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The compiler as an optimizing compiler
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Compiler optimization and the volatile keyword
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Stack use in C and C++
Benefits of reducing debug information in objects
Methods of reducing debug information in objects a
Guarding against multiple inclusion of header file
Methods of minimizing function parameter passing o
Returning structures from functions through regist
Functions that return the same result when called
Comparison of pure and impure functions
Recommendation of postfix syntax when qualifying f
Inline functions
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Inline functions and debugging
Types of data alignment
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Compiler storage of data objects by natural byte a
Relevance of natural data alignment at compile tim
Unaligned data access in C and C++ code
The __packed qualifier and unaligned data access i
Unaligned fields in structures
Performance penalty associated with marking whole
Unaligned pointers in C and C++ code
Unaligned Load Register (LDR) instructions generat
Comparisons of an unpacked struct, a __packed stru
Compiler support for floating-point arithmetic
Default selection of hardware or software floating
Example of hardware and software support differenc
Vector Floating-Point (VFP) architectures
Limitations on hardware handling of floating-point
Implementation of Vector Floating-Point (VFP) supp
Compiler and library support for half-precision fl
Half-precision floating-point number format
Compiler support for floating-point computations a
Types of floating-point linkage
Compiler options for floating-point linkage and co
Floating-point linkage and computational requireme
Processors and their implicit Floating-Point Units
Integer division-by-zero errors in C code
Software floating-point division-by-zero errors in
About trapping software floating-point division-by
Identification of software floating-point division
Software floating-point division-by-zero debugging
New language features of C99
New library features of C99
// comments in C99 and C90
Compound literals in C99
Designated initializers in C99
Hexadecimal floating-point numbers in C99
Flexible array members in C99
__func__ predefined identifier in C99
inline functions in C99
long long data type in C99 and C90
Macros with a variable number of arguments in C99
Mixed declarations and statements in C99
New block scopes for selection and iteration state
_Pragma preprocessing operator in C99
Restricted pointers in C99
Additional Complex numbers in C99
Boolean type and Extended integer types and functions in How to prevent uninitialized data from being initi
Compiler Diagnostic Messages
Using the Inline and Embedded Assemblers of the AR
Compiler Command-line Options
Language Extensions
Compiler-specific Features
C and C++ Implementation Details
What is Semihosting?
Via File Syntax
Summary Table of GNU Language Extensions
Standard C Implementation Definition
Standard C++ Implementation Definition
C and C++ Compiler Implementation Limits
Unaligned pointers in C and C++ code
| Non-Confidential |  PDF version | ARM DUI0375H | ||
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| Home > Compiler Coding Practices > Unaligned pointers in C and C++ code | ||||
4.38 Unaligned pointers in C and C++ code
If you want to define a pointer that can point to a word at any address, you must specify the __packed qualifier.
By default, the compiler expects conventional C and C++ pointers to point to naturally
aligned words in memory because this enables the compiler to generate more efficient code.
For example, to specify an unaligned pointer:
__packed int *pi; // pointer to unaligned int
When a pointer is declared as
__packed, the compiler generates code that
correctly accesses the dereferenced value of the pointer, regardless of its alignment. The
generated code consists of a sequence of byte accesses, or variable alignment-dependent
shifting and masking instructions, rather than a simple LDR instruction.
Consequently, declaring a pointer as __packed incurs a performance and code
size penalty. | Non-Confidential | PDF version | ARM DUI0375H |
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