targets: let specific targets inherit more general targets

This avoids a ton of duplication and makes it easier to change a generic
target (for example, the "cortex-m" target) for all boards that use it.

Also, by making it possible to inherit properties from a parent target
specification, it is easier to support out-of-tree boards that don't
have to be updated so often. A target specification for a
special-purpose board can simply inherit the specification of a
supported chip and override the properites it needs to override (like
the programming interface).

target.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"encoding/json"
+	"io"
 	"os"
 	"os/user"
 	"path/filepath"
@@ -17,6 +18,7 @@ import (
 // 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 {
+	Inherits    []string `json:"inherits"`
 	Triple      string   `json:"llvm-target"`
 	BuildTags   []string `json:"build-tags"`
 	Linker      string   `json:"linker"`
@@ -30,13 +32,117 @@ type TargetSpec struct {
 	GDBCmds     []string `json:"gdb-initial-cmds"`
 }
 
-// Load a target specification
+// copyProperties copies all properties that are set in spec2 into itself.
+func (spec *TargetSpec) copyProperties(spec2 *TargetSpec) {
+	// TODO: simplify this using reflection? Inherits and BuildTags are special
+	// cases, but the rest can simply be copied if set.
+	spec.Inherits = append(spec.Inherits, spec2.Inherits...)
+	if spec2.Triple != "" {
+		spec.Triple = spec2.Triple
+	}
+	spec.BuildTags = append(spec.BuildTags, spec2.BuildTags...)
+	if spec2.Linker != "" {
+		spec.Linker = spec2.Linker
+	}
+	if spec2.RTLib != "" {
+		spec.RTLib = spec2.RTLib
+	}
+	if len(spec2.PreLinkArgs) != 0 {
+		spec.PreLinkArgs = spec2.PreLinkArgs
+	}
+	if spec2.Objcopy != "" {
+		spec.Objcopy = spec2.Objcopy
+	}
+	if len(spec2.Emulator) != 0 {
+		spec.Emulator = spec2.Emulator
+	}
+	if spec2.Flasher != "" {
+		spec.Flasher = spec2.Flasher
+	}
+	if len(spec2.OCDDaemon) != 0 {
+		spec.OCDDaemon = spec2.OCDDaemon
+	}
+	if spec2.GDB != "" {
+		spec.GDB = spec2.GDB
+	}
+	if len(spec2.GDBCmds) != 0 {
+		spec.GDBCmds = spec2.GDBCmds
+	}
+}
+
+// load reads a target specification from the JSON in the given io.Reader. It
+// may load more targets specified using the "inherits" property.
+func (spec *TargetSpec) load(r io.Reader) error {
+	err := json.NewDecoder(r).Decode(spec)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// loadFromName loads the given target from the targets/ directory inside the
+// compiler sources.
+func (spec *TargetSpec) loadFromName(name string) error {
+	path := filepath.Join(sourceDir(), "targets", strings.ToLower(name)+".json")
+	fp, err := os.Open(path)
+	if err != nil {
+		return err
+	}
+	defer fp.Close()
+	return spec.load(fp)
+}
+
+// resolveInherits loads inherited targets, recursively.
+func (spec *TargetSpec) resolveInherits() error {
+	// First create a new spec with all the inherited properties.
+	newSpec := &TargetSpec{}
+	for _, name := range spec.Inherits {
+		subtarget := &TargetSpec{}
+		err := subtarget.loadFromName(name)
+		if err != nil {
+			return err
+		}
+		err = subtarget.resolveInherits()
+		if err != nil {
+			return err
+		}
+		newSpec.copyProperties(subtarget)
+	}
+
+	// When all properties are loaded, make sure they are properly inherited.
+	newSpec.copyProperties(spec)
+	*spec = *newSpec
+
+	return nil
+}
+
+// Load a target specification.
 func LoadTarget(target string) (*TargetSpec, error) {
 	if target == "" {
 		target = llvm.DefaultTargetTriple()
 	}
 
-	spec := &TargetSpec{
+	// See whether there is a target specification for this target (e.g.
+	// Arduino).
+	spec := &TargetSpec{}
+	err := spec.loadFromName(target)
+	if err == nil {
+		// Successfully loaded this target from a built-in .json file. Make sure
+		// it includes all parents as specified in the "inherits" key.
+		err = spec.resolveInherits()
+		if err != nil {
+			return nil, err
+		}
+		return spec, nil
+	} else if !os.IsNotExist(err) {
+		// Expected a 'file not found' error, got something else. Report it as
+		// an error.
+		return nil, err
+	} else {
+		// No target spec available. Use the default one, useful on most systems
+		// with a regular OS.
+		*spec = TargetSpec{
 			Triple:      target,
 			BuildTags:   []string{runtime.GOOS, runtime.GOARCH},
 			Linker:      "cc",
@@ -45,26 +151,9 @@ func LoadTarget(target string) (*TargetSpec, error) {
 			GDB:         "gdb",
 			GDBCmds:     []string{"run"},
 		}
-
-	// See whether there is a target specification for this target (e.g.
-	// Arduino).
-	path := filepath.Join(sourceDir(), "targets", strings.ToLower(target)+".json")
-	if fp, err := os.Open(path); err == nil {
-		defer fp.Close()
-		*spec = TargetSpec{} // reset all fields
-		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. Use the default one.
-	}
-
 		return spec, nil
 	}
+}
 
