Marco van de Voort wrote: > > Do you mean that only one level of dependencies must be checked with > > "uses", whereas even the indirectly #included files must be checked for > > changes? > > You always have to do the #include. Always. Pre-compiled headers are > possible, not trivial, since it requires a comparison of the entire > preprocessor state to see if a header needs recompilation.
With precompiled headers it's almost like with compiled units: every compiled file contains a list of dependend files, dates and/or checksums. If something has changed, the affected unit or module must be recompiled. If nothing has changed, the preprocessor states can not have changed as well. > > > That allows the compiler to auto-find the compilation order.. A compilation order only applies to units. A Pascal compiler or linker starts with the root unit, and then descends into all used units. C projects instead consist of a single (flat) list of modules, which can be compiled in any order. > > A C project doesn't require any compilation order, every module can be > > compiled independently from other modules. > > Implementation: yes. Header: no. Hmm, perhaps we have a different idea of precompiled header files. Let me explain what BC 3.1 did: Header files are not precompiled for themselves, one by one, instead there must exist something like a root module (*.c) which contains a list of #include's. These included files are precompiled together, in the given order. Multiple modules can share the same precompiled headers only if the modules #include the same header files in the same sequence. The precompiled header files, for all modules of a project, reside in the same precompiled file. This file in fact is a library, containing distinct sets of precompiled headers, in different extent or order. IOW modules with exactly the same #include's in sequence share a single precompiled header module. The file may be organized as a tree of header modules, with bifurcations when different files are #included after a preceding identical sequence. Borland also introduced a #pragma headerstop, that delimits the list of precompiled header files. All modules with the same #include's before this pragma can share the same header module, eventually following #includes are not precompiled. This feature allows to precompile a reasonable number of header files into one header module. AFAIR it was easier to #include exactly the same header files in all modules of a project, regardless of the actual needs of every module. The resulting precompiled header file, for a Win3.1 project, then could be as small as 20 MB, in contrast to > 80 MB for less, but different, #includes in the source modules. Regardless of such optimization, as long as the header files are not touched themselves, and the #include's in the source modules are unchanged, a recompilation of the header files is never required. DoDi _______________________________________________ fpc-devel maillist - fpc-devel@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-devel
