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> Currently, there's too many instances to add any sort of warning for this: using some relatively basic heuristics (a copy is considered potentially-expensive if the type is not trivially copyable or trivially copy constructible—or if it is, but the size of the type is > 32 bytes: this was chosen somewhat arbitrarily to avoid warning about iterating across a map of string_view to string_view), there's about 400+ potentially-expensive copies
For my education: how did you gather this data? Via a clang plugin or something?
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As a reviewer I would find "explicit type means intentional copy" to be overly obscure - it's one more rule of thumb I'd need to remember in a huge list of existing C++ corner cases.
On Tue, Apr 8, 2025 at 11:56 AM 'Joe Mason' via cxx <c...@chromium.org> wrote:As a reviewer I would find "explicit type means intentional copy" to be overly obscure - it's one more rule of thumb I'd need to remember in a huge list of existing C++ corner cases.+1Also +1 to doing whatever we can with existing clang-tidy checks like performance-for-range-copy
Also +1 to doing things with clang-tidy + Tricium (as opposed to clang plugins) in general
PK
On Tue, 8 Apr 2025 at 15:02, Peter Kasting <pkas...@chromium.org> wrote:On Tue, Apr 8, 2025 at 11:56 AM 'Joe Mason' via cxx <c...@chromium.org> wrote:As a reviewer I would find "explicit type means intentional copy" to be overly obscure - it's one more rule of thumb I'd need to remember in a huge list of existing C++ corner cases.+1Also +1 to doing whatever we can with existing clang-tidy checks like performance-for-range-copyI've already noted that the existing clang-tidy check isn't sufficient for catching the sort of bugs we're interested in.
Also +1 to doing things with clang-tidy + Tricium (as opposed to clang plugins) in generalI suspect we'll end up here eventually, but I think it's important to point out that clang-tidy does not cover all platforms. I'm not sure if there's a plan to fix that.
On Tue, Apr 8, 2025, 3:06 PM Daniel Cheng <dch...@chromium.org> wrote:On Tue, 8 Apr 2025 at 15:02, Peter Kasting <pkas...@chromium.org> wrote:On Tue, Apr 8, 2025 at 11:56 AM 'Joe Mason' via cxx <c...@chromium.org> wrote:As a reviewer I would find "explicit type means intentional copy" to be overly obscure - it's one more rule of thumb I'd need to remember in a huge list of existing C++ corner cases.+1Also +1 to doing whatever we can with existing clang-tidy checks like performance-for-range-copyI've already noted that the existing clang-tidy check isn't sufficient for catching the sort of bugs we're interested in.I saw the comment about not catching code that mutates a copy when it intended to mutate the original (by non const ref). I assume this isn't frequent, since (a) it feels unusual and (b) I would think getting this wrong would result in functional errors that would actually make the code visibly broken the majority of the time.
Were there other problems this also doesn't catch?My tough-love take is: I'm way more worried about fixing unintentional copies due to bad arg passing (e.g. passing large types by value and not moving on the caller side) than unintentional copies due to value type in range-for loops. I'm sure the former happens much more than a few hundred places in the codebase, given how frequently I run into it. I don't know whether there's any opportunity cost here -- maybe people willing to fix one of these would never have worked on fixing the other anyway -- but if there is, then the latter issue is where I'd choose to spend resources.