Floating Point Services

The kernel allows threads to use floating point registers on board configurations that support these registers.

Note

Floating point services are currently available only for boards based on ARM Cortex-M SoCs supporting the Floating Point Extension, the Intel x86 architecture, the SPARC architecture and ARCv2 SoCs supporting the Floating Point Extension. The services provided are architecture specific.

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

Concepts 

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 threads that use floating point registers. It is the kernel’s default floating point services mode.

If a thread 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 thread that uses floating point registers.

On x86 platforms, the kernel initializes the floating point registers so they can be used by any thread (initialization in skipped on ARM Cortex-M platforms and ARCv2 platforms). The floating point registers are left unchanged whenever a context switch occurs.

Note

The behavior is undefined, if two or more threads attempt to use the 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 and enables access to the floating point registers, so they can be used by any thread, 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.

ARM Cortex-M architecture (with the Floating Point Extension)

Note

The Shared FP registers mode is the default Floating Point Services mode in ARM Cortex-M.

On the ARM Cortex-M architecture with the Floating Point Extension, the kernel treats all threads as FPU users when shared FP registers mode is enabled. This means that any thread is allowed to access the floating point registers. The ARM kernel automatically detects that a given thread is using the floating point registers the first time the thread accesses them.

Pretag a thread that intends to use the FP registers by using one of the techniques listed below.

A statically-created ARM thread can be pretagged by passing the K_FP_REGS option to K_THREAD_DEFINE.
A dynamically-created ARM thread can be pretagged by passing the K_FP_REGS option to k_thread_create().

Pretagging a thread with the K_FP_REGS option instructs the MPU-based stack protection mechanism to properly configure the size of the thread’s guard region to always guarantee stack overflow detection, and enable lazy stacking for the given thread upon thread creation.

During thread context switching the ARM kernel saves the callee-saved floating point registers, if the switched-out thread has been using them. Additionally, the caller-saved floating point registers are saved on the thread’s stack. If the switched-in thread has been using the floating point registers, the kernel restores the callee-saved FP registers of the switched-in thread and the caller-saved FP context is restored from the thread’s stack. Thus, the kernel does not save or restore the FP context of threads that are not using the FP registers.

Each thread that intends to use the floating point registers must provide an extra 72 bytes of stack space where the callee-saved FP context can be saved.

Lazy Stacking is currently enabled in Zephyr applications on ARM Cortex-M architecture, minimizing interrupt latency, when the floating point context is active.

When the MPU-based stack protection mechanism is not enabled, lazy stacking is always active in the Zephyr application. When the MPU-based stack protection is enabled, the following rules apply with respect to lazy stacking:

Lazy stacking is activated by default on threads that are pretagged with K_FP_REGS
Lazy stacking is activated dynamically on threads that are not pretagged with K_FP_REGS, as soon as the kernel detects that they are using the floating point registers.

If an ARM thread does not require use of the floating point registers any more, it can call k_float_disable(). This instructs the kernel not to save or restore its FP context during thread context switching.

ARM64 architecture

Note

The Shared FP registers mode is the default Floating Point Services mode on ARM64. The compiler is free to optimize code using FP/SIMD registers, and library functions such as memcpy are known to make use of them.

On the ARM64 (Aarch64) architecture the kernel treats each thread as a FPU 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.

FPU register usage by ISRs is supported although not recommended. When an ISR uses floating point or SIMD registers, then the access is trapped, the current FPU user context is saved in the thread object and the ISR is resumed with interrupts disabled so to prevent another IRQ from interrupting the ISR and potentially requesting FPU usage. Because ISR don’t have a persistent register context, there are no provision for saving an ISR’s FPU context either, hence the IRQ disabling.

Each thread object becomes 512 bytes larger when Shared FP registers mode is enabled.

ARCv2 architecture

On the ARCv2 architecture, the kernel treats each thread as a non-user or FPU user and the thread must be tagged by one of the following techniques.

A statically-created ARC thread can be tagged by passing the K_FP_REGS option to K_THREAD_DEFINE.
A dynamically-created ARC thread can be tagged by passing the K_FP_REGS to k_thread_create().

If an ARC thread does not require use of the floating point registers any more, it can call k_float_disable(). This instructs the kernel not to save or restore its FP context during thread context switching.

