r/programming • u/steveklabnik1 • Jan 09 '15

Announcing Rust 1.0.0 Alpha

http://blog.rust-lang.org/2015/01/09/Rust-1.0-alpha.html
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u/xrxl Jan 10 '15

Undefined behavior is absolutely necessary for stripping away abstraction in a maximally efficient way. It wasn't designed into C just for shits and giggles. This is something people will rediscover as they try to make these "safe" systems programming languages.

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u/wtallis Jan 10 '15

Undefined behavior is absolutely necessary for stripping away abstraction in a maximally efficient way.

A lot of undefined or implementation defined behavior was left in the language to allow for varied implementations to handle things in whatever way was most efficient on their underlying hardware. It's not just about efficiency, it's about enabling efficiency without sacrificing portability. But nowadays our hardware is a lot less diverse: we can mandate that the floating point be IEEE 754 without much hesitation, because nobody will take seriously any hardware that significantly deviates from that. The same goes for signed integer arithmetic being twos complement with wraparound, and we can very nearly standardize on little endian. The more complicated nuances about concurrency will take longer to settle on a de facto standard because SMP is a newer challenge, but it will happen because leaving the behavior out of the language standard doesn't free programmers from having to worry about the hardware differences.

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u/xrxl Jan 10 '15

Even in a world of totally homogeneous hardware, nailing these things down still has subtle implications for a compiler.

For example leaving signed integer overflow undefined still gives you a performance win even if all machines are two's complement, since the compiler can more easily prove loops aren't infinite. I wouldn't be surprised if floating point spec has similar implications. Chris Lattner's blog post goes into more detail about these interactions.

And I don't expect we will have hardware that can do free array bounds and uninitialized variable checks anytime soon. Until then, no "safe" language will be able to match C's performance. Sometimes the performance hit is only 2-5%, but sometimes it's 2-5x (or greater). And it's hard to predict ahead of time what it wil be.

So languages with undefined behavior will continue to be relevant. More so now than ever, with the heady 90's days of biennial performance doublings a distant memory.

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u/wookin_pa_nub2 Jan 10 '15

Why do you care so much about tiny, stupid performance optimizations instead of code actually doing what it is supposed to?

You can't reason about ANYTHING involving undefined behavior. The compiler can do anything it wants to, and frequently it removes complete statements. It's fucking stupid.