3.4. Using VFP Floating Point
Prev Chapter 3. Sourcery CodeBench Lite for ARM EABI Next

3.4. Using VFP Floating Point

3.4.1. Enabling Hardware Floating Point

GCC provides three basic options for compiling floating-point code:

  • Software floating point emulation, which is the default. In this case, the compiler implements floating-point arithmetic by means of library calls.
  • VFP hardware floating-point support using the soft-float ABI. This is selected by the -mfloat-abi=softfp option. When you select this variant, the compiler generates VFP floating-point instructions, but the resulting code uses the same call and return conventions as code compiled with software floating point.
  • VFP hardware floating-point support using the VFP ABI, which is the VFP variant of the Procedure Call Standard for the ARM® Architecture (AAPCS). This ABI uses VFP registers to pass function arguments and return values, resulting in faster floating-point code. To use this variant, compile with -mfloat-abi=hard.

You can freely mix code compiled with either of the first two variants in the same program, as they both use the same soft-float ABI. However, code compiled with the VFP ABI is not link-compatible with either of the other two options. If you use the VFP ABI, you must use this option to compile your entire program, and link with libraries that have also been compiled with the VFP ABI. For example, you may need to use the VFP ABI in order to link your program with other code compiled by the ARM RealView® compiler, which uses this ABI.

Sourcery CodeBench Lite for ARM EABI includes libraries built with software floating point, which are compatible with VFP code compiled using the soft-float ABI. While the compiler is capable of generating code using the VFP ABI, no compatible runtime libraries are provided in Sourcery CodeBench Lite. However, VFP hard-float libraries built with both ABIs are available to Sourcery CodeBench Standard and Professional Edition subscribers.

Note that, in addition to selecting hard/soft float and the ABI via the -mfloat-abi option, you can also compile for a particular FPU using the -mfpu option. For example, -mfpu=neon selects VFPv3 with NEON coprocessor extensions.

3.4.2. NEON SIMD Code

Sourcery CodeBench includes support for automatic generation of NEON SIMD vector code. Autovectorization is a compiler optimization in which loops involving normal integer or floating-point code are transformed to use NEON SIMD instructions to process several data elements at once.

To enable generation of NEON vector code, use the command-line options -ftree-vectorize -mfpu=neon -mfloat-abi=softfp. The -mfpu=neon option also enables generation of VFPv3 scalar floating-point code.

Sourcery CodeBench also includes support for manual generation of NEON SIMD code using C intrinsic functions. These intrinsics, the same as those supported by the ARM RealView® compiler, are defined in the arm_neon.h header and are documented in the 'ARM NEON Intrinsics' section of the GCC manual. The command-line options -mfpu=neon -mfloat-abi=softfp must be specified to use these intrinsics; -ftree-vectorize is not required.

3.4.3. Half-Precision Floating Point

Sourcery CodeBench for ARM EABI includes support for half-precision (16-bit) floating point, including the new __fp16 data type in C and C++, support for generating conversion instructions when compiling for processors that support them, and library functions for use in other cases.

To use half-precision floating point, you must explicitly enable it via the -mfp16-format command-line option to the compiler. For more information about __fp16 representations and usage from C and C++, refer to the GCC manual.

Prev Up Next
3.3. Using Flash Memory Home 3.5. Fixed-Point Arithmetic