[PATCH] D103750: [analyzer][WIP] Handle std::make_unique for SmartPtrModeling

[Artem Dergachev via Phabricator via cfe-commits]

NoQ added a comment.

In D103750#2804438 <https://reviews.llvm.org/D103750#2804438>, @RedDocMD wrote:

> Nice. Looks like I have something to read up on. Turns out, I end up learning something new in C++ every now and then.  😃

Yeah, it's also pretty interesting ABI-wise. As you know, C++ doesn't have stable ABI. That said, one popular calling convention for functions that return C++ objects by value consists in passing a hidden argument to the function that contains the address at which to construct the return value. Like "this" is an implicit argument to methods, the return address is yet another implicit argument. So you can think of `getReturnValueUnderConstruction()` as of an accessor for that implicit argument value similarly to how `getCXXThisVal()` is the accessor for the implicit this-argument value.

This is completely different from C where you can return the structure on stack or even in registers (which can't be pointed to at all); this becomes possible in C because structures can be copied arbitrarily without invoking any sort of copy constructor.

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Random general advice! When writing patches, you typically explore the design space of potential solutions - which you did great in this case. When you do so, it's a great idea to include the results of such exploration in the commit message (and/or in later comments). Especially if the first / most obvious thing to try didn't work. If you publish your second-choice solution, people will most likely ask you why you chose it instead of the obvious first-choice solution so it's an act of friendliness to anticipate this question. Similarly, if you have concerns about your solution, please communicate them openly; they'll either be discovered anyway but a lot more effort will be spent on it, or they won't be discovered during review which could result in future problems; both variants are worse than communicating openly. Ideally you carry more responsibility for proving that your approach is right than the reviewer has a responsibility to find problems in it. The reviewer is the last line of defense when it comes to applying critical thinking but the process really starts with you as you challenge your own assumptions and approaches by questioning both their correctness and their architectural harmony. Trying to find at least two solutions to each problem and compare them against each other is such a good exercise in critical thinking that almost every patch deserves such discussion.


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