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0 pointsScottBurson8y ago0 comments

> But in a GC:ed language, the n variable would be bump allocated once every loop.

Citation needed. I don't know of any GCed language that heap-allocates local variables in this way (well, maybe SML/NJ does, but I doubt it). Certainly not Java or any Common Lisp I've ever used.

But I agree with your larger point: heap allocation is never quite as fast as stack allocation, once you factor in the additional GC load. I don't actually know how close it gets with modern collectors; would love to see some numbers.

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bjourne8y ago

C# does and Java did before the escape analysis optimization became default. You can find numbers in this old article from 1999: https://www.cc.gatech.edu/~harrold/6340/cs6340_fall2009/Read...

"and the overall execution time reduction ranges from 2% to 23% (with a median of 7%) on a 333 MHz PowerPC workstation with 128 MB memory."

The benchmark is 20 years old so it is kind of out of date. I don't know of any modern benchmarks. I suspect that the difference would be much bigger nowadays because programmers don't avoid allocating small local objects as much.

ScottBursonOP8y ago

Oh, now I understand what you're saying. I got thrown off when you said "the address of the variable n [...] moves a bit for each loop". This isn't quite right; it's not the address of the variable n itself that changes, it's the address of the allocated object that it points to.

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