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Chapter 2 C and C++ Source-Level Optimizations 7
Software Optimization Guide for AMD64 Processors
25112 Rev. 3.06 September 2005
Chapter 2 C and C++ Source-Level
Optimizations
Although C and C++ compilers generally produce very compact object code, many performance
improvements are possible by careful source code optimization. Most such optimizations result from
taking advantage of the underlying mechanisms used by C and C++ compilers to translate source
code into sequences of AMD64 instructions. This chapter includes guidelines for writing C and C++
source code that result in the most efficiently optimized AMD64 code.
This chapter covers the following topics:
Topic Page
Declarations of Floating-Point Values 9
Using Arrays and Pointers 10
Unrolling Small Loops 13
Expression Order in Compound Branch Conditions 14
Long Logical Expressions in If Statements 16
Arrange Boolean Operands for Quick Expression Evaluation 17
Dynamic Memory Allocation Consideration 19
Unnecessary Store-to-Load Dependencies 20
Matching Store and Load Size 22
SWITCH and Noncontiguous Case Expressions 25
Arranging Cases by Probability of Occurrence 28
Use of Function Prototypes 29
Use of const Type Qualifier 30
Generic Loop Hoisting 31
Local Static Functions 34
Explicit Parallelism in Code 35
Extracting Common Subexpressions 37
Sorting and Padding C and C++ Structures 39
Sorting Local Variables 41
Replacing Integer Division with Multiplication 43
Frequently Dereferenced Pointer Arguments 44
Array Indices 46
32-Bit Integral Data Types 47
Sign of Integer Operands 48