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Both GCC and Clang generate strange/inefficient code

16 points by rsf ago | 5 comments

the_fall |next [-]

It's common for compilers to generate mildly unusual code because they translate high-level code into an abstract intermediate notation, run a variety optimization steps on that notation, and then emit machine-specific code to perform whatever the optimizations yielded. There's no constraint along the lines of "but select the most logical opcode for this task".

The claim that the code is inefficient is really not substantiated well in this blog post. Sometimes, long-winded assembly actually runs faster because of pipelining, register aliasing, and other quirks. Other times, a "weird" way of zeroing a register may actually take up less space in memory, etc.

rsf |root |parent [-]

> The claim that the code is inefficient is really not substantiated well in this blog post.

I didn't run benchmarks, but in the case of clang writing zeros to memory (which are never used thereafter), there's no way that particular code is optimal.

For the gcc output, it seems unlikely that the three versions are all optimal, given the inconsistent strategies used. In particular, the code that sets the output value to 0 or 1 in the size = 3 version is highly unlikely to be optimal in my opinion. I'd be amazed if it is!

Your point that unintuitive code is sometimes actually optimal is well taken though :)

its_magic |root |parent [-]

Stefan Kanthak has previously noted that GCC's code generator is quite horrible, in these extensive investigations:

https://skanthak.hier-im-netz.de/gcc.html

btdmaster |previous [-]

In my experience C++ abstractions give the optimizer a harder job and thus it generates worse code. In this case, different code is emitted by clang if you write a C version[0] versus C++ original[1].

Usually abstraction like this means that the compiler has to emit generic code which is then harder to flow through constraints and emit the same final assembly since it's less similar to the "canonical" version of the code that wouldn't use a magic `==` (in this case) or std::vector methods or something else like that.

[0] https://godbolt.org/z/vso7xbh61

[1] https://godbolt.org/z/MjcEKd9Tr

pjmlp |root |parent [-]

Except that the C++ version doesn't need to be like that.

Abstractions are welcome when it doesn't matter, when it matters there are other ways to write the code and it keeps being C++ compliant.