conversions between int, long, jlong, int64_t etc.
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What is the best way to do this?

int is a compiler specific type, at least 16 bits
long is a compiler specific type, at least 32 bits
jlong is a C type representing 64-bit Java long quantities
int64_t is a C type, representing 64-bit quantities within the JVM

Do we need conversion macros to convert between jlong and int64_t
quantities?
Casts between them are not a good idea, because on some platforms
they may have to be implemented using structures.

src/share/javavm/runtime/Threadruntime.c

#if defined IBM_OS2
    tid->eetop = ptr2ll((void *)val);
#else
    tid->eetop = (int64_t)val;
#endif