AMD x86 ユーザーズマニュアル

AMD

iv

Contents

AMD Athlon™ Processor x86 Code Optimization

22007E/0—November 1999

Switch Statement Usage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Optimize Switch Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Use Prototypes for All Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Use Const Type Qualifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Generic Loop Hoisting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Generalization for Multiple Constant Control Code. . . . . . . . 23

Declare Local Functions as Static . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Dynamic Memory Allocation Consideration . . . . . . . . . . . . . . . . . . . 25

Introduce Explicit Parallelism into Code . . . . . . . . . . . . . . . . . . . . . . 25

Explicitly Extract Common Subexpressions . . . . . . . . . . . . . . . . . . . 26

C Language Structure Component Considerations . . . . . . . . . . . . . . 27

Sort Local Variables According to Base Type Size . . . . . . . . . . . . . . 28

Accelerating Floating-Point Divides and Square Roots . . . . . . . . . . 29

Avoid Unnecessary Integer Division. . . . . . . . . . . . . . . . . . . . . . . . . . 31

Copy Frequently De-referenced Pointer Arguments to
Local Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Instruction Decoding Optimizations

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Select DirectPath Over VectorPath Instructions. . . . . . . . . . . . . . . . 34

Load-Execute Instruction Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Use Load-Execute Integer Instructions . . . . . . . . . . . . . . . . . . 34

Use Load-Execute Floating-Point Instructions with
Floating-Point Operands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Avoid Load-Execute Floating-Point Instructions with
Integer Operands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Align Branch Targets in Program Hot Spots . . . . . . . . . . . . . . . . . . . 36

Use Short Instruction Lengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Avoid Partial Register Reads and Writes. . . . . . . . . . . . . . . . . . . . . . 37

Replace Certain SHLD Instructions with Alternative Code. . . . . . . 38

Use 8-Bit Sign-Extended Immediates . . . . . . . . . . . . . . . . . . . . . . . . . 38