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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