mips: add GOMIPS=softfloat support

Previously, the compiler would default to hardfloat. This is not
supported by some MIPS CPUs.

This took me much longer than it should have because of a quirk in the
LLVM Mips backend: if the target-features string is not set (like during
LTO), the Mips backend picks the first function in the module and uses
that. Unfortunately, in the case of TinyGo this first function is
`llvm.dbg.value`, which is an LLVM intrinsic and doesn't have the
target-features string. I fixed it by adding a `-mllvm -mattr=` flag to
the linker.

compileopts/config.go

@@ -72,6 +72,12 @@ func (c *Config) GOARM() string {
 	return c.Options.GOARM
 }
 
+// GOMIPS will return the GOMIPS environment variable given to the compiler when
+// building a program.
+func (c *Config) GOMIPS() string {
+	return c.Options.GOMIPS
+}
+
 // BuildTags returns the complete list of build tags used during this build.
 func (c *Config) BuildTags() []string {
 	tags := append([]string(nil), c.Target.BuildTags...) // copy slice (avoid a race)
@@ -240,6 +246,9 @@ func (c *Config) LibcPath(name string) (path string, precompiled bool) {
 	if c.ABI() != "" {
 		archname += "-" + c.ABI()
 	}
+	if c.Target.SoftFloat {
+		archname += "-softfloat"
+	}
 
 	// Try to load a precompiled library.
 	precompiledDir := filepath.Join(goenv.Get("TINYGOROOT"), "pkg", archname, name)

compileopts/options.go

@@ -23,6 +23,7 @@ type Options struct {
 	GOOS            string // environment variable
 	GOARCH          string // environment variable
 	GOARM           string // environment variable (only used with GOARCH=arm)
+	GOMIPS          string // environment variable (only used with GOARCH=mips and GOARCH=mipsle)
 	Directory       string // working dir, leave it unset to use the current working dir
 	Target          string
 	Opt             string

compileopts/target.go

@@ -30,6 +30,7 @@ type TargetSpec struct {
 	Features         string   `json:"features,omitempty"`
 	GOOS             string   `json:"goos,omitempty"`
 	GOARCH           string   `json:"goarch,omitempty"`
+	SoftFloat        bool     // used for non-baremetal systems (GOMIPS=softfloat etc)
 	BuildTags        []string `json:"build-tags,omitempty"`
 	GC               string   `json:"gc,omitempty"`
 	Scheduler        string   `json:"scheduler,omitempty"`
@@ -86,6 +87,10 @@ func (spec *TargetSpec) overrideProperties(child *TargetSpec) error {
 			if src.Uint() != 0 {
 				dst.Set(src)
 			}
+		case reflect.Bool:
+			if src.Bool() {
+				dst.Set(src)
+			}
 		case reflect.Ptr: // for pointers, copy if not nil
 			if !src.IsNil() {
 				dst.Set(src)
@@ -290,13 +295,22 @@ func defaultTarget(options *Options) (*TargetSpec, error) {
 		}
 	case "mips", "mipsle":
 		spec.CPU = "mips32r2"
-		spec.Features = "+fpxx,+mips32r2,+nooddspreg,-noabicalls"
 		spec.CFlags = append(spec.CFlags, "-fno-pic")
 		if options.GOOS == "mips" {
 			llvmarch = "mips" // big endian
 		} else {
 			llvmarch = "mipsel" // little endian
 		}
+		switch options.GOMIPS {
+		case "hardfloat":
+			spec.Features = "+fpxx,+mips32r2,+nooddspreg,-noabicalls"
+		case "softfloat":
+			spec.SoftFloat = true
+			spec.Features = "+mips32r2,+soft-float,-noabicalls"
+			spec.CFlags = append(spec.CFlags, "-msoft-float")
+		default:
+			return nil, fmt.Errorf("invalid GOMIPS=%s: must be hardfloat or softfloat", options.GOMIPS)
+		}
 	case "wasm":
 		llvmarch = "wasm32"
 		spec.CPU = "generic"