Restrictions on VFP usage

The VFP support implements only the RunFast mode of operation:

trapped floating point exceptions are disabled
the Round-to-nearest mode is enabled
the Flush-to-zero mode is enabled
default NaNs are enabled

Note: Both Round-to-nearest and Flush-to-zero are defined in the IEEE754 standard.

This mode of operation doesn't require any software support code. However, it doesn't provide full IEEE754 compliance. For full details about the RunFast mode of operation, see the ARM Architecture Reference Manual, published by ARM.

No software emulation or support code is provided for the VFP instruction set. This means that code using VFP instructions can be run only on a processor that implements VFP hardware. Executing VFP instructions without the presence of VFP hardware will result in a SIGFPE signal.

The standard QNX Neutrino libraries are compiled to use a soft-float implementation for floating-point operations to ensure the code can run on all supported ARM processors.

The soft-float implementation passes floating-point arguments and results in ARM registers, or on the stack. The code that uses VFP instructions for floating point must use the same argument (or result mechanism) to ensure that it can interoperate correctly with code compiled for soft-float.

A VFP-enabled math library, called libm-vfp.so, is provided and can be used on targets that implement VFP hardware in two ways:

If your development will target only systems that implement VFP hardware, you can replace the armle/lib/libm.so link to point at libm-vfp.so.2 instead of libm.so.2. This means that linking with -lm will use libm-vfp.so instead, and mkifs will build libm-vfp.so into an image instead of usinglibm.so.2
If your development targets a variety of ARM processors, some of which don't implement VFP hardware, you can use the following when targeting VFP processors:
- use -lm-vfp instead of -lm when linking your application, as well as specifying the compilation flags described below
- specify libm.so=libm-vfp.so in your target build file to ensure that mkifs builds the image with libm-vfp.so instead of using libm.so