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28 C and C++ Source-Level Optimizations Chapter 2
25112 Rev. 3.06 September 2005
Software Optimization Guide for AMD64 Processors
2.11 Arranging Cases by Probability of Occurrence
Optimization
Arrange switch statement cases by probability of occurrence, from most probable to least probable.
Application
This optimization applies to:
• 32-bit software
• 64-bit software
Rationale
Arranging switch statement cases by probability of occurrence improves performance when the
switch statement is translated as a comparison chain; this arrangement has no negative impact when
the statement is translated as a jump table.
Example
Avoid switch statements such as the following, in which the cases are not arranged by probability of
occurrence:
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");
}
Instead, arrange the cases to test for frequently occurring values first:
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");
}