Comment by gcr | Hacker News Reader

Comment by gcr | Hacker News Reader

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I strongly disagree with your sentiment.

If you wrote an application that's as efficient as possible without any wasted bits in the size_t type, it then only works on your machine.

If I wanted to run such an application on my supercomputer with 2TB of RAM (such machines exist), I would then have to recompile for a 41-bit size_t.

We use machine-neutral (but architecture-specific) size_t for these kinds of things explicitly to avoid recompiling on different machines that are instances of the same platform.

Said another way, binary distributions could not exist if everything was made efficient for the underlying hardware. It would stink to have to recompile the world after upgrading RAM.

I'd rather have a few bits of wasted space (which are typically lost anyway due to struct packing) than lose intra-platform comparability.

A 48-bit size_t like I suggested could address up to 256TB of RAM. All modern x86_64 cpus are limited to 48-bits of address space, you're not losing any portability here.

Also consider my strlen example. Say you compute strlen by iterating through the whole string until you find a 0, then you return a size_t for the number of bytes you iterated through. That operation is O(n) in the length of the string. If you were to use my strlen function on a string whose length is greater than would fit into a 32-bit integer, say a 1TB string or something, then the function would take so long to compute it that it would be useless. To be efficient, you'd probably have to redesign your program to do some special things to handle 1TB strings, maybe some special algorithms, or some kind of indexing. Returning a 64-bit integer type does not mean that the function can actually handle working with 64-bit sized quantities. So if you have a lot of datastructures where you keep string length, why store them as a 64-bit size_t when your application would be completely unable to handle strings of that size without keeling over?

Of course you wouldn't really use a 48-bit size_t, because x86_64 cpus don't work well with 48-bit quantities.

2TB of RAM isn't close to a supercomputer nowadays, you can even get one of those at AWS now (the appropriately named x1.32xlarge instance).

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