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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

Compiler options for floating-point linkage and computations

4.51 Compiler options for floating-point linkage and computations

Compiler options determine the type of floating-point linkage and floating-point computations.

By specifying the type of floating-point linkage and floating-point computations you require, you can determine, from the following table, the associated compiler command-line options that are available.

Table 4-13 Compiler options for floating-point linkage and floating-point computations

Linkage Computations    
Hardware FP linkage Software FP linkage Hardware FP coprocessor Software FP library (fplib) Compiler options
No Yes No Yes
No Yes Yes No
Yes No Yes No
softvfp specifies software floating-point linkage. When software floating-point linkage is used, either:
  • The calling function and the called function must be compiled using one of the options --softvfp, --fpu softvfp+vfpv2, --fpu softvfp+vfpv3, --fpu softvfp+vfpv3_fp16, softvfp+vfpv3_d16, softvfp+vfpv3_d16_fp16, softvfp+vfpv4, softvfp+vfpv4_d16, or softvfp+fpv4-sp.
  • The calling function and the called function must be declared using the __softfp keyword.
Each of the options --fpu softvfp, --fpu softvfp+vfpv2,--fpu softvfp+vfpv3, --fpu softvfp+vfpv3_fp16, --fpu softvfpv3_d16, --fpu softvfpv3_d16_fp16, --fpu softvfp+vfpv4, softvfp+vfpv4_d16 and softvfp+fpv4-sp specify software floating-point linkage across the whole file. In contrast, the __softfp keyword enables software floating-point linkage to be specified on a function by function basis.


Rather than having separate compiler options to select the type of floating-point linkage you require and the type of floating-point computations you require, you use one compiler option, --fpu, to select both. For example, --fpu=softvfp+vfpv2 selects software floating-point linkage, and a hardware coprocessor for the computations. Whenever you use softvfp, you are specifying software floating-point linkage.
If you use the --fpu option, you must know the VFP architecture version implemented in the target processor. An alternative to --fpu=softvfp+... is --apcs=/softfp. This gives software linkage with whatever VFP architecture version is implied by --cpu. --apcs=/softfp and --apcs=/hardfp are alternative ways of requesting the integer or floating-point variant of the Procedure Call Standard for the ARM Architecture (AAPCS).
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