Confusing error message with useless spill

[eponier]

The compilation of this program

export fn main () -> reg u64 {
  reg u64 res;
  res = 0;
  () = #spill(res);
  return res;
}

fails with error

compilation error:
variable allocation: variable “res.167” (declared at "file.jazz", line 4 (14-17)) may not be initialized

The spill is useless, but should the compiler fail? And the error message is wrong, the variable is not used, but it is initialized.

Using AT_none in the code generated by spill_prog (as hinted by the FIXME in lower_spill.v) fixes the issue. It enables the compiler to remove the useless spills. Is this the correct fix?

[eponier]

Debugging with Benjamin, we discovered that we don't need #spill to get the error.

export fn main () -> reg u64 {
  reg u64 res;
  stack u64 s;
  res = 0;
  s = #MOV(res);
  return res;
}

There are two orthogonal problems:

• variables not removed by dead code elimination (because they appear in an instruction with tag AT_keep) but otherwise not used make varalloc fail
• spill_prog produces such instructions, probably we don't want that but this is orthogonal

[vbgl]

The first problem is duplicate of #680.