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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 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

Automatic function inlining and static functions

4.22 Automatic function inlining and static functions

At -O2 and -O3 levels of optimization, or when --autoinline is specified, the compiler can automatically inline functions if it is practical and possible to do so, even if the functions are not declared as __inline or inline.

This works best for static functions, because if all use of a static function can be inlined, no out-of-line copy is required. Unless a function is explicitly declared as static (or __inline), the compiler has to retain the out-of-line version of it in the object file in case it is called from some other module.
It is best to mark all non-inline functions as static if they are not used outside the translation unit where they are defined (a translation unit being the preprocessed output of a source file together with all of the headers and source files included as a result of the #include directive). Typically, you do not want to place definitions of non-inline functions in header files.
If you fail to declare functions that are never called from outside a module as static, code can be adversely affected. In particular, you might have:
  • A larger code size, because out-of-line versions of functions are retained in the image.
    When a function is automatically inlined, both the in-line version and an out-of-line version of the function might end up in the final image, unless the function is declared as static. This might increase code size.
  • An unnecessarily complicated debug view, because there are both inline versions and out-of-line versions of functions to display.
    Retaining both inline and out-of-line copies of a function in code can sometimes be confusing when setting breakpoints or single-stepping in a debug view. The debugger has to display both in-line and out-of-line versions in its interleaved source view so that you can see what is happening when stepping through either the in-line or out-of-line version.
Because of these problems, declare non-inline functions as static when you are sure that they can never be called from another module.
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