8eb0dfe5b6
See https://clang.llvm.org/docs/StandardCPlusPlusModules.html#experimental-non-cascading-changes for the full background. In short, we want to cut off the dependencies to improve the recompilation time. And namespace is special, as the same namespace can occur in many TUs. This patch tries to clean up unneeded reference to namespace from other module file. The touched parts are: - When write on disk lookup table, don't write the merged table. This makes the ownership/dependency much cleaner. For performance, I think the intention was to make the cost of mergin table amortized. But in our internal workloads, I didn't observe any regression if I turn off writing the merged table. - When writing redeclarations, only write the ID of the first namespace and avoid referenece to all other previous namespaces. (I don't want to write the first namespace either... but it is more complex). - When writing the name lookup table, try to write the local namespace. @jtstogel @mpark I want to invite you to test this with your internal workloads to figure out the correctness and the performance impact on this. I know I can make the change clean for you by inserting code guarded by "if writing named module" but I think it will be much better if we can make the underlying implementation more homogeneous if possible.
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