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

Thu Apr 30 17:20:23 PDT 2009

On Apr 30, 2009, at 5:02 PM, Gavin Barraclough wrote:

> Maciej,
>
> This sounds good, and sounds like it could clean things up a lot.
>
> In the breakdown below you don't explicitly mention what would  
> happen to h/w specific macros like PLATFORM_X86, though you do  
> mention 'CPU' in your email, OOI are you thinking something like?:
>
> CPU()
>    Examples:
>        CPU(X86)
>        CPU(X86_64)

Yes, pretty much. And we can have hierarchies of CPU defines for  
families of related instruction sets should that ever be useful. Not  
sure it will be. Endianness would be another example of a potential  
platform adaptation macro, and possibly also pointer size, although  
the latter is detectable from code fairly simply and so perhaps not  
worth the effort.

  - Maciej

>
> cheers,
> G.
>
>
> On Apr 30, 2009, at 4:12 PM, Maciej Stachowiak 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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>