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This reverts commit 33be50daa9ce1074c3b423a4ab27c70c0722113a, effectively reapplying: - 260a856c2abcef49c7cb3bdcd999701db3e2af38 - 3043e5a5c33c4c871f4a1dfd621a8839f9a1f0b3 - 49142991a685bd427d7e877c29c77371dfb7634c ... with a fix to skip a call to `SmallVector::isReferenceToStorage()` when we know the parameter had been taken by value for small, POD-like `T`. See https://reviews.llvm.org/D93779 for the discussion on the revert. At a high-level, these commits fix reference invalidation in SmallVector's push_back, append, insert (one or N), and resize operations. For more details, please see the original commit messages. This commit fixes a bug that crept into `SmallVectorTemplateCommon::reserveForAndGetAddress()` during the review process after performance analysis was done. That function is now called `reserveForParamAndGetAddress()`, clarifying that it only works for parameter values. It uses that knowledge to bypass `SmallVector::isReferenceToStorage()` when `TakesParamByValue`. This is `constexpr` and avoids adding overhead for "small enough", trivially copyable `T`. Performance could potentially be tuned further by increasing the threshold for `TakesParamByValue`, which is currently defined as: ``` bool TakesParamByValue = sizeof(T) <= 2 * sizeof(void *); ``` in the POD-like version of SmallVectorTemplateBase (else, `false`). Differential Revision: https://reviews.llvm.org/D94800
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