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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 library functions in C99 Complex numbers in C99 Boolean type and in C99 Extended integer types and functions in floating-point environment access in C99 snprintf family of functions in C99 type-generic math macros in C99 wide character I/O functions in C99 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

Recommendation of postfix syntax when qualifying functions with ARM function modifiers

4.19 Recommendation of postfix syntax when qualifying functions with ARM function modifiers

You can use function modifiers such as __pure either prefix or postfix, that is before the function declaration or after the parameter list. ARM recommends using the more precise postfix syntax.

Many ARM keyword extensions modify the behavior or calling sequence of a function. For example, __pure, __irq, __swi, __swi_indirect , __softfp, and __value_in_regs all behave in this way.
These function modifiers all have a common syntax. A function modifier such as __pure can qualify a function declaration either:
  • Before the function declaration. For example:
    __pure int foo(int);
  • After the closing parenthesis on the parameter list. For example:
    int foo(int) __pure;
For simple function declarations, each syntax is unambiguous. However, for a function whose return type or arguments are function pointers, the prefix syntax is imprecise. For example, the following function returns a function pointer, but it is not clear whether __pure modifies the function itself or its returned pointer type:
__pure int (*foo(int)) (int); /* declares 'foo' as a (pure?) function
                                 that returns a pointer to a (pure?)
                                 It is ambiguous which of the two
                                 function types is pure. */
In fact, the single __pure keyword at the front of the declaration of foo modifies both foo itself and the function pointer type returned by foo.
In contrast, the postfix syntax enables clear distinction between whether __pure applies to the argument, the return type, or the base function, when declaring a function whose argument and return types are function pointers. For example:
int (*foo1(int) __pure) (int);        /* foo1 is a pure function
                                         returning a pointer to
                                         a normal function */
int (*foo2(int)) (int) __pure;        /* foo2 is a function
                                         returning a pointer to
                                         a pure function */
int (*foo3(int) __pure) (int) __pure; /* foo3 is a pure function
                                         returning a pointer to
                                         a pure function */
In this example:
  • foo1 and foo3 are modified themselves.
  • foo2 and foo3 return a pointer to a modified function.
  • The functions foo3 and foo are identical.
Because the postfix syntax is more precise than the prefix syntax, ARM recommends that, where possible, you make use of the postfix syntax when qualifying functions with ARM function modifiers.
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