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Both GCC and Clang generate strange/inefficient code

14 points by rsf

by btdmaster

1 subcomments

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

by the_fall

1 subcomments

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.