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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

Comparison of pure and impure functions

4.18 Comparison of pure and impure functions

The two sample routines in the following table illustrate the use of the __pure keyword.

Both routines call a function fact() to calculate the sum of n! and n!. fact() depends only on its input argument n to compute n!. Therefore, fact() is a pure function.
The first routine shows a naive implementation of the function fact(), where fact() is not declared __pure. In the second implementation, fact() is qualified as __pure to indicate to the compiler that it is a pure function.

Table 4-7 C code for pure and impure functions

A pure function not declared __pure A pure function declared __pure
int fact(int n)
{
    int f = 1;
    while (n > 0)
        f *= n--;
    return f;
} 
int foo(int n)
{
    return fact(n)+fact(n);
}
int fact(int n) __pure
{
    int f = 1;
    while (n > 0)
        f *= n--;
    return f;
}
int foo(int n)
{
    return fact(n)+fact(n);
}
The following table shows the corresponding disassembly of the machine code produced by the compiler for each of the sample implementations above, where the C code for each implementation has been compiled using the option -O2, and inlining has been suppressed.

Table 4-8 Disassembly for pure and impure functions

A pure function not declared __pure A pure function declared __pure
fact PROC
    ...
foo PROC
    MOV      r3, r0
    PUSH     {lr}
    BL       fact
    MOV      r2, r0
    MOV      r0, r3
    BL       fact
    ADD      r0, r0, r2
    POP      {pc}
    ENDP
fact PROC
    ...
foo PROC
    PUSH     {lr}
    BL       fact
    LSL      r0,r0,#1
    POP      {pc}
    ENDP
In the disassembly where fact() is not qualified as __pure, fact() is called twice because the compiler does not know that the function is a candidate for Common Subexpression Elimination (CSE). In contrast, in the disassembly where fact() is qualified as __pure, fact() is called only once, instead of twice, because the compiler has been able to perform CSE when adding fact(n) + fact(n).
Related reference
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