Software Optimization Guide for AMD Athlon™ 64 and AMD Opteron™ Processors Publication # 25112 Revision: 3.03 Issue Date: September 2003 © 2001 – 2003 Advanced Micro Devices, Inc. All rights reserved. The contents of this document are provided in connection with Advanced Micro Devices, Inc. (“AMD”) products. AMD makes no representations or warranties with respect to the accuracy or completeness of the contents of this publication and reserves the right to make changes to specifications and product descriptions at any time without notice. No license, whether express, implied, arising by estoppel or otherwise, to any intellectual property rights is granted by this publication. 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Other product names used in this publication are for identification purposes only and may be trademarks of their respective companies. 25112 Rev. 3.03 September 2003 Software Optimization Guide for AMD Athlon™ 64 and AMD Opteron™ Processors Contents Revision History . .xv Chapter 1 Introduction . .1 1.1 Intended Audience . .1 1.2 Getting Started Quickly . .1 1.3 Using This Guide . .2 1.4 Important New Terms . .4 1.5 Key Optimizations . .6 Chapter 2 C and C++ Source-Level Optimizations . .7 2.1 Declarations of Floating-Point Values . .9 2.2 Using Arrays and Pointers . .10 2.3 Unrolling Small Loops . .13 2.4 Expression Order in Compound Branch Conditions . .14 2.5 Long Logical Expressions in If Statements . .16 2.6 Arrange Boolean Operands for Quick Expression Evaluation . .17 2.7 Dynamic Memory Allocation Consideration . .19 2.8 Unnecessary Store-to-Load Dependencies . .20 2.9 Matching Store and Load Size . .22 2.10 SWITCH and Noncontiguous Case Expressions . .25 2.11 Arranging Cases by Probability of Occurrence . .28 2.12 Use of Function Prototypes . .29 2.13 Use of const Type Qualifier . .30 2.14 Generic Loop Hoisting . .31 2.15 Local Static Functions . .34 2.16 Explicit Parallelism in Code . .35 2.17 Extracting Common Subexpressions . .37 2.18 Sorting and Padding C and C++ Structures . .39 2.19 Sorting Local Variables . .41 Contents iii Software Optimization Guide for AMD Athlon™ 64 and AMD Opteron™ 25112 Rev. 3.03 September 2003 Processors 2.20 Replacing Integer Division with Multiplication . .43 2.21 Frequently Dereferenced Pointer Arguments . .44 2.22 Using Signed Integers for 32-Bit Array Indices . .46 2.23 32-Bit Integral Data Types . .47 2.24 Sign of Integer Operands . .48 2.25 Accelerating Floating-Point Division and Square Root . .50 2.26 Fast Floating-Point-to-Integer Conversion . .52 2.27 Speeding Up Branches Based on Comparisons Between Floats . .54 Chapter 3 General 64-Bit Optimizations . .57 3.1 64-Bit Registers and Integer Arithmetic . .58 3.2 64-Bit Arithmetic and Large-Integer Multiplication . .60 3.3 128-Bit Media Instructions and Floating-Point Operations . .65 3.4 32-Bit Legacy GPRs and Small Unsigned Integers . .66 Chapter 4 Instruction-Decoding Optimizations . .69 4.1 DirectPath Instructions . .70 4.2 Load-Execute Instructions . .71 4.2.1 Load-Execute Integer Instructions . .71 4.2.2 Load-Execute Floating-Point Instructions with Floating-Point Operands . .72 4.2.3 Load-Execute Floating-Point Instructions with Integer Operands . .72 4.3 Branch Targets in Program Hot Spots . .74 4.4 32/64-Bit vs. 16-Bit Forms of the LEA Instruction . .75 4.5 Short Instruction Encodings . .76 4.6 Partial-Register Reads and Writes . .77 4.7 Using LEAVE for Function Epilogues . .79 4.8 Alternatives to SHLD Instruction . .81 4.9 8-Bit Sign-Extended Immediate Values . .83 4.10 8-Bit Sign-Extended Displacements . .84 4.11 Code Padding with Operand-Size Override and NOP . .85 Chapter 5 Cache and Memory Optimizations . .87 5.1 Memory-Size Mismatches . .88 iv Contents 25112 Rev. 3.03 September 2003 Software Optimization Guide for AMD Athlon™ 64 and AMD Opteron™ Processors 5.2 Natural Alignment of Data Objects . .91 5.3 Multiprocessor Considerations . .92 5.4 Store-to-Load Forwarding Restrictions . .93 5.5 Prefetch Instructions . .97 5.6 Write-combining . .105 5.7 L1 Data Cache Bank Conflicts . .106 5.8 Placing Code and Data in the Same 64-Byte Cache Line . .108 5.9 Sorting and Padding C and C++ Structures . .109 5.10 Sorting Local Variables . .111 5.11 Appropriate Memory Copying Routines . .112 5.12 Stack Considerations . .123 5.13 Interleave Loads and Stores . .124 Chapter 6 Branch Optimizations . .125 6.1 Density of Branches . .126 6.2 Two-Byte Near-Return RET Instruction . .128 6.3 Branches That Depend on Random Data . .130 6.4 Pairing CALL and RETURN . .132 6.5 Recursive Functions . .133 6.6 Nonzero Code-Segment Base Values . ..