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arm: cortex_m: select by default FP sharing mode when using the FPU

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For applications that make use of the FPU in cortex m,
we enforce the FPU sharing registers mode, because the
compiler, under certain optimization regimes, may use
FP instructions and create FP context in any thread,
so the unshared registers mode is not practically
supported.

In addition to that we force FPU_SHARING to depend on
MULTITHREADING, as FPU sharing mode does not make sense
outside the normal multi-threaded builds.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
This commit is contained in:
Ioannis Glaropoulos 2021-01-27 14:24:59 +01:00 • committed by Anas Nashif
commit 86c1b57103
1 changed files with 11 additions and 1 deletions
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12
arch/Kconfig
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@ -764,12 +764,22 @@ config FPU
config FPU_SHARING
bool "FPU register sharing"
depends on FPU
depends on FPU && MULTITHREADING
default y if ARM && ARMV7_M_ARMV8_M_FP
help
This option enables preservation of the hardware floating point registers
across context switches to allow multiple threads to perform concurrent
floating point operations.
Note that on Cortex-M processors with the floating point extension we
enable by default the FPU register sharing mode, as some GCC compilers
may activate a floating point context by generating FP instructions for
any thread, and that context must be preserved when switching such
threads in and out. The developers can still disable the FP sharing
mode in their application projects, and switch to Unshared FP registers
mode, if it is guaranteed that the image code does not generate FP
instructions outside the single thread context that is allowed to do so.
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