The HGPR option specifies that the compiler is permitted to exploit
64-bit General Purpose Registers (GPRs) in 32-bit programs targeting
z/Architecture hardware.

.-NOHGPR-------------------------.
>>-+-HGPR--+----------------------+-+--------------------------><
| .-NOPRESERVE-. |
'-(--+-PRESERVE---+--)-'
- PRESERVE
- PRESERVE Instructs the compiler to preserve the high halves
of the 64-bit GPRs that a function is using, by saving them in the
prolog for the function and restoring them in the epilog. The PRESERVE
suboption is necessary only if the caller is not known to be Enterprise
PL/I and/or the z/OS XL C/C++ compiler-generated code.
- NOPRESERVE
- NOPRESERVE lets the compiler omit preserving the high-halves
of the 64-bit GPRs that a function is using. Because of performance
considerations, the default suboption for HGPR is NOPRESERVE.
HGPR means "High-half of 64-bit GPR", which refers to the use of
native 64-bit instructions. In particular, if the application use
8-byte integers, it should benefit from the native 64-bit instructions.
Note:
The NOHGPR option must be used in all CICS and SQL
applications.
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