NaN boxing on 64 bit systems

[AE9RB]

I noticed that NaN boxing was originally supporting 64 bit pointers and later removed. I couldn't find why this was the case but thought maybe it had to do with this:

AMD architecture uses only the lower 48 bits of an address, and bits 48 through 63 must be a copy of bit 47 or the processor raises an exception.

So maybe it's possible to add it back with this knowledge. I don't have an AMD core to test.

[cremno]

It was actually only 32-bit at first. But when was it removed? Looking at the header, NaN boxing seems to be supported by (some) 64-bit architectures, or am I missing something?

[AE9RB]

You can't compile NaN boxing on 64-bit right now because of:

mruby/src/state.c

Lines 32 to 34 in d17506c

	#ifdef MRB_NAN_BOXING
	mrb_static_assert(sizeof(void*) == 4, "when using NaN boxing sizeof pointer must be 4 byte");
	#endif

[cremno]

Ah! Yes, that assertion should be removed. It was added before 64-bit support was merged.

ae288fe (Jun 2013)
#1441 (Aug 2013)

Do you want to create a PR or should I? 64-bit NaN boxing compiles and works fine for me.

[matz]

64bit NaN boxing never had been implemented for mruby, but for LuaJIT which I steal NaN boxing idea. Since I don't have 64bit machine that only use lower 48 bits for sure, it's too hard for me to implement it. If someone comes with the patch, I am happy to merge.

[matz]

Oops, I forgot about #1441. I will fix.