On Thu, 16 Jan 2025 11:28:55 GMT, Stefan Karlsson <stef...@openjdk.org> wrote:
> HotSpot uses precompiled headers to speed up non-incremental builds. The > current mechanism requires all .cpp files to include precompiled.hpp as the > first non-comment line. This requires HotSpot devs to think about precompiled > headers when they create new files and when they need to update/manage the > include sections. > > All the compilers that we use support using precompiled headers without > explicitly including the associated header in the source code. I propose that > we use that capability instead. > > The make files changes to support this are: > 1) Remove share/precompiled/ from the list of include directories, > 2) Compiler-specific changes to support this: > > *Windows cl*: > * Add the share/precompiled/ include directory when generating the PCH file. > * Use /FI to indicate that we should include precompiled.hpp. /Yu says that > it should use a PCH for precompiled.hpp. /Fp tells the compiler that it > should pick up the named precompiled header. > > From experiments it seems like it doesn't matter what name I pass in to /FI > and /Yu, they just have to be the same and the specified file doesn't even > have to exist, as long as we also pass in the /Fp flag we get the benefits of > the precompiled header. > > *gcc*: > * Use -include to include the precompiled.hpp file. Note that the compilation > command line will not have share/precompiled in the include path and that the > precompiled header will still be picked up correctly. > > *clang*: > * No changes needed, the -include-pch line is all that we need. > > I've verified that these changes give the expected compilation time > reductions both by comparing compilation times in our CI but also by running > individual compilations of .cpp files on Linux, Mac, and Windows. > > Note that the compiler will complain if someone now tries to include > precompiled.hpp explicitly. > > Note also that we have a section about precompiled.hpp in the HotSpot style > guide. If this proposal is accepted I would like to update the style guide as > a separate RFE. > > Tested by letting GHA build and running tier1-2 in our CI pipeline. I believe I had cases where I explicitly needed to avoid using the precompiled mechanism for a file. That is still possible, right? ------------- PR Comment: https://git.openjdk.org/jdk/pull/23146#issuecomment-2595605730
