compiler: move settings to a separate Config struct

Moving settings to a separate config struct has two benefits:
  - It decouples the compiler a bit from other packages, most
    importantly the compileopts package. Decoupling is generally a good
    thing.
  - Perhaps more importantly, it precisely specifies which settings are
    used while compiling and affect the resulting LLVM module. This will
    be necessary for caching the LLVM module.
    While it would have been possible to cache without this refactor, it
    would have been very easy to miss a setting and thus let the
    compiler work with invalid/stale data.

compiler/goroutine.go

@@ -21,10 +21,10 @@ import (
 func (b *builder) createGoInstruction(funcPtr llvm.Value, params []llvm.Value, prefix string, pos token.Pos) llvm.Value {
 	paramBundle := b.emitPointerPack(params)
 	var callee, stackSize llvm.Value
-	switch b.Scheduler() {
+	switch b.Scheduler {
 	case "none", "tasks":
 		callee = b.createGoroutineStartWrapper(funcPtr, prefix, pos)
-		if b.AutomaticStackSize() {
+		if b.AutomaticStackSize {
 			// The stack size is not known until after linking. Call a dummy
 			// function that will be replaced with a load from a special ELF
 			// section that contains the stack size (and is modified after
@@ -34,7 +34,7 @@ func (b *builder) createGoInstruction(funcPtr llvm.Value, params []llvm.Value, p
 		} else {
 			// The stack size is fixed at compile time. By emitting it here as a
 			// constant, it can be optimized.
-			stackSize = llvm.ConstInt(b.uintptrType, b.Target.DefaultStackSize, false)
+			stackSize = llvm.ConstInt(b.uintptrType, b.DefaultStackSize, false)
 		}
 	case "coroutines":
 		callee = b.CreatePtrToInt(funcPtr, b.uintptrType, "")
@@ -90,7 +90,7 @@ func (c *compilerContext) createGoroutineStartWrapper(fn llvm.Value, prefix stri
 		entry := c.ctx.AddBasicBlock(wrapper, "entry")
 		builder.SetInsertPointAtEnd(entry)
 
-		if c.Debug() {
+		if c.Debug {
 			pos := c.program.Fset.Position(pos)
 			diFuncType := c.dibuilder.CreateSubroutineType(llvm.DISubroutineType{
 				File:       c.getDIFile(pos.Filename),
@@ -147,7 +147,7 @@ func (c *compilerContext) createGoroutineStartWrapper(fn llvm.Value, prefix stri
 		entry := c.ctx.AddBasicBlock(wrapper, "entry")
 		builder.SetInsertPointAtEnd(entry)
 
-		if c.Debug() {
+		if c.Debug {
 			pos := c.program.Fset.Position(pos)
 			diFuncType := c.dibuilder.CreateSubroutineType(llvm.DISubroutineType{
 				File:       c.getDIFile(pos.Filename),