Справочник Пользователя для AMD 250

25112

Rev. 3.06

September 2005

Use 

if-else

 statements in place of 

switch

 statements that have noncontiguous case expressions. 

(Case expressions are the individual expressions to which the single 

switch

 expression is compared.)

This optimization applies to:

•

32-bit software

•

64-bit software

If the case expressions are contiguous or nearly contiguous integer values, most compilers translate 
the 

switch

 statement as a jump table instead of a comparison chain. Jump tables generally improve 

performance because:

•

They reduce the number of branches to a single procedure call.

•

The size of the control-flow code is the same no matter how many cases there are.

•

The amount of control-flow code that the processor must execute is the same for all values of the 

switch

 expression.

However, if the case expressions are noncontiguous values, most compilers translate the 

switch

 

statement as a comparison chain. Comparison chains are undesirable because:

•

They use dense sequences of conditional branches, which interfere with the processor’s ability to 
successfully perform branch prediction.

•

The size of the control-flow code increases with the number of cases.

•

The amount of control-flow code that the processor must execute varies with the value of the 

switch

 expression.