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The motivating use case is to support import the function declaration across modules to construct call graph edges for indirect calls [1] when importing the function definition costs too much compile time (e.g., the function is too large has no `noinline` attribute). 1. Currently, when the compiled IR module doesn't have a function definition but its postlink combined summary contains the function summary or a global alias summary with this function as aliasee, the function definition will be imported from source module by IRMover. The implementation is in FunctionImporter::importFunctions [2] 2. In order for FunctionImporter to import a declaration of a function, both function summary and alias summary need to carry the def / decl state. Specifically, all existing summary fields doesn't differ across import modules, but the def / decl state of is decided by `<ImportModule, Function>`. This change encodes the def/decl state in `GlobalValueSummary::GVFlags`. In the subsequent changes 1. The indexing step `computeImportForModule` [3] will compute the set of definitions and the set of declarations for each module, and passing on the information to bitcode writer. 2. Bitcode writer will look up the def/decl state and sets the state when it writes out the flag value. This is demonstrated in https://github.com/llvm/llvm-project/pull/87600 3. Function importer will read the def/decl state when reading the combined summary to figure out two sets of global values, and IRMover will be updated to import the declaration (aka linkGlobalValuePrototype [4]) into the destination module. - The next change is https://github.com/llvm/llvm-project/pull/87600 [1] mentioned in rfc https://discourse.llvm.org/t/rfc-for-better-call-graph-sort-build-a-more-complete-call-graph-by-adding-more-indirect-call-edges/74029#support-cross-module-function-declaration-import-5 [2]3b337242ee/llvm/lib/Transforms/IPO/FunctionImport.cpp (L1608-L1764)[3]3b337242ee/llvm/lib/Transforms/IPO/FunctionImport.cpp (L856)[4]3b337242ee/llvm/lib/Linker/IRMover.cpp (L605)
//===----------------------------------------------------------------------===// // C Language Family Front-end //===----------------------------------------------------------------------===// Welcome to Clang. This is a compiler front-end for the C family of languages (C, C++, Objective-C, and Objective-C++) which is built as part of the LLVM compiler infrastructure project. Unlike many other compiler frontends, Clang is useful for a number of things beyond just compiling code: we intend for Clang to be host to a number of different source-level tools. One example of this is the Clang Static Analyzer. If you're interested in more (including how to build Clang) it is best to read the relevant web sites. Here are some pointers: Information on Clang: http://clang.llvm.org/ Building and using Clang: http://clang.llvm.org/get_started.html Clang Static Analyzer: http://clang-analyzer.llvm.org/ Information on the LLVM project: http://llvm.org/ If you have questions or comments about Clang, a great place to discuss them is on the Clang forums: https://discourse.llvm.org/c/clang/ If you find a bug in Clang, please file it in the LLVM bug tracker: http://llvm.org/bugs/