Top-level constants in the LLVM backend

[Jiří Beneš]

Motivation

The LLVM backend currently does not support top-level let bindings of any kind.
As a first step, it would be nice to at least support top-level bindings of literals that don't use any capabilities (what I'll call "top-level constants" here), like for example:

val myString = "Hello, world!"
// ^^^
// [error] Toplevel def and let bindings not yet supported: Let(myString,Literal(Hello, world!,Data(String,List())))

def main() = {
  println(myString)
}

Existing use in the standard library

We even use top-level constants in the standard library in the string module:

effekt/libraries/common/string.effekt

Lines 206 to 227 in acbec87

 // ANSI escape codes 
 namespace ANSI { 
   val BLACK = "\u001b[30m" 
   val RED = "\u001b[31m" 
   val GREEN = "\u001b[32m" 
   val YELLOW = "\u001b[33m" 
   val BLUE = "\u001b[34m" 
   val MAGENTA = "\u001b[35m" 
   val CYAN = "\u001b[36m" 
   val WHITE = "\u001b[37m" 
  
   val BG_BLACK = "\u001b[40m" 
   val BG_RED = "\u001b[41m" 
   val BG_GREEN = "\u001b[42m" 
   val BG_YELLOW = "\u001b[43m" 
   val BG_BLUE = "\u001b[44m" 
   val BG_MAGENTA = "\u001b[45m" 
   val BG_CYAN = "\u001b[46m" 
   val BG_WHITE = "\u001b[47m" 
  
   val RESET = "\u001b[0m" 
 } 

which are then used by the test module:

effekt/libraries/common/test.effekt

Line 79 in acbec87

 println(ANSI::RED ++ "- (FAIL) " ++ name ++ "\n    " ++ msg ++ ANSI::RESET); 

Unfortunately since the LLVM backend doesn't support these top-level constants, one may use neither ANSI escape codes nor, by transitivity, the test module. :(

Implementation details

Based on my Clang-based testing, I think it should be enough to do the following to declare a top-level Pos constant:

@topLevelPos = constant %struct.Pos { i64 42, ptr null }, align 8

The biggest problem might be that we'd need to add these top-level constants to the Machine representation as that's where the current error is being thrown from:

effekt/effekt/shared/src/main/scala/effekt/machine/Transformer.scala

Lines 39 to 47 in acbec87

   val transformedDefinitions = definitions.foldLeft(mainEntry) { 
     case (rest, lifted.Definition.Def(id, lifted.BlockLit(tparams, params, body))) => 
       Def(Label(transform(id), params.map(transform)), transform(body), rest) 
     case (rest, d) => 
       ErrorReporter.abort(s"Toplevel def and let bindings not yet supported: ${d}") 
   } 
  
   Program(declarations, transformedDefinitions) 
 } 

(One could start by pattern-matching on something like (rest, lifted.Definition.Let(constantName, lit@Literal(_, _)))...)

Technically, we should also be able to instead inline all of these top-level constants wherever they appear, but that's quite ad-hoc.