During thread context switching the ARC kernel saves the callee-saved floating point registers, if the switched-out thread has been using them. Additionally, the caller-saved floating point registers are saved on the thread’s stack. If the switched-in thread has been using the floating point registers, the kernel restores the callee-saved FP registers of the switched-in thread and the caller-saved FP context is restored from the thread’s stack. Thus, the kernel does not save or restore the FP context of threads that are not using the FP registers. An extra 16 bytes (single floating point hardware) or 32 bytes (double floating point hardware) of stack space is required to load and store floating point registers.

RISC-V architecture

On the RISC-V architecture the kernel treats each thread as an FPU user on a case-by-case basis with the FPU access allocated on demand. A “lazy save” algorithm is used during context switching which updates the floating point registers only when it is absolutely necessary. For example, the FPU registers are not saved when switching from an FPU user to a non-user thread (or an FPU user that doesn’t touch the FPU during its scheduling slot), and then back to the original FPU user.

FPU register usage by ISRs is supported although not recommended. When an ISR uses floating point or SIMD registers, then the access is trapped, the current FPU user context is saved in the thread object and the ISR is resumed with interrupts disabled so to prevent another IRQ from interrupting the ISR and potentially requesting FPU usage. Because ISR don’t have a persistent register context, there are no provision for saving an ISR’s FPU context either, hence the IRQ disabling.

As an optimization, the FPU context is preemptively restored upon scheduling back an “active FPU user” thread that had its FPU context saved away due to FPU usage by another thread. Active FPU users are so designated when they make the FPU state “dirty” during their most recent scheduling slot before being scheduled out. So if a thread doesn’t modify the FPU state within its scheduling slot and another thread claims the FPU for itself afterwards then that first thread will be subjected to the on-demand regime and won’t have its FPU context restored until it attempts to access it again. But if that thread does modify the FPU before being scheduled out then it is likely to continue using it when scheduled back in and preemptively restoring its FPU context saves on the exception trap overhead that would occur otherwise.

Each thread object becomes 136 bytes (single-precision floating point hardware) or 264 bytes (double-precision floating point hardware) larger when Shared FP registers mode is enabled.

SPARC architecture

On the SPARC architecture, the kernel treats each thread as a non-user or FPU user and the thread must be tagged by one of the following techniques:

A statically-created thread can be tagged by passing the K_FP_REGS option to K_THREAD_DEFINE.
A dynamically-created thread can be tagged by passing the K_FP_REGS to k_thread_create().

During thread context switch at exit from interrupt handler, the SPARC kernel saves all floating point registers, if the FPU was enabled in the switched-out thread. Floating point registers are saved on the thread’s stack. Floating point registers are restored when a thread context is restored iff they were saved at the context save. Saving and restoring of the floating point registers is synchronous and thus not lazy. The FPU is always disabled when an ISR is called (independent of CONFIG_FPU_SHARING).

Floating point disabling with k_float_disable() is not implemented.

When CONFIG_FPU_SHARING is used, then 136 bytes of stack space is required for each FPU user thread to load and store floating point registers. No extra stack is required if CONFIG_FPU_SHARING is not used.

x86 architecture

On the x86 architecture the kernel treats each thread 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
cooperative	any	0 bytes
preemptive	none	0 bytes
preemptive	FPU	108 bytes
preemptive	SSE	464 bytes

The x86 kernel automatically detects that a given thread 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 pretag a thread using one of the techniques listed below.

A statically-created x86 thread can be pretagged by passing the K_FP_REGS or K_SSE_REGS option to K_THREAD_DEFINE.
A dynamically-created x86 thread can be pretagged by passing the K_FP_REGS or K_SSE_REGS option to k_thread_create().
An already-created x86 thread can pretag itself once it has started by passing the K_FP_REGS or K_SSE_REGS option to k_float_enable().

If an x86 thread uses the floating point registers infrequently it can call k_float_disable() 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 by calling k_float_enable().

Implementation 

Performing Floating Point Arithmetic 

No special coding is required for a thread to use floating point arithmetic if the kernel is properly configured.

The following code shows how a routine can use floating point arithmetic to avoid overflow issues when computing the average of a series of integer values.

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);
}

Suggested Uses 

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

Configuration Options 

To configure unshared FP registers mode, enable the CONFIG_FPU configuration option and leave the CONFIG_FPU_SHARING configuration option disabled.

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

For x86, use the CONFIG_X86_SSE configuration option to enable support for SSEx instructions.

API Reference 

Floating Point APIs