Avoiding Branch Operations in Switch Statements

Question

Instead of a switch statement running iterating through branch after branch, is there away to make assembly look up a list in an array for a goto statement? Or is this usually optimized in the compiler?

Such an optimization would help immensely for large switch statements with constant values.

Ex:

switch(test) {
case 1:
// Do something
break;
case 2:
// Do something
break;
}

"Optimized":

action_link[] = {action_1, action_2};
goto action_link[test];

action_1:
// Do Something
action_2:
// Do Something

http://stackoverflow.com/questions/2596320/how-does-switch-compile-and-how-optimized-and-fast-is-it — Steephen, Jun 15 '15 at 01:39

score 1 · Answer 1 · answered Jun 15 '15 at 01:39

The compiler will make this decision for you, based on your optimisation settings and its heuristics about what might make a good jump table.

In many simple situations, the compiler might decide that a naive test-and-jump chain would be faster or smaller than the equivalent jump table.

score 1 · Answer 2 · edited Jun 15 '15 at 02:25

1

That is indeed what you should expect from a decent compiler. In fact, the limitations of the switch statement were based on making it easy to translate to a "jump table" instead of multiple branches. Back at the dawn of time, FORTRAN had the "computed GOTO" for the same reason.

edited Jun 15 '15 at 02:25

Greg Hewgill

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answered Jun 15 '15 at 01:44

Charlie Martin

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Avoiding Branch Operations in Switch Statements

2 Answers2