 // Return the source directory of this package, or "." when it cannot be
 // recovered.

targets/bluepill.json

@@ -1,9 +1,18 @@
 {
+	"inherits": ["cortex-m"],
 	"llvm-target": "armv7m-none-eabi",
-	"build-tags": ["bluepill", "stm32f103xx", "stm32", "tinygo.arm", "js", "wasm"],
-	"linker": "arm-none-eabi-gcc",
-	"rtlib": "compiler-rt",
-	"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",
+	"build-tags": ["bluepill", "stm32f103xx", "stm32"],
+	"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"
+	],
 	"flash": "openocd -f interface/stlink-v2.cfg -f target/stm32f1x.cfg -c 'program {hex} reset exit'"
 }

targets/cortex-m.json

@@ -0,0 +1,7 @@
+{
+	"build-tags": ["tinygo.arm", "js", "wasm"],
+	"linker": "arm-none-eabi-gcc",
+	"rtlib": "compiler-rt",
+	"objcopy": "arm-none-eabi-objcopy",
+	"gdb": "arm-none-eabi-gdb"
+}

targets/microbit.json

@@ -1,26 +1,7 @@
 {
-	"llvm-target": "armv6m-none-eabi",
-	"build-tags": ["microbit", "nrf51822", "nrf51", "nrf", "tinygo.arm", "js", "wasm"],
-	"linker": "arm-none-eabi-gcc",
-	"rtlib": "compiler-rt",
-	"pre-link-args": [
-		"-nostdlib",
-		"-nostartfiles",
-		"-mcpu=cortex-m0",
-		"-mthumb",
-		"-T", "targets/nrf51.ld",
-		"-Wl,--gc-sections",
-		"-fno-exceptions", "-fno-unwind-tables",
-		"-ffunction-sections", "-fdata-sections",
-		"-Os",
-		"-DNRF51",
-		"-Ilib/CMSIS/CMSIS/Include",
-		"lib/nrfx/mdk/system_nrf51.c",
-		"src/device/nrf/nrf51.s"
-	],
-	"objcopy": "arm-none-eabi-objcopy",
+	"inherits": ["nrf51"],
+	"build-tags": ["microbit"],
 	"flash": "openocd -f interface/cmsis-dap.cfg -f target/nrf51.cfg -c 'program {hex} reset exit'",
 	"ocd-daemon": ["openocd", "-f", "interface/cmsis-dap.cfg", "-f", "target/nrf51.cfg"],
-	"gdb": "arm-none-eabi-gdb",
 	"gdb-initial-cmds": ["target remote :3333", "monitor halt", "load", "monitor reset", "c"]
 }

targets/nrf51.json

@@ -0,0 +1,20 @@
+{
+	"inherits": ["cortex-m"],
+	"llvm-target": "armv6m-none-eabi",
+	"build-tags": ["nrf51822", "nrf51", "nrf"],
+	"pre-link-args": [
+		"-nostdlib",
+		"-nostartfiles",
+		"-mcpu=cortex-m0",
+		"-mthumb",
+		"-T", "targets/nrf51.ld",
+		"-Wl,--gc-sections",
+		"-fno-exceptions", "-fno-unwind-tables",
+		"-ffunction-sections", "-fdata-sections",
+		"-Os",
+		"-DNRF51",
+		"-Ilib/CMSIS/CMSIS/Include",
+		"lib/nrfx/mdk/system_nrf51.c",
+		"src/device/nrf/nrf51.s"
+	]
+}

targets/nrf52.json

@@ -0,0 +1,20 @@
+{
+	"inherits": ["cortex-m"],
+	"llvm-target": "armv7em-none-eabi",
+	"build-tags": ["nrf52", "nrf"],
+	"pre-link-args": [
+		"-nostdlib",
+		"-nostartfiles",
+		"-mcpu=cortex-m4",
+		"-mthumb",
+		"-T", "targets/nrf52.ld",
+		"-Wl,--gc-sections",
+		"-fno-exceptions", "-fno-unwind-tables",
+		"-ffunction-sections", "-fdata-sections",
+		"-Os",
+		"-DNRF52832_XXAA",
+		"-Ilib/CMSIS/CMSIS/Include",
+		"lib/nrfx/mdk/system_nrf52.c",
+		"src/device/nrf/nrf52.s"
+	]
+}

