[clang][deps] Introduce copy-on-write CompilerInvocation
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swiftlang/swift#68503 |
This function has the same semantics as `std::shared_ptr<T>::use_count()`, and enables implementing copy-on-write semantics.
This enables making the whole `CompilerInvocation` more efficient through copy-on-write.
This PR introduces new copy-on-write `CompilerInvocation` class (`CowCompilerInvocation`), which will be used by the dependency scanner to reduce the number of copies performed when generating command lines for discovered modules.
We create one `CompilerInvocation` for each modular dependency we discover. This means we create a lot of copies, even though most of the invocation is the same between modules. This patch makes use of the copy-on-write flavor of `CompilerInvocation` to share the common parts, reducing memory usage and speeding up the scan.
This patch makes the generation of command lines for modular dependencies lazy/on-demand. That operation is somewhat expensive and prior to this patch used to be performed multiple times for the identical `ModuleDeps` (i.e. when they were imported from multiple different TUs).
…lvm#66301) This avoids making copies at the end of `makeCommonInvocationForModuleBuild()` (in `ModuleDepCollector.cpp`).
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