[Webkit-unassigned] [Bug 65401] New: Weird macros for use in "#if" in Platform.h

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=65401

           Summary: Weird macros for use in "#if" in Platform.h
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: UNCONFIRMED
          Severity: Normal
          Priority: P2
         Component: Platform
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: schaub.johannes at googlemail.com


Source/JavaScriptCore/wtf/Platform.h contains macros like this one:

#define PLATFORM(WTF_FEATURE) (defined WTF_PLATFORM_##WTF_FEATURE  && WTF_PLATFORM_##WTF_FEATURE)

and their use is for code as follows

#if PLATFORM(MAC) || (PLATFORM(QT) && USE(QTKIT))
#include "MediaPlayerPrivateQTKit.h"
#if USE(AVFOUNDATION)
#include "MediaPlayerPrivateAVFoundationObjC.h"
#endif

There are two things that make me thing that the first part of that logical-AND in the definition of "PLATFORM" isn't needed and may actually be harmful. 

1. Why is it harmful? The C++ spec says at 16.1p4 "If the token 'defined' is generated as a result of this replacement process ..., the behavior is undefined.". That's what's happening here.

2. Why is it not needed? Because "#if" replaces any unknown macros by "0". And the macro already relies on this to happen. If it wouldn't happen, the "&& WTF_PLATFORM_##WTF_FEATURE" would be broken if the macro doesn't exist. 

I don't know why that code is chosen to be that way, but I recommend to change it to the following, if it works on all targets:

    #define PLATFORM(WTF_FEATURE) (WTF_PLATFORM_##WTF_FEATURE)

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