[llvm] WIP [ADT][NFC] SmallVector performance improvements (PR #169291)

[via llvm-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Linux-->
# :penguin: Linux x64 Test Results

* 96688 tests passed
* 1758 tests skipped

All tests passed but another part of the build **failed**. Click on a failure below to see the details.

<details>
<summary>unittests/ADT/CMakeFiles/ADTTests.dir/SmallVectorTest.cpp.o</summary>

```
FAILED: unittests/ADT/CMakeFiles/ADTTests.dir/SmallVectorTest.cpp.o
sccache /opt/llvm/bin/clang++ -DGTEST_HAS_RTTI=0 -DLLVM_BUILD_STATIC -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_USE_CXX11_ABI=1 -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/unittests/ADT -I/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/unittests/ADT -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/third-party/unittest/googletest/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/third-party/unittest/googlemock/include -gmlt -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wno-pass-failed -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=c++17  -Wno-variadic-macros -Wno-gnu-zero-variadic-macro-arguments -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -Wno-suggest-override -MD -MT unittests/ADT/CMakeFiles/ADTTests.dir/SmallVectorTest.cpp.o -MF unittests/ADT/CMakeFiles/ADTTests.dir/SmallVectorTest.cpp.o.d -o unittests/ADT/CMakeFiles/ADTTests.dir/SmallVectorTest.cpp.o -c /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/unittests/ADT/SmallVectorTest.cpp
In file included from /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/unittests/ADT/SmallVectorTest.cpp:13:
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include/llvm/ADT/SmallVector.h:634:27: error: first argument in call to 'memset' is a pointer to non-trivially copyable type '(anonymous namespace)::NonCopyable' [-Werror,-Wnontrivial-memcall]
634 |         std::memset(this->end(), 0, (N - this->size()) * sizeof(T));
|                           ^
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include/llvm/ADT/SmallVector.h:648:30: note: in instantiation of function template specialization 'llvm::SmallVectorImpl<(anonymous namespace)::NonCopyable>::resizeImpl<false>' requested here
648 |   void resize(size_type N) { resizeImpl<false>(N); }
|                              ^
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/unittests/ADT/SmallVectorTest.cpp:172:5: note: in instantiation of member function 'llvm::SmallVectorImpl<(anonymous namespace)::NonCopyable>::resize' requested here
172 |   V.resize(42);
|     ^
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include/llvm/ADT/SmallVector.h:634:27: note: explicitly cast the pointer to silence this warning
634 |         std::memset(this->end(), 0, (N - this->size()) * sizeof(T));
|                           ^
|                     (void*)
1 error generated.
```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/169291