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As suggested by @efriedma in: https://reviews.llvm.org/D76096#4370369 This should speed up evaluating whether an expression is constant or not, but due to the complexity of these two different implementations, we may start getting different answers for edge cases for which we do not yet have test cases in-tree (or perhaps even performance regressions for some cases). As such, contributors have carte blanche to revert if necessary. For additional historical context about ExprConstant vs CGExprConstant, here's snippets from a private conversation on discord: ndesaulniers: why do we have clang/lib/AST/ExprConstant.cpp and clang/lib/CodeGen/CGExprConstant.cpp? Does clang constant fold during ast walking/creation AND during LLVM codegen? efriedma: originally, clang needed to handle two things: integer constant expressions (the "5" in "int x[5];"), and constant global initializers (the "5" in "int x = 5;"). pre-C++11, the two could be handled mostly separately; so we had the code for integer constants in AST/, and the code for globals in CodeGen/. C++11 constexpr sort of destroyed that separation, though. so now we do both kinds of constant evaluation on the AST, then CGExprConstant translates the result of that evaluation to LLVM IR. but we kept around some bits of the old cgexprconstant to avoid performance/memory usage regressions on large arrays. Reviewed By: efriedma Differential Revision: https://reviews.llvm.org/D151587
//===----------------------------------------------------------------------===// // 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/