 +60,12 @@ func (c *Config) GOARCH() string {
 	return c.Target.GOARCH
 }
 
+// GOARM will return the GOARM environment variable given to the compiler when
+// building a program.
+func (c *Config) GOARM() string {
+	return c.Options.GOARM
+}
+
 // BuildTags returns the complete list of build tags used during this build.
 func (c *Config) BuildTags() []string {
 	tags := append(c.Target.BuildTags, []string{"tinygo", "math_big_pure_go", "gc." + c.GC(), "scheduler." + c.Scheduler(), "serial." + c.Serial()}...)

compileopts/options.go

@@ -21,6 +21,7 @@ var (
 type Options struct {
 	GOOS            string // environment variable
 	GOARCH          string // environment variable
+	GOARM           string // environment variable (only used with GOARCH=arm)
 	Target          string
 	Opt             string
 	GC              string

compileopts/target.go

@@ -5,6 +5,7 @@ package compileopts
 import (
 	"encoding/json"
 	"errors"
+	"fmt"
 	"io"
 	"os"
 	"os/exec"
@@ -165,13 +166,26 @@ func LoadTarget(options *Options) (*TargetSpec, error) {
 	if options.Target == "" {
 		// Configure based on GOOS/GOARCH environment variables (falling back to
 		// runtime.GOOS/runtime.GOARCH), and generate a LLVM target based on it.
-		llvmarch := map[string]string{
-			"386":   "i386",
-			"amd64": "x86_64",
-			"arm64": "aarch64",
-			"arm":   "armv7",
-		}[options.GOARCH]
-		if llvmarch == "" {
+		var llvmarch string
+		switch options.GOARCH {
+		case "386":
+			llvmarch = "i386"
+		case "amd64":
+			llvmarch = "x86_64"
+		case "arm64":
+			llvmarch = "aarch64"
+		case "arm":
+			switch options.GOARM {
+			case "5":
+				llvmarch = "armv5"
+			case "6":
+				llvmarch = "armv6"
+			case "7":
+				llvmarch = "armv7"
+			default:
+				return nil, fmt.Errorf("invalid GOARM=%s, must be 5, 6, or 7", options.GOARM)
+			}
+		default:
 			llvmarch = options.GOARCH
 		}
 		llvmos := options.GOOS
@@ -245,7 +259,14 @@ func defaultTarget(goos, goarch, triple string) (*TargetSpec, error) {
 		spec.Features = "+cx8,+fxsr,+mmx,+sse,+sse2,+x87"
 	case "arm":
 		spec.CPU = "generic"
-		spec.Features = "+armv7-a,+dsp,+fp64,+vfp2,+vfp2sp,+vfp3d16,+vfp3d16sp,-thumb-mode"
+		switch strings.Split(triple, "-")[0] {
+		case "armv5":
+			spec.Features = "+armv5t,+strict-align,-thumb-mode"
+		case "armv6":
+			spec.Features = "+armv6,+dsp,+fp64,+strict-align,+vfp2,+vfp2sp,-thumb-mode"
+		case "armv7":
+			spec.Features = "+armv7-a,+dsp,+fp64,+vfp2,+vfp2sp,+vfp3d16,+vfp3d16sp,-thumb-mode"
+		}
 	case "arm64":
 		spec.CPU = "generic"
 		spec.Features = "+neon"