ST20-C1 datasheet(180/205 Pages) STMICROELECTRONICS

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C.2

Compiling switch statements

The switch statement is a special form of conditional transfer where the transfer is

determined by comparing an expression to a number of constants.

When compiling the process

switch

...

the expression

x is evaluated and stored in a local variable by

x; stl selector

Then each branch of the switch statement

c1, ..., cn

can be compiled by

ldl selector; ldc c1; sub; cj L;

ldl selector; ldc c2; sub; cj L;

...

ldl selector; eqc cn;cj M;

P; j END;

where the label

END: is placed at the end of the switch statement.

C.2.3 Optimal compilation of switch

The compilation method given above will produce inefficient code for large switch

statements. To produce more efficient code the following rules can be used.

First build up a set of pairs of selector values and processes, consisting of every

selector value in the switch statement along with its associated process — the

process part of each pair can be represented by the offset to the start of the compiled

code for that process. Then the following rules can be used.

If there are 3 entries or less then use the method as described above.

If there are 12 entries or less then use a binary search to limit the number of

comparisons required.

For more than 12 entries attempt to use a jump table. The offset of the start of

each selected process is placed in the table against each selector value.

Entries that do not match a selector in the switch statement must contain the

offset of an error handler process. This jump table should be the largest table

such that about one third of the entries are filled. This compilation strategy is

then recursively called to handle the two ends. The

jab (section 4.10) and ldpi

(section 4.5) instructions, can be used to jump to the selected piece of code.

The choice of 3 or less entries, 12 or less entries and one third filled tab le are the

values used in current ST20 compilers.

ST20-C1 Datasheet(PDF) 180 Page - STMicroelectronics