These variable attributes are supported by the Nvidia PTX back end:
shared
Use this attribute to place a variable in the .shared
memory space. This memory space is private to each cooperative thread array; only threads within one thread block refer to the same instance of the variable. The runtime does not initialize variables in this memory space.
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https://gcc.gnu.org/onlinedocs/gcc-7.3.0/gcc/Nvidia-PTX-Variable-Attributes.html