Floating Point Services

Note

Floating point services are currently available only for boards based on the ARM Cortex-M4 or the Intel x86 architectures. The services provided are architecture specific.

Concepts

The kernel allows an application’s tasks and fibers to use floating point registers on board configurations that support these registers.

Note

The kernel does not support the use of floating point registers by ISRs.

The kernel can be configured to provide only the floating point services required by an application. Three modes of operation are supported, which are described below. In addition, the kernel’s support for the SSE registers can be included or omitted, as desired.

No FP registers mode

This mode is used when the application has no tasks or fibers that use floating point registers. It is the kernel’s default floating point services mode.

If a task or fiber uses any floating point register, the kernel generates a fatal error condition and aborts the thread.

Unshared FP registers mode

This mode is used when the application has only a single task or fiber that uses floating point registers.

The kernel initializes the floating point registers so they can be used by any task or fiber. The floating point registers are left unchanged whenever a context switch occurs.

Note

Incorrect operation may result if two or more tasks or fibers use floating point registers, as the kernel does not attempt to detect (or prevent) multiple threads from using these registers.

Shared FP registers mode

This mode is used when the application has two or more threads that use floating point registers. Depending upon the underlying CPU architecture, the kernel supports one or more of the following thread sub-classes:

• non-user: A thread that cannot use any floating point registers
• FPU user: A thread that can use the standard floating point registers
• SSE user: A thread that can use both the standard floating point registers and SSE registers

The kernel initializes the floating point registers so they can be used by any task or fiber, then saves and restores these registers during context switches to ensure the computations performed by each FPU user or SSE user are not impacted by the computations performed by the other users.

On the ARM Cortex-M4 architecture the kernel treats all tasks and fibers as FPU users when shared FP registers mode is enabled. This means that the floating point registers are saved and restored during a context switch, even when the associated threads are not using them. Each task and fiber must provide an extra 132 bytes of stack space where these register values can be saved.

On the x86 architecture the kernel treats each task and fiber as a non-user, FPU user or SSE user on a case-by-case basis. A “lazy save” algorithm is used during context switching which updates the floating point registers only when it is absolutely necessary. For example, the registers are not saved when switching from an FPU user to a non-user thread, and then back to the original FPU user. The following table indicates the amount of additional stack space a thread must provide so the registers can be saved properly.

Thread type	FP register use	Extra stack space required
fiber	any	0 bytes
task	none	0 bytes
task	FPU	108 bytes
task	SSE	464 bytes

The x86 kernel automatically detects that a given task or fiber is using the floating point registers the first time the thread accesses them. The thread is tagged as an SSE user if the kernel has been configured to support the SSE registers, or as an FPU user if the SSE registers are not supported. If this would result in a thread that is an FPU user being tagged as an SSE user, or if the application wants to avoid the exception handling overhead involved in auto-tagging threads, it is possible to pre-tag a thread using one of the techniques listed below.

• An x86 task or fiber can tag itself as an FPU user or SSE user by calling task_float_enable() or fiber_float_enable() once it has started executing.
• An x86 fiber can be tagged as an FPU user or SSE user by its creator by calling fiber_start() with the USE_FP or USE_SSE option, respectively.
• A microkernel task can be tagged as an FPU user or SSE user by adding it to the FPU task group or the SSE task group when the task is defined.

Note

Adding the task to the FPU or SSE task groups by calling task_group_join() does not tag the task as an FPU user or SSE user.

If an x86 thread uses the floating point registers infrequently it can call task_float_disable() or fiber_float_disable() as appropriate to remove its tagging as an FPU user or SSE user. This eliminates the need for the kernel to take steps to preserve the contents of the floating point registers during context switches when there is no need to do so. When the thread again needs to use the floating point registers it can re-tag itself as an FPU user or SSE user using one of the techniques listed above.

Purpose

Use the kernel floating point services when an application needs to perform floating point operations.

Usage

Configuring Floating Point Services

To configure unshared FP registers mode, enable the CONFIG_FLOAT configuration option and leave the CONFIG_FP_SHARING configuration option disabled.

To configure shared FP registers mode, enable both the CONFIG_FLOAT configuration option and the CONFIG_FP_SHARING configuration option. Also, ensure that any task that uses the floating point registers has sufficient added stack space for saving floating point register values during context switches, as described above.

Use the CONFIG_SSE configuration option to enable support for SSEx instructions (x86 only).

Example: Performing Floating Point Arithmetic

This code shows how a routine can use floating point arithmetic to avoid overflow issues when computing the average of a series of integer values. Note that no special coding is required if the kernel is properly configured.

int average(int *values, int num_values)
{
    double sum;
    int i;

    sum = 0.0;

    for (i = 0; i < num_values; i++) {
        sum += *values;
        values++;
    }

    return (int)((sum / num_values) + 0.5);
}

APIs

The following floating point services APIs (x86 only) are provided by microkernel.h and by nanokernel.h:

fiber_float_enable()

Tells the kernel that the specified task or fiber is now an FPU user or SSE user.

task_float_enable()

Tells the kernel that the specified task or fiber is now an FPU user or SSE user.

fiber_float_disable()

Tells the kernel that the specified task or fiber is no longer an FPU user or SSE user.

task_float_disable()

Tells the kernel that the specified task or fiber is no longer an FPU user or SSE user.