all: add compiler support for interrupts

This commit lets the compiler know about interrupts and allows
optimizations to be performed based on that: interrupts are eliminated
when they appear to be unused in a program. This is done with a new
pseudo-call (runtime/interrupt.New) that is treated specially by the
compiler.

compiler/compiler.go

@@ -217,7 +217,7 @@ func (c *Compiler) Compile(mainPath string) []error {
 				path = path[len(tinygoPath+"/src/"):]
 			}
 			switch path {
-			case "machine", "os", "reflect", "runtime", "runtime/volatile", "sync", "testing", "internal/reflectlite":
+			case "machine", "os", "reflect", "runtime", "runtime/interrupt", "runtime/volatile", "sync", "testing", "internal/reflectlite":
 				return path
 			default:
 				if strings.HasPrefix(path, "device/") || strings.HasPrefix(path, "examples/") {
@@ -828,9 +828,6 @@ func (c *Compiler) parseFunc(frame *Frame) {
 		frame.fn.LLVMFn.SetLinkage(llvm.InternalLinkage)
 		frame.fn.LLVMFn.SetUnnamedAddr(true)
 	}
-	if frame.fn.IsInterrupt() && strings.HasPrefix(c.Triple(), "avr") {
-		frame.fn.LLVMFn.SetFunctionCallConv(85) // CallingConv::AVR_SIGNAL
-	}
 
 	// Some functions have a pragma controlling the inlining level.
 	switch frame.fn.Inline() {
@@ -1314,6 +1311,8 @@ func (c *Compiler) parseCall(frame *Frame, instr *ssa.CallCommon) (llvm.Value, e
 			return c.emitVolatileLoad(frame, instr)
 		case strings.HasPrefix(name, "runtime/volatile.Store"):
 			return c.emitVolatileStore(frame, instr)
+		case name == "runtime/interrupt.New":
+			return c.emitInterruptGlobal(frame, instr)
 		}
 
 		targetFunc := c.ir.GetFunction(fn)