Ping: RFR(M): 8086069: Adapt runtime calls to recent intrinsics to pass ints as long

[John Rose]

On Jun 15, 2015, at 5:07 PM, Dean Long <dean.long at oracle.com> wrote:
> 
> This may be a dumb (but hopefully related) question, but why do we need to add top() for _LP64:
> 
> 4364 #ifdef _LP64
> 4365 #define XTOP ,top() /*additional argument*/
> 4366 #else  //_LP64
> 4367 #define XTOP        /*no additional argument*/
> 4368 #endif //_LP64
> 
> 4396  make_runtime_call(RC_LEAF|RC_NO_FP,
> 4397                    OptoRuntime::fast_arraycopy_Type(),
> 4398                    StubRoutines::unsafe_arraycopy(),
> 4399                    "unsafe_arraycopy",
> 4400                    TypeRawPtr::BOTTOM,
> 4401                    src, dst, size XTOP);
> 
> And why only for"size", but not "src" and "dst"?


That is one of the awkward places we jam in LP64-specific code.
Java has no size_t type; the closest it gets is "long".
But the compiler uses Java types to set up runtime stub call signatures.
So if we want the compiler to pass a size_t value to a stub, it has to pass a long on !LP64 and int on ILP32.
(There's no need for an int-in-a-long here, since size_t is never 32 bits on an int-in-a-long machine.)
To complete the embarrassment, the compiler has an internal convention of always representing the twin word for longs and doubles (see JVMS).
Net result is that if you want to ask for a size_t in the JIT on LP64, you have to #ifdef in a jlong, and pass a "top" twin word.
— John