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Compiler User Guide
Preface
Overview of the Compiler
Getting Started with the Compiler
Compiler Features
Compiler Coding Practices
The compiler as an optimizing compiler
Compiler optimization for code size versus speed
Compiler optimization levels and the debug view
Selecting the target processor at compile time
Enabling FPU for bare-metal
Optimization of loop termination in C code
Loop unrolling in C code
Compiler optimization and the volatile keyword
Code metrics
Code metrics for measurement of code size and data
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
Compiler decisions on function inlining
Automatic function inlining and static functions
Inline functions and removal of unused out-of-line
Automatic function inlining and multifile compilat
Restriction on overriding compiler decisions about
Compiler modes and inline functions
Inline functions in C++ and C90 mode
Inline functions in C99 mode
Inline functions and debugging
Types of data alignment
Advantages of natural data alignment
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 fields in structures
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4.36 Unaligned fields in structures
You can use the __packed qualifier to create unaligned fields in structures. This saves space because the compiler does not need to pad fields to their natural size boundary.
For efficiency, fields in a structure are positioned on their natural size boundary. This
means that the compiler often inserts padding between fields to ensure that they are
naturally aligned.
When space is at a premium, you can use the
__packed qualifier to create
structures without padding between fields. Structures can be packed in the following
ways:- The entire
structcan be declared as__packed. For example:__packed struct mystruct { char c; short s; } // not recommendedEach field of the structure inherits the__packedqualifier.Declaring an entirestructas__packedtypically incurs a penalty both in code size and performance. - Individual non-aligned fields within the
structcan be declared as__packed. For example:struct mystruct { char c; __packed short s; // recommended }This is the recommended approach to packing structures.
Note
The same principles apply to unions. You can declare either an entire union as
__packed, or use the __packed attribute to identify
components of the union that are unaligned in memory.Related concepts
| Non-Confidential | PDF version | ARM DUI0375H |
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