User's Manual

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

Replace Divides with Multiplies 77

Integer Optimizations

This chapter describes ways to improve integer performance

through optimized programming techniques. The guidelines are

listed in order of importance.

Replace Divides with Multiplies

Replace integer division by constants with multiplication by

the reciprocal. Because the AMD Athlon™ processor has a very

fast integer multiply (5–9 cycles signed, 4–8 cycles unsigned)

and the integer division delivers only one bit of quotient per

cycle (22–47 cycles signed, 17–41 cycles unsigned), the

equivalent code is much faster. The user can follow the

examples in this chapter that illustrate the use of integer

division by constants, or access the executables in the

opt_utilities directory in the AMD documentation CD-ROM

(order# 21860) to find alternative code for dividing by a

constant.

Multiplication by Reciprocal (Division) Utility

The code for the utilities can be found at “Derivation of

Multiplier Used for Integer Division by Constants” on page 93.

All utilities were compiled for the Microsoft Windows

95,

Windows 98, and Windows NT

environments. All utilities are

provided ‘as is’ and are not supported by AMD.