compiler: fix constant globals on AVR

Turn const globals into non-const globals, at least until pointers have
the correct address space attribute.

compiler.go

@@ -3129,6 +3129,17 @@ func (c *Compiler) ApplyFunctionSections() {
 	}
 }
 
+// Turn all global constants into global variables. This works around a
+// limitation on Harvard architectures (e.g. AVR), where constant and
+// non-constant pointers point to a different address space.
+func (c *Compiler) NonConstGlobals() {
+	global := c.mod.FirstGlobal()
+	for !global.IsNil() {
+		global.SetGlobalConstant(false)
+		global = llvm.NextGlobal(global)
+	}
+}
+
 func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) {
 	builder := llvm.NewPassManagerBuilder()
 	defer builder.Dispose()

main.go

@@ -49,6 +49,18 @@ func Compile(pkgName, outpath string, spec *TargetSpec, printIR, dumpSSA bool, a
 		return err
 	}
 
+	// On the AVR, pointers can point either to flash or to RAM, but we don't
+	// know. As a temporary fix, load all global variables in RAM.
+	// In the future, there should be a compiler pass that determines which
+	// pointers are flash and which are in RAM so that pointers can have a
+	// correct address space parameter (address space 1 is for flash).
+	if strings.HasPrefix(spec.Triple, "avr") {
+		c.NonConstGlobals()
+		if err := c.Verify(); err != nil {
+			return err
+		}
+	}
+
 	// Generate output.
 	if strings.HasSuffix(outpath, ".o") {
 		return c.EmitObject(outpath)