generating-match-conditions.md

Generating match conditions and blocks

Scalar constants

Scalar constants should be disallowed as the head of a match expression. Otherwise, we will have to perform calculations to prune dead code branches - and it serves little to no purpose.

Scalar type values

NumberType values

Example

some-num ?
| 0 -> "None"
| 1 -> "One"
| 2 -> "Two"
| 3 -> "Three"
| _ -> "A lot"

Should generate code similar to this:

    ; some-num's value is saved to %3 in the entry block
    switch i64 %3, label %3.default []
b3.clause0:
    br %3.return
b3.clause1:
    br %3.return
b4.clause2:
    br %3.return
b4.clause3:
    br %3.return
b4.default:
    br %3.return
b4.return:
    %4 = phi i8* [ @const.None, %b3.clause0 ], [ @const.One, %b3.clause1 ], [ @const.Two, %b3.clause2 ], [ @const.Three, %b3.clause3], [ @const.Many, %b3.default ]

Without guards, this should be the simplest case. Each clause that has a TCPNumber pattern type should contribute to the building of an LLVM switch instruction, like this.

1. Save the last instruction from the head expression as exprId.
2. Fold over the clauses with an initial state of (exprId, [], irState). It will:
   1. match the pattern type
   2. On TCPNumber:
      1. Extract the number from the pattern
      2. Create a new basic block
      3. Save the block id as blockId
      4. generate the expr list into the new basic block
      5. Save the last expression id as lastId
      6. Return a state of (exprId, (blockId, lastId)::clauseInfo, irState')
   3. On 'TCVar':
      1. Create a new basic block