Explicitly mark global variable definition as having 'undefined' value

[heksterb]

With the code as is, global variable definitions have no initializer, producing invalid IR. For example, for the following input program to tinylang:

MODULE Point;

TYPE Point = RECORD X, Y: INTEGER END;

VAR p: Point;

PROCEDURE AssignX(a: INTEGER);
BEGIN
  p.X := a;
END AssignX;

END Point.

invoking build/tools/driver/tinylang --emit-llvm Point.mod produces this IR for the global variable definition:

@_t5Point1p = private global %Point

Attempting to process this produces an error:

$ llc Point.ll
llc: error: llc: Point.ll:10:1: error: expected value token
define void @_t5Point7AssignX(i64 %a) {
^

The simplest way to resolve this is to provide the variable with an 'undefined' initializer; now the IR reads

@_t5Point1p = private global %Point undef

which can be processed.

[OrchidRock]

How about use zeroinitializer instead of undef? I had pulled a PR #20 for this bug.

[heksterb]

Hi @OrchidRock ; thanks for looking at this. I guess the answer to your question depends on what the semantics of global variables in 'tinylang' are, which are probably similar to those of Modula-2, which itself are probably similar to those of Pascal. I couldn't easily find any information about variable initialization in Modula-2, but in Pascal variables appear to be uninitialized (see for example freepascal.org).

Ultimately it doesn't matter so much for a toy language. and I know this isn't rock-solid evidence, but I think "uninitialized" looks like the correct answer.