On Mon, Apr 11, 2022 at 12:14 PM Aaron Ballman via cfe-commits < cfe-commits@lists.llvm.org> wrote:
> On Mon, Apr 11, 2022 at 10:50 AM Joerg Sonnenberger via Phabricator < > revi...@reviews.llvm.org> wrote: > > > > joerg added a comment. > > > > The patch contains at least one user visible change that would be quite > surprising: it is no longer possible to intentionally set a break point on > `std::move`. > > Thank you, that is a really good point. It would be a surprise to set > a breakpoint on the code I can see in debug mode (-O0 -g) and never > hit the breakpoint. > The same is true of `std::strlen`: if it is builtin'ed, then you don't get a call instruction. This is perfectly expected and natural. https://godbolt.org/z/jYzTM4a7c Passing -fno-builtin successfully disables the optimization for std::strlen. It should also disable the proposed optimization for std::move. Contra Aaron, I do not see *any difference at all* between what happens today for std::strlen and what's proposed to happen tomorrow for std::move. I don't think std::move is somehow "more advanced" or "more STL'ish" or "more C++'ish" than std::strlen. They're both part of the standard library, and the compiler in hosted mode is allowed to assume it knows what they do — but in freestanding mode (when -fno-builtin is passed) the compiler should *not* assume it knows what they do. > Thinking more about it, what about a slightly different implementation > strategy? Provide a compiler built-in `__builtin_std_move`. If it is > present, libc++ can alias it into `namespace std` using regular C++. FWIW, Joerg, I don't understand this suggestion. By what means can libc++ "alias [a compiler builtin] into namespace std"? The only thing I have imagined so far is something like inline auto move(auto&& t) { return __builtin_std_move(t); } but that would defeat the whole purpose of adding the builtin. The point of adding the builtin is to eliminate the cost of instantiating+codegenning+inlining, a whole new function for every call. In C you could get away with #define move(x) __builtin_std_move(x) but that's a non-starter in C++, because namespaces. –Arthur
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