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[FE] Manipulate the first byte of guard variable type in both load and store…

Authored by Xiangling_L on Feb 8 2021, 8:14 AM.

Description

[FE] Manipulate the first byte of guard variable type in both load and store operation

As Itanium ABI[http://itanium-cxx-abi.github.io/cxx-abi/abi.html#once-ctor]
points out:

"The size of the guard variable is 64 bits. The first byte (i.e. the byte at
the address of the full variable) shall contain the value 0 prior to
initialization of the associated variable, and 1 after initialization is complete."

Differential Revision: https://reviews.llvm.org/D95822