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This is extended to all `std::` functions that take a reference to a value and return a reference (or pointer) to that same value: `move`, `forward`, `move_if_noexcept`, `as_const`, `addressof`, and the libstdc++-specific function `__addressof`. We still require these functions to be declared before they can be used, but don't instantiate their definitions unless their addresses are taken. Instead, code generation, constant evaluation, and static analysis are given direct knowledge of their effect. This change aims to reduce various costs associated with these functions -- per-instantiation memory costs, compile time and memory costs due to creating out-of-line copies and inlining them, code size at -O0, and so on -- so that they are not substantially more expensive than a cast. Most of these improvements are very small, but I measured a 3% decrease in -O0 object file size for a simple C++ source file using the standard library after this change. We now automatically infer the `const` and `nothrow` attributes on these now-builtin functions, in particular meaning that we get a warning for an unused call to one of these functions. In C++20 onwards, we disallow taking the addresses of these functions, per the C++20 "addressable function" rule. In earlier language modes, a compatibility warning is produced but the address can still be taken. The same infrastructure is extended to the existing MSVC builtin `__GetExceptionInfo`, which is now only recognized in namespace `std` like it always should have been. This is a re-commit of fc3090109643af8d2da9822d0f99c84742b9c877, a571f82a50416b767fd3cce0fb5027bb5dfec58c, and 64c045e25b8471bbb572bd29159c294a82a86a25 which were reverted in e75d8b70370435b0ad10388afba0df45fcf9bfcc due to a crasher bug where CodeGen would emit a builtin glvalue as an rvalue if it constant-folds. Reviewed By: aaron.ballman Differential Revision: https://reviews.llvm.org/D123345
//===----------------------------------------------------------------------===// // C Language Family Front-end //===----------------------------------------------------------------------===// Welcome to Clang. This is a compiler front-end for the C family of languages (C, C++, Objective-C, and Objective-C++) which is built as part of the LLVM compiler infrastructure project. Unlike many other compiler frontends, Clang is useful for a number of things beyond just compiling code: we intend for Clang to be host to a number of different source-level tools. One example of this is the Clang Static Analyzer. If you're interested in more (including how to build Clang) it is best to read the relevant web sites. Here are some pointers: Information on Clang: http://clang.llvm.org/ Building and using Clang: http://clang.llvm.org/get_started.html Clang Static Analyzer: http://clang-analyzer.llvm.org/ Information on the LLVM project: http://llvm.org/ If you have questions or comments about Clang, a great place to discuss them is on the Clang forums: https://discourse.llvm.org/c/clang/ If you find a bug in Clang, please file it in the LLVM bug tracker: http://llvm.org/bugs/