1 Commits

Author	SHA1	Message	Date
Malcolm Parsons	321f24ec42	Diagnose cases of "return x" that should be "return std::move(x)" for efficiency ... Summary: This patch adds two new diagnostics, which are off by default: **-Wreturn-std-move** This diagnostic is enabled by `-Wreturn-std-move`, `-Wmove`, or `-Wall`. Diagnose cases of `return x` or `throw x`, where `x` is the name of a local variable or parameter, in which a copy operation is performed when a move operation would have been available. The user probably expected a move, but they're not getting a move, perhaps because the type of "x" is different from the return type of the function. A place where this comes up in the wild is `stdext::inplace_function<Sig, N>` which implements conversion via a conversion operator rather than a converting constructor; see https://github.com/WG21-SG14/SG14/issues/125#issue-297201412 Another place where this has come up in the wild, but where the fix ended up being different, was try { ... } catch (ExceptionType ex) { throw ex; } where the appropriate fix in that case was to replace `throw ex;` with `throw;`, and incidentally to catch by reference instead of by value. (But one could contrive a scenario where the slicing was intentional, in which case throw-by-move would have been the appropriate fix after all.) Another example (intentional slicing to a base class) is dissected in https://github.com/accuBayArea/Slides/blob/master/slides/2018-03-07.pdf **-Wreturn-std-move-in-c++11** This diagnostic is enabled only by the exact spelling `-Wreturn-std-move-in-c++11`. Diagnose cases of "return x;" or "throw x;" which in this version of Clang *do* produce moves, but which prior to Clang 3.9 / GCC 5.1 produced copies instead. This is useful in codebases which care about portability to those older compilers. The name "-in-c++11" is not technically correct; what caused the version-to-version change in behavior here was actually CWG 1579, not C++14. I think it's likely that codebases that need portability to GCC 4.9-and-earlier may understand "C++11" as a colloquialism for "older compilers." The wording of this diagnostic is based on feedback from @rsmith. **Discussion** Notice that this patch is kind of a negative-space version of Richard Trieu's `-Wpessimizing-move`. That diagnostic warns about cases of `return std::move(x)` that should be `return x` for speed. These diagnostics warn about cases of `return x` that should be `return std::move(x)` for speed. (The two diagnostics' bailiwicks do not overlap: we don't have to worry about a `return` statement flipping between the two states indefinitely.) I propose to write a paper for San Diego that would relax the implicit-move rules so that in C++2a the user //would// see the moves they expect, and the diagnostic could be re-worded in a later version of Clang to suggest explicit `std::move` only "in C++17 and earlier." But in the meantime (and/or forever if that proposal is not well received), this diagnostic will be useful to detect accidental copy operations. Reviewers: rtrieu, rsmith Reviewed By: rsmith Subscribers: lebedev.ri, Rakete1111, rsmith, cfe-commits Tags: #clang Differential Revision: https://reviews.llvm.org/D43322 Patch by Arthur O'Dwyer. llvm-svn: 329914	2018-04-12 14:48:48 +00:00