[wtallis]

It's not even clear to me that there's any overhead to the Rust version. Checking error codes that should be checked isn't overhead. Checking them inefficiently would be overhead, but the Rust version looks like it should compile down to something pretty similar to what the equivalent C switch blocks would produce.

[masklinn]

> It's not even clear to me that there's any overhead to the Rust version.

There is a slight bit of stack overhead: Option<ForkResult> is at least {tag:u8, {tag:u8, pid:i32}}, and due to alignment constraints it's actually {tag: u32, {tag: u32, pid: i32 }}). A nonzero wrapper[0] would allow folding either ForkResult or Option into a 0-valued pid_t and remove one level of tagging: http://is.gd/yxStW1

Beyond that you'd need generalised enum folding in order to fold two tags into the underlying value (you'd denote that pid_t is nonzero and nonnegative for instance)

[0] which is unstable, so not really an option

[arielb1]

We do have a planned optimization that would fold the tags for cases like `Option<ForkResult>` to give a word pair, which should be returned in %eax:%edx (or %rax:%rdx).

[Jweb_Guru]

Really? That's exciting. Missed enum layout optimizations are one of my few issues with Rust right now.

[pcwalton]

But if the wrappers get inlined (which they should be) then SROA kicks in and promotes the tags to SSA values, where other optimizations such as SCCP can eliminate them. Optimizing compilers are awesome :)

[ant6n]

Theoretically It should be possible to have a union based on the value of the ints {-1, 0, positive}, which should use only one 32bit integer.

[sergiolp]

I'm not talking about the efficency of the resulting binary, but the "distance" from what the programmer is thinking, to what the machine will really do.

Compiler optimizations aside, C does a pretty good job at this. It's way more efficient than writing assembly, but your still basically just moving memory around, while doing some arithmethic. Easy to understand in "machine" terms.

Of course, this is only relevant when you're doing low-level stuff, like kernel or drivers programming. For the userland, Rust really looks like a nice language (I've played with it just a bit), and I'd be really happy if it pushes C++ away ;-)

[MaulingMonkey]

Compiler optimizations included, C does a terrible job at this. It puts forward a seductive but terrible mirage of simple mappings and understandings which are just plain broken. And then you add multithreading into the mix and it gets even worse, even without optimizations.

We live in a world of many cores, and multiple CPUs all over the place - in your GPUs, your hard drives, motherboard controllers - and the intrinsic language support for multithreading literally does not exist as part of the C99 standard? One has to reach out to a mixture of POSIX, and the compiler extensions the POSIX implementation uses to annotate memory barriers so the optimizer won't break things, and intrinsics that introduce atomic operations, and... gah!

C and C++ do such a terrible job of this I have to resort to disassembly to debug program behavior far too frequently. These are the only languages I'm forced to do this with. If C or C++ were really "close to what the machine will really do", I'd expect the opposite result.

Even simple things like class and structure layouts and type sizes are controlled by a mess of compiler and architecture specific rules and extensions to control the application of those rules with regards to padding, alignment, etc. which I get to debug. Ever had to debug differences in class layout between MSVC and Clang due to differently handling EBCO in a multiple inheritance environment? What about handling alignment of 8-byte types on 32-bit architectures differently? At least you've replaced all uses of "long" because of the mixture of LP64 and LLP64 compilers out there...? And what about when two incompatible versions of the standard library with different type layouts get linked in by a coworker? These are the symptoms of a language that doesn't control what the machine is really doing very well at all.

When I really need tight control over what the machine will do at a low level, my tools are actual (dis)assembly, intrinsics, an understanding of the underlying hardware itself, and simple code that eschews features requiring significant runtime support or underpinnings. None of those are C or C++ specific. The last one requires some knowledge of how a language's features are implemented - C and C++ might be broken enough that you're forced to wrestle with that topic, when it's more optional in other languages, but... that still doesn't make it C or C++ specific.

</rant>

[iopq]

Rust is just as close to the hardware as C, it just checks your code more.