AMD x86 Switch Statement Usage, Use Prototypes for All Functions, Optimize Switch Statements

22007E/0 — November 1999 AMD Athlon™ Processor x86 Code Optimization

Switch Statement Usage

Optimize Switch Statements

Switch statements are translated using a variety of algorithms. The most common of these are jump tables and comparison chains/trees. It is recommended to sort the cases of a switch statement according to the probability of occurrences, with the most probable first. This will improve performance when the switch is translated as a comparison chain. It is further recommended to make the case labels small, contiguous integers, as this will allow the switch to be translated as a jump table.

Example 1 (Avoid):

int days_in_month, short_months, normal_months, long_months;

switch	(days_in_month) {
case	28:
case	29: short_months++; break;
case	30: normal_months++; break;
case	31: long_months++; break;

default: printf ("month has fewer than 28 or more than 31 days\n");

}

Example 2 (Preferred):

int days_in_month, short_months, normal_months, long_months;

switch	(days_in_month) {
case	31: long_months++; break;
case	30: normal_months++; break;
case	28:
case	29: short_months++; break;

default: printf ("month has fewer than 28 or more than 31 days\n");

}

Use Prototypes for All Functions

In general, use prototypes for all functions. Prototypes can convey additional information to the compiler that might enable more aggressive optimizations.

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