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15.4.2 Single-Pass Code Generation

All the code generated by the parser is emitted in a single pass. For example, consider the following SQL logic:

CREATE FUNCTION func_4(i int)
  DECLARE str CHAR(10);

  CASE i
    WHEN 1 THEN SET str="1";
    WHEN 2 THEN SET str="2";
    WHEN 3 THEN SET str="3";
    ELSE SET str="unknown";

  RETURN str;

The compiled program for this Stored Function is:

Pos     Instruction
0       set str@1 NULL
1       set_case_expr (12) 0 i@0
2       jump_if_not 5(12) (case_expr@0 = 1)
3       set str@1 _latin1'1'
4       jump 12
5       jump_if_not 8(12) (case_expr@0 = 2)
6       set str@1 _latin1'2'
7       jump 12
8       jump_if_not 11(12) (case_expr@0 = 3)
9       set str@1 _latin1'3'
10      jump 12
11      set str@1 _latin1'unknown'
12      freturn 254 str@1

Note the instruction at position 4: jump 12. How can the compiler generate this instruction in a single pass, when the destination (12) is not known yet ? This instruction is a forward jump. What happens during code generation is that, by the time the compiler has generated the code for positions [0] to [11], the generated code looks like this:

Pos     Instruction
0       set str@1 NULL
1       set_case_expr ( ?? ) 0 i@0
2       jump_if_not 5( ?? ) (case_expr@0 = 1)
3       set str@1 _latin1'1'
4       jump ??
5       jump_if_not 8( ?? ) (case_expr@0 = 2)
6       set str@1 _latin1'2'
7       jump ??
8       jump_if_not 11( ?? ) (case_expr@0 = 3)
9       set str@1 _latin1'3'
10      jump ??
11      set str@1 _latin1'unknown'

The final destination of the label for the END CASE is not known yet, and the list of all the instructions (1, 2, 4, 5, 7, 8 and 10) that need to point to this unknown destination (represented as ??) is maintained in a temporary structure used during code generation only. This structure is called the context back patch list.

When the destination label is finally resolved to a destination (12), all the instructions pointing to that label, which have been already generated (but with a bogus destination) are back patched to point to the correct location. See the comments marked BACKPATCH in the code for more details.

As a side note, this generated code also shows that some temporary variables can be generated implicitly, such as the operand of the CASE expression, labeled case_expr@0.


Numbering of case expressions in the symbol table uses a different namespace than variables, so that case_expr@0 and i@0 are two different variables, even when both internally numbered with offset zero.

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