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6:18:42
hayley
Well, I sent an email to Cliff Click asking about what he thought about callee saved registers, and I expected to not get a response any time soon. I was amazingly proved wrong.
6:23:27
hayley
The context was the "Bits of advice for VM writers" presentation, where he is also critical of callee-saves registers on x86 <https://www.youtube.com/watch?v=vzzABBxo44g>. I asked if he thought inlining (or call site optimization) changes the assumptions made, and if he thinks callee-saves is a good idea on x86-64 or ARM.
6:24:49
hayley
He said callee-saves was very messy in concurrent stack scanning on HotSpot, and "it wasn't really worth it for all the very-low-frequency GC bugs that entailed." And that callee-saves can sometimes get a small win on x86-64, because spilling is optimized. But on a RISC there is a larger win.
6:25:36
hayley
But, as he said it was "since leaf routines make up (nearly by definition) 50% of all calls", I wonder if I didn't communicate that I was interested in how these change under inlining.