[webkit-dev] -Wpessimizing-move and -Wredundant-move

Andy Estes aestes at apple.com
Mon Mar 18 14:43:25 PDT 2019

Hi Michael,

> On Mar 18, 2019, at 2:35 PM, Michael Catanzaro <mcatanzaro at igalia.com> wrote:
> Hi,
> GCC 9 has new -Wpessimizing-move ("warning: moving a local object in a return statement prevents copy elision") and -Wredundant-move ("warning: redundant move in return statement") warnings. These are enabled by -Wextra (which we use) and will be triggered by code like:
> return WTFMove(foo);
> when foo is a copyable type. This idiom is only appropriate if foo is a noncopyable (move-only) type. We have many, many instances of such code. I'm trying to fix them. Please don't add more! (It's easy enough to disable the warnings, but they're there for a reason.)

It might not even be appropriate for move-only types because of NRVO.

FWIW, Apple’s ports use the equivalent clang warning for pessimizing and redundant moves, and we cleaned up a bunch of these mistakes in our ports a few years ago. Hopefully you aren’t finding any of these mistakes in shared code.

(WTFMove was written to accommodate these warnings, in fact.)

Really glad you’re turning these warnings on!


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