Today Java has pointer compression where you use a 32 bit reference but shift it a few places to the left to make a 64-bit address which saves space on pointers but wastes it on alignment

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xxs 8 months ago | parent | next [-]

All allocated objects would have the three least significant bits as 0. Any java object cannot be 'too small' as they all have object headers (more if you need a fully blown synchronized/mutex). So with compressed pointers (up to 32GB Heaps) all objects are aligned but then again, each pointer is 4 bytes only (instead of 8). Overall it's a massive win.

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kstrauser 8 months ago | parent [-]

Huh, that’s clever! Do you have to choose that at compile or launch time, or does a program start like that and then “grow” when it uses more than 32GB of heap?

	▲	xxs 8 months ago \| parent [-]
		In Java you have to set max heap somehow - either ergonomics or just -Xmx command line option. Max heap is given (many a reason, and it sets before running the main method), so if you pick under the 32GB it'd auto use compressed pointers (optimize for size - optimize for speed). That option (compressed pointers) can be switched off, of course, via a command line option as well.

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o11c 8 months ago | parent | prev | next [-]

It's not wasted on alignment, since that alignment is already required (unless you need a very large heap). Remember that Java's GC heap is only used to allocate Objects, not raw bytes. There are ways to allocate memory outside of the heap and if you're dealing with that much raw data you should probably be using them.

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layer8 8 months ago | parent | prev [-]

Alignment is required anyway to prevent word tearing, for the atomicity guarantees.