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

[Mario Bensi]

We pursued the same goal for a couple of years. Since our porting  
targets are various middleware & CE platforms, we had to identify and  
adapt WebKit needs at a better grained level than platform.
In order to do this we collected all dependencies in a Browser  
Abstraction Layer (BAL directory). The configuration is handled by a  
Base directory (definition of types, platform specifications) and we  
use CMake to define platform specificities (and it's a great cross- 
platform tool).

Sure the BAL model has still improvements ahead of it, but it has the  
merit of existing, being widely tester on a quite wide range of  
targets, configurations and libraries.

Regards
Mario

Le Friday 01 May 2009 01:12:54 Maciej Stachowiak, vous avez écrit :
> 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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