[clang] [clang][deps] Generate command lines lazily (PR #65691)

[Jan Svoboda via cfe-commits]

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@@ -136,9 +138,15 @@ struct ModuleDeps {
   /// determined that the differences are benign for this compilation.
   std::vector<ModuleID> ClangModuleDeps;
 
-  /// Compiler invocation that can be used to build this module. Does not
-  /// include argv[0].
-  std::vector<std::string> BuildArguments;
+  /// Get (or compute) the compiler invocation that can be used to build this
+  /// module. Does not include argv[0].
+  const std::vector<std::string> &getBuildArguments();
+
+private:
+  friend class ModuleDepCollectorPP;
+
+  std::variant<std::shared_ptr<CowCompilerInvocation>, std::vector<std::string>>
+      BuildInvocationOrArguments;
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jansvoboda11 wrote:

My concern is that if I use `std::shared_ptr<CowCompilerInvocation>`, `ModuleDeps` stops having value semantics. I guess that would be solved by making it a `std::shared_ptr<const CowCompilerInvocation>` to prevent mutation of the invocation.

Yes, both inlined and indirect storage use the same amount of storage (+- 8B), but I was thinking about this from memory locality perspective (storing the rarely used invocation away from frequently used data). But I guess this data structure is full of pointers to heap anyways, so that doesn't really matter.

I guess I'll just go with the inline approach to save on boilerplate.

https://github.com/llvm/llvm-project/pull/65691