[webkit-dev] Platform.h vs. makefiles

[Maciej Stachowiak]

I guess it depends on value of this (and feasibility of IDE highlighting based on a generated source file) vs value of having a common approach for compile-time and runtime flags. Runtime flags and their default values likely can’t be effectively expressed in headers.

Separate file also lets us do things like make build systems exclude files based on feature flag values, though of course, an alternative is to surround whole files with #include guards and always build everything.

> On May 11, 2020, at 9:34 AM, Alexey Proskuryakov <ap at webkit.org> wrote:
> 
> 
> I see substantial appeal in having a separate data file, however I'm not sure if it can inform IDE parsing and syntax highlighting for code enabled/disabled at compile time. Header files seem like they would get that right more often.
> 
> - Alexey
> 
> 
>> 10 мая 2020 г., в 10:07 PM, Maciej Stachowiak <mjs at apple.com <mailto:mjs at apple.com>> написал(а):
>> 
>> 
>> I think the best way to configure WebKit is to use a separate data file, neither a header nor a makefile or similar, that defines the options in a single place with clear syntax. Then everything else is generated from that. Such a system could cover runtime flags as well, which are even more scattered around the project than compile-time flags.
>> 
>> Moving logic from build files to the header is probably a move in the right direction, though of course it carries risk, particularly for less tested configurations.
>> 
>> -  Maciej
>> 
>>> On May 10, 2020, at 5:13 PM, Darin Adler <darin at apple.com <mailto:darin at apple.com>> wrote:
>>> 
>>> Hi folks.
>>> 
>>> The Platform.h configuration file family has been useful for WebKit for a long time. We created it to try to organize configuration options in WebKit so the would not be spread out through the whole project.
>>> 
>>> One way to look at these, particularly the ENABLE options, is as a set of configuration options that let each consumer of the WebKit source code create a unique flavor of WebKit with the particular features they want enabled turned on and others turned off. Another is to think of this as a mechanism for keeping decisions made by the WebKit contributors organized and easy to understand.
>>> 
>>> If these truly are WebKit consumer options, then it’s important they be set as part of the build process. The macros can be defined with a build and configuration system outside WebKit, and the Platform.h headers should interpret those values correctly.
>>> 
>>> On the other hand, if we are just trying to keep our decisions straight, then it would be best if the logic for what is on and what is off by in the header files, written with preprocessor macro logic, and not spread across multiple make files and scripts.
>>> 
>>> Thus I think the pattern on macOS, for example, of setting these in .xcconfig files doesn’t make a lot of sense. I think the .xcconfig files should compute the things that need to be determined about the build environment, but the configuration decisions should be in files like PlatformHaveCocoa.h, for example.
>>> 
>>> I think the guideline should be like this:
>>> 
>>> All code to compute configuration should be in the Platform.h files, not in makefiles, with only the following exceptions:
>>> 
>>> 1) Options like ENABLE(XXX) that some ports wish to offer as options to people building WebKit can have overridden values in makefiles. But even these options should have sensible defaults in the Platform.h headers that express the current status of the port from the point of view of the port’s maintainers. Ideally we’d find a way to not repeat these default settings twice.
>>> 
>>> 2) In some cases, the build machinery needs to contribute to things like feature detection. So on some platforms, things like HAVE(READLINE) would be set correctly by the build system.
>>> 
>>> Options intended to be set by the environment would carefully be wrapped in #ifndef.
>>> 
>>> But other options, which simply express relationships between configuration elements, are designed to be set by Platform.h and not overridden by the environment, and so they would not be wrapped in #ifndef. Many HAVE options and most USE options would fall into this category.
>>> 
>>> What do you all think?
>>> 
>>> — Darin
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>> 
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