0

Section 6.6.3 of the dragon book (Compilers: Principles, Techniques, and Tools 2nd Edition) describes how to translate the flow-of-control statements like if/if-else/while into intermediate code.

For example,

The translation of if (B) S1 consists of B.code followed by S1.code. Within B.code are jumps based on the value of B. If B is true, control flows to the first instruction of S1.code, and if B is false, control flows to the instruction immediately following S1.code.

The labels for the jumps in B.code and S.code are managed using inherited attributes. With a boolean expression B, we associate two labels: B.true, the label to which control flows if B is true, and B.false, the label to which control flows if B is false. With a statement S, we associate an inherited attribute S.next denoting a label for the instruction immediately after the code for S.

I understand the translation. It lets B to generate the goto jumps based on the value of B. Since B itself does not know where to jump. Therefore, we associate it with two inherited attributes B.true and B.false, which are provided by the node S.

I am wondering is this translation unnecessarily complicated? Why do not let S generate the branch instruction based on the value of B. Similary, is the inherited attribute S.next necessary?

hengxin
  • 1,867
  • 2
  • 21
  • 42

1 Answers1

0

I find an answer in Section 7.4.1 of the book "Engineering a Compiler (2nd Edition)":

Traditionally, two representations have been proposed for boolean values: a numerical encoding and a positional encoding. The former assigns specific values to true and false and manipulates them using the target machine’s arithmetic and logical operations. The latter approach encodes the value of the expression as a position in the executable code. It uses comparisons and conditional branches to evaluate the expression; the different control-flow paths represent the result of evaluation. Each approach works well for some examples, but not for others.

The dragon book uses the positional encoding.

hengxin
  • 1,867
  • 2
  • 21
  • 42