Fix the debug exercise in the "Borrowing a Value" section #2083
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Trying to reproduce this RVO shows that actually the optimization that is used here is simple inlining. Once inlining is disabled, the addresses change, even with `-O -C opt-level=3`. The return of the values is also never changed to an "efficient memcpy", but instead was returned on eax+edx, although this result is of course specific to the calling convention of the platform ABI, but no memcpy on the most popular amd64 architecture.
djmitche
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May 17, 2024
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Looks good! I think the course should avoid getting too into the weeds on code generation in general. Folks who are interested in such things can dig down on their own.
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Trying to reproduce this RVO shows that actually the optimization that is used here is simple inlining. Once inlining is disabled, the addresses change, even with
-O -C opt-level=3.The return of the values is also never changed to an "efficient memcpy", but instead was returned on eax+edx, although this result is of course specific to the calling convention of the platform ABI, but no memcpy on the most popular amd64 architecture.
I don't think it's educationally important to teach RVO here, so I didn't go into any length trying to force Rust to do a real RVO (passing Point address into the function on the assembly level, so the function can fill in the addition). In my opinion the only important thing is, that if a student is actually clicking the Playground link and looks into the assembly, then our description should match the generated code. That's why I just fixed the content instead of trying to fix the example to be RVO.