[webkit-dev] Proposal for a new way to handle porting #ifdefs

[Ojan Vafai]

This looks great to me. It will definitely make ports more maintainable.
For thoroughness sake, it would be good to add a bit about what should be a
USE define versus a setting. For example, the EditingBehavior enum in
Settings.h could just as easily be a USE define. I think it makes sense in
this case to have it be runtime settable because then we can write layout
tests that test both Windows and Mac editing behaviors.
Ojan

On Thu, Apr 30, 2009 at 4:12 PM, Maciej Stachowiak <mjs at apple.com> wrote:

>
> I think our set of porting macros has become somewhat confused.
>
> Originally, our idea was that a port represents primarily adaptation to a
> particular platform. However, over time it has become clear that most of
> what is decided by a port is not platform adaptation, but rather policy
> decisions. For example, ports decide to have different features enabled, or
> to use different sets of system functionality on the same underlying OS.
>
> In addition, I think the catchall top-level PLATFORM create confusion,
> because it is not totally clear if they are policy decisions, platform
> adaptation decisions, or what.
>
> Third, it seems wrong that the policy choices of every port are represented
> as a bunch of ifdef tomfoolery inside a single Platform.h file.
>
> And fourth, many ports often run on the same OS, but with a different set
> of choices - for example on Mac OS X it is possible to build the Mac,
> Chromium, Gtk, Qt and Wx ports (at least).
>
>
> Therefore, I propose that we change as follows:
>
> 1) Strictly separate platform adaptation (mandatory to run on a given OS,
> compiler, or CPU at all) from policy choices (what features to enable, what
> optional libraries to use).
>
> 2) Phase out PLATFORM macros completely - each use should be converted to a
> policy choice, or a platform adaptation decision.
>
> 3) Instead of ports being defined by a top-level PLATFORM macro, I propose
> that each port should have its own header file to define policy decisions.
> For example, I'd propose that the system Mac OS X WebKit should use
> PortCocoa.h, and the WebKit used by Safari for Windows should use
> PortWinCG.h. There may also be a PortIPhone.h. These port definition headers
> would live in their own top-level WebKit module. Each one would be
> completely owned by whoever is generally considered the "owner" of a given
> port. Because related ports on different platforms may wish to share policy
> choices, it's ok for Port headers to include shared headers for some
> choices. For example, all Apple-maintained ports may include PortApple.h. We
> could go even further and have PortDefault.h to make default choices of what
> features are enabled, that ports would have to explicitly override.
>
> 4) Platform adaptation macros would still be defined in Platform.h based on
> sniffing the environment, this would include things like the compiler, the
> underlying OS, available libc functions, and so forth.
>
>
> Platform adaptation macros would be:
>
> OS() - underlying operating system; only to be used for mandated low-level
> services like virtual memory, not to choose a GUI toolkit
>    Examples:
>        OS(UNIX) - Any Unix-like OS
>        OS(DARWIN) - Underlying OS is the base OS X environment
>        OS(FREEBSD) - FreeBSD
>        OS(WIN) - Any version of Windows
>        OS(WINCE) - The embedded version of Windows
>
> COMPILER() - the compiler being used to build the project
>    Examples:
>        COMPILER(GCC) - GNU Compiler Collection
>        COMPILER(MSVC) - Microsoft Visual C++
>        COMPILER(RVCT) - ARM compiler
>
> HAVE() - specific system features (headers, functions or similar) that are
> present or not
>    Examples:
>        HAVE(MMAP) - mmap() function is available
>        HAVE(ERRNO_H) - errno.h header is available
>        HAVE(MADV_FREE) - madvise(MADV_FREE) is available
>
>
> Policy decision macros would be:
>
> USE() - use a particular third-party library or optional OS service
>    Examples:
>        USE(SKIA) - Use the Skia graphics library
>        USE(CG) - Use CoreGraphics
>        USE(V8) - Use the V8 JavaScript implementation
>        USE(CFNET) - Use CFNetwork networking
>        USE(NSURL_NET) - Use NSURLConnection-based networking
>        USE(APPKIT) - Use AppKit views and events
>        USE(GTK) - Use Gtk+
>        USE(QT) - Use Qt
>        USE(QUICKTIME) - Use the QuickTime media engine
>        USE(QTKIT) - Use the QuickTime media engine via the Mac QTKit API
>        USE(QUICKTIME_WIN) - Use the QuickTime media engine via its Windows
> API
>
> ENABLE() - turn on a specific feature of WebKit
>    Examples:
>       ENABLE(ACCESSIBILITY) - Enable support for assistive technologies
> (currently wrongly a HAVE)
>       ENABLE(XSLT) - Include XSLT support
>       ENABLE(OBJC_MAC_API) - Include Objective C API based on NSViews
> (current WebKit Mac)
>       ENABLE(OBJC_DOM_API) - Include Objective C DOM bindings (may apply to
> other ObjC toolkits than AppKit)
>       ENABLE(JSC) - Enable use of the JavaScriptCore implementation
> (inconsistent with V8 because JSC is a WebKit feature but V8 is an external
> dependency, even though they serve similar purposes)
>       ENABLE(VIDEO) - Enable support for the HTML5 Video element
>       ENABLE(SVG) - Enable support for SVG (Scalable Vector Graphics)
>       ENABLE(WML) - Enable support for WML
>
>
>
> Some macros that would be completely phased out, in favor of platform and
> policy decisions:
>
> PLATFORM(MAC) - A mix of things that should be USE(APPKIT), USE(NSURL_NET),
> ENABLE(OBJC_MAC_API) and a host of other things
> PLATFORM(WIN) - Hodgepodge of mandatory platform adaptation, optional
> platform adaptation, and choices specific to Apple's Mac Port
> PLATFORM(GTK) - Most of this would be replaced by USE(GTK) but perhaps
> different policy macros are appropriate in some cases.
> PLATFORM(CHROMIUM) - Grab-bag of various policy choices.
>
>
> I believe that with this new proposal, ifdefs in the code would be much
> more understandable. Any time something is ifdef'd, it would be clear why -
> is this to support a given public API? Is it to support a particular feature
> or variant behavior? Is it to make use of an underlying library? Is it just
> something you *have* to do on the OS? As a side effect, it would somewhat
> discourage scattered trivial behavior differences, since it would be
> necessary to name and explain them instead of just putting them behind a
> catchall ifdef. I believe every porter has been an offender on this front,
> Apple included, and it's probably best to minimize this sort of thing.
>
>
> This is not a new policy yet. Right now I am just proposing it for
> discussion. Thoughts?
>
>
> Regards,
> Maciej
>
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> webkit-dev at lists.webkit.org
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>
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