tl;dr Build times on all platforms, in particular for incremental builds, have decreased in the last weeks by landing several cleanups to C++ include dependencies that reduce the aggregated number of included files by about 30%.
Hi, Did you notice a reduction in build times lately? This might not be a coincidence. In this mail, I want to provide some details on the improvements made. I want to thank everyone who contributed to this through up-front discussions and reviews. Recently I landed a number of patches on Bug 1676346  that in various ways clean up C++ include directives in our codebase. Some landed ca. 3 weeks ago, some landed last week. Overall, these reduce the aggregated number of included non-system header files in a full build by about 30% on Linux. I don't have numbers for other platforms, but they should benefit as well. On my machine, this reduced the time for a clobber build by about 10%. While that might go unnoticed, incremental builds are often sped up even a lot more, since the number of translation units that need to be rebuilt decreases. E.g. the number of translation units that include dom/base/Document.h reduced by ca. 52%, resulting in a build time reduction of 48% on my machine after modifying that. Your mileage may vary. While this might not spare you from buying new hardware, it will make builds faster regardless of the hardware you are running on, and hopefully increase productivity. If you want to share your experiences with me, please get in touch! You might be curious what I did to achieve that, or how you can contribute to reducing build times yourself. It's a combination of things, most importantly three things: 1. Remove unused include directives 2. Split some headers 3. Use forward declarations more 4. Hide definitions to allow using forward declarations even more About 1: I found there are several include directives that are not needed at all. They could simply be removed. However, the majority of cases were a bit more complex, because of a lot of missing include directives. When removing an include for X.h from a header file Y.h that doesn't need it, another file that included Y.h might need X.h. Or, Y.h itself might need something from a header indirectly included from X.h. Or similar cases. This meant quite a lot of more include directives for more basic things needed to be added to ensure the build doesn't break. About 2: Some headers have a lot of dependencies, but only relatively few users of that header need them. One example was IPCMessageUtils.h, which is included by all files generated by IPDL, which also contained a lot of specializations of the ParamTraits template that are needed only by few files. Apart from some very basic specializations, these were moved to the new EnumSerializers.h and IPCMessageUtilsSpecializations.h as well as some existing headers, so that the remaining IPCMessageUtils.h header has only much more limited dependencies. About 3: Our coding style favors forward declarations over inclusion of the full definition of types where possible. I replaced the inclusion of header files containing full definitions of types by forward declarations at a number of places where this is sufficient, e.g. because they are only used in function signatures. It's worth noting that there were also a number of cases where a forward declaration was present, but actually the full definition is required, e.g. when a type is used as a base class or as the value type of a data member, or an inline function body dereferences into a member. About 4: As mentioned in the last point, inline function bodies often require the inclusion of additional headers because they dereference into types of which otherwise only forward declarations were necessary. Similar considerations apply to private (nested) classes. Some of those were moved to the corresponding implementation files to hide these dependencies, and reduce the number of necessary includes in the header files. I was using some tools to support this, notably ClangBuildAnalyzer  and include-what-you-use . ClangBuildAnalyzer helped to detect headers that are expensive to parse throughout the build, and direct efforts to reduce those specifically. But there remains a long tail of similar things that can and should be fixed. include-what-you-use helps to identify the headers from which a file uses declarations to avoid depending on indirectly included files which might disappear from the include chain by unrelated changes. These tools are not readymade for everyday use, but I will try to provide some ideas for using them effectively later on. Best wishes Simon  https://bugzilla.mozilla.org/show_bug.cgi?id=1676346  https://github.com/aras-p/ClangBuildAnalyzer  https://github.com/include-what-you-use/include-what-you-use _______________________________________________ dev-platform mailing list firstname.lastname@example.org https://lists.mozilla.org/listinfo/dev-platform