targets/nrf52840-mdk.json

@@ -1,25 +1,7 @@
 {
-	"llvm-target": "armv7em-none-eabi",
-	"build-tags": ["nrf52840_mdk", "nrf52840", "nrf", "tinygo.arm", "js", "wasm"],
-	"linker": "arm-none-eabi-gcc",
-	"pre-link-args": [
-		"-nostdlib",
-		"-nostartfiles",
-		"-mcpu=cortex-m4",
-		"-mthumb",
-		"-T", "targets/nrf52840.ld",
-		"-Wl,--gc-sections",
-		"-fno-exceptions", "-fno-unwind-tables",
-		"-ffunction-sections", "-fdata-sections",
-		"-Os",
-		"-DNRF52840_XXAA",
-		"-Ilib/CMSIS/CMSIS/Include",
-		"lib/nrfx/mdk/system_nrf52840.c",
-		"src/device/nrf/nrf52840.s"
-	],
-	"objcopy": "arm-none-eabi-objcopy",
+	"inherits": ["nrf52840"],
+	"build-tags": ["nrf52840_mdk"],
 	"flash": "openocd -f interface/cmsis-dap.cfg -f target/nrf51.cfg -c 'program {hex} reset exit'",
 	"ocd-daemon": ["openocd", "-f", "interface/cmsis-dap.cfg", "-f", "target/nrf51.cfg"],
-	"gdb": "arm-none-eabi-gdb",
 	"gdb-initial-cmds": ["target remote :3333", "monitor halt", "load", "monitor reset", "c"]
 }

targets/nrf52840.json

@@ -0,0 +1,20 @@
+{
+	"inherits": ["cortex-m"],
+	"llvm-target": "armv7em-none-eabi",
+	"build-tags": ["nrf52840", "nrf"],
+	"pre-link-args": [
+		"-nostdlib",
+		"-nostartfiles",
+		"-mcpu=cortex-m4",
+		"-mthumb",
+		"-T", "targets/nrf52840.ld",
+		"-Wl,--gc-sections",
+		"-fno-exceptions", "-fno-unwind-tables",
+		"-ffunction-sections", "-fdata-sections",
+		"-Os",
+		"-DNRF52840_XXAA",
+		"-Ilib/CMSIS/CMSIS/Include",
+		"lib/nrfx/mdk/system_nrf52840.c",
+		"src/device/nrf/nrf52840.s"
+	]
+}

targets/pca10040.json

@@ -1,26 +1,7 @@
 {
-	"llvm-target": "armv7em-none-eabi",
-	"build-tags": ["pca10040", "nrf52832", "nrf52", "nrf", "tinygo.arm", "js", "wasm"],
-	"linker": "arm-none-eabi-gcc",
-	"rtlib": "compiler-rt",
-	"pre-link-args": [
-		"-nostdlib",
-		"-nostartfiles",
-		"-mcpu=cortex-m4",
-		"-mthumb",
-		"-T", "targets/nrf52.ld",
-		"-Wl,--gc-sections",
-		"-fno-exceptions", "-fno-unwind-tables",
-		"-ffunction-sections", "-fdata-sections",
-		"-Os",
-		"-DNRF52832_XXAA",
-		"-Ilib/CMSIS/CMSIS/Include",
-		"lib/nrfx/mdk/system_nrf52.c",
-		"src/device/nrf/nrf52.s"
-	],
-	"objcopy": "arm-none-eabi-objcopy",
+	"inherits": ["nrf52"],
+	"build-tags": ["pca10040"],
 	"flash": "nrfjprog -f nrf52 --sectorerase --program {hex} --reset",
 	"ocd-daemon": ["openocd", "-f", "interface/jlink.cfg", "-c", "transport select swd", "-f", "target/nrf51.cfg"],
-	"gdb": "arm-none-eabi-gdb",
 	"gdb-initial-cmds": ["target remote :3333", "monitor halt", "load", "monitor reset", "c"]
 }

targets/qemu.json

@@ -1,8 +1,7 @@
 {
+	"inherits": ["cortex-m"],
 	"llvm-target": "armv7m-none-eabi",
-	"build-tags": ["qemu", "lm3s6965", "tinygo.arm", "js", "wasm"],
-	"linker": "arm-none-eabi-gcc",
-	"rtlib": "compiler-rt",
+	"build-tags": ["qemu", "lm3s6965"],
 	"pre-link-args": [
 		"-nostdlib",
 		"-nostartfiles",
@@ -15,6 +14,5 @@
 		"-Os",
 		"targets/cortex-m.s"
 	],
-	"objcopy": "arm-none-eabi-objcopy",
 	"emulator": ["qemu-system-arm", "-machine", "lm3s6965evb", "-semihosting", "-nographic", "-kernel"]
 }