From type/Scalar

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multi sub atomic-assign($target is rw, $value)

Performs an atomic assignment of $value into the Scalar $target. The atomic-assign routine ensures that any required barriers are performed such that the changed value will be "published" to other threads.

From type/atomicint

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multi sub atomic-assign(atomicint $ is rw, int $value)
multi sub atomic-assign(atomicint $ is rw, Int() $value)

Performs an atomic assignment to a native integer, which may be in a lexical, attribute, or native array element. If $value cannot unbox to a 64-bit native integer due to being too large, an exception will be thrown. If the size of atomicint is only 32 bits, then an out of range $value will be silently truncated. The atomic-assign routine ensures that any required barriers are performed such that the changed value will be "published" to other threads.