targets: add CPU property everywhere

This is for consistency with Clang, which always adds a CPU flag even if
it's not specified in CFLAGS.

This commit also adds some tests to make sure the Clang target-cpu
matches the CPU property in the JSON files.

This does have an effect on the generated binaries. The effect is very
small though: on average just 0.2% increase in binary size, apparently
because Cortex-M3 and Cortex-M4 are compiled a bit differently. However,
when rebased on top of https://github.com/tinygo-org/tinygo/pull/2218
(minsize), the difference drops to -0.1% (a slight decrease on average).

compileopts/config.go

@@ -220,6 +220,19 @@ func (c *Config) CFlags() []string {
 	cflags = append(cflags, "-O"+c.Options.Opt)
 	// Set the LLVM target triple.
 	cflags = append(cflags, "--target="+c.Triple())
+	// Set the -mcpu (or similar) flag.
+	if c.Target.CPU != "" {
+		if c.GOARCH() == "amd64" || c.GOARCH() == "386" {
+			// x86 prefers the -march flag (-mcpu is deprecated there).
+			cflags = append(cflags, "-march="+c.Target.CPU)
+		} else if strings.HasPrefix(c.Triple(), "avr") {
+			// AVR MCUs use -mmcu instead of -mcpu.
+			cflags = append(cflags, "-mmcu="+c.Target.CPU)
+		} else {
+			// The rest just uses -mcpu.
+			cflags = append(cflags, "-mcpu="+c.Target.CPU)
+		}
+	}
 	return cflags
 }
 

compileopts/target.go

@@ -221,8 +221,15 @@ func defaultTarget(goos, goarch, triple string) (*TargetSpec, error) {
 		GDB:              []string{"gdb"},
 		PortReset:        "false",
 	}
-	if goarch == "386" {
+	switch goarch {
+	case "386":
 		spec.CPU = "pentium4"
+	case "amd64":
+		spec.CPU = "x86-64"
+	case "arm":
+		spec.CPU = "generic"
+	case "arm64":
+		spec.CPU = "generic"
 	}
 	if goos == "darwin" {
 		spec.CFlags = append(spec.CFlags, "-isysroot", "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk")