Author: Chuanqi Xu Date: 2023-07-19T15:49:56+08:00 New Revision: 0d6ea6fc3b137b92809be740544e2715a7b0ba65
URL: https://github.com/llvm/llvm-project/commit/0d6ea6fc3b137b92809be740544e2715a7b0ba65 DIFF: https://github.com/llvm/llvm-project/commit/0d6ea6fc3b137b92809be740544e2715a7b0ba65.diff LOG: [NFC] [C++20] [Modules] Add a performance tip to the document It is a known problem that the compiler isn't efficiency when there are a lot of duplications in the modules to developers. But it may not be verbose to the (potential) new users of C++20 modules. It may be necessary to mention this in the document. Added: Modified: clang/docs/StandardCPlusPlusModules.rst Removed: ################################################################################ diff --git a/clang/docs/StandardCPlusPlusModules.rst b/clang/docs/StandardCPlusPlusModules.rst index 0e0e7967511869..f7778ecaeff670 100644 --- a/clang/docs/StandardCPlusPlusModules.rst +++ b/clang/docs/StandardCPlusPlusModules.rst @@ -501,6 +501,95 @@ is attached to the global module fragments. For example: Now the linkage name of ``NS::foo()`` will be ``_ZN2NS3fooEv``. +Performance Tips +---------------- + +Reduce duplications +~~~~~~~~~~~~~~~~~~~ + +While it is legal to have duplicated declarations in the global module fragments +of diff erent module units, it is not free for clang to deal with the duplicated +declarations. In other word, for a translation unit, it will compile slower if the +translation unit itself and its importing module units contains a lot duplicated +declarations. + +For example, + +.. code-block:: c++ + + // M-partA.cppm + module; + #include "big.header.h" + export module M:partA; + ... + + // M-partB.cppm + module; + #include "big.header.h" + export module M:partB; + ... + + // other partitions + ... + + // M-partZ.cppm + module; + #include "big.header.h" + export module M:partZ; + ... + + // M.cppm + export module M; + export import :partA; + export import :partB; + ... + export import :partZ; + + // use.cpp + import M; + ... // use declarations from module M. + +When ``big.header.h`` is big enough and there are a lot of partitions, +the compilation of ``use.cpp`` may be slower than +the following style significantly: + +.. code-block:: c++ + module; + #include "big.header.h" + export module m:big.header.wrapper; + export ... // export the needed declarations + + // M-partA.cppm + export module M:partA; + import :big.header.wrapper; + ... + + // M-partB.cppm + export module M:partB; + import :big.header.wrapper; + ... + + // other partitions + ... + + // M-partZ.cppm + export module M:partZ; + import :big.header.wrapper; + ... + + // M.cppm + export module M; + export import :partA; + export import :partB; + ... + export import :partZ; + + // use.cpp + import M; + ... // use declarations from module M. + +The key part of the tip is to reduce the duplications from the text includes. + Known Problems -------------- _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
