Add __builtin_unreachable() to CRASH() when using Clang on Windows.

Source/wtf/Assertions.h

 /*
  * Copyright (C) 2003, 2006, 2007 Apple Inc.  All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 109 matching lines @@
     const char* m_name;
     char* m_cxaDemangled;
 };
 
 } // namespace WTF
 
 using WTF::FrameToNameScope;
 
 /* IMMEDIATE_CRASH() - Like CRASH() below but crashes in the fastest, simplest possible way with no attempt at logging. */
 #ifndef IMMEDIATE_CRASH
 #if COMPILER(GCC)

[Nico, 2015/07/02 04:54:37]

Isn't it a better fix to change this to COMIPLER(GCC) || COMIPLER(CLANG)
instead?

[Sam McNally, 2015/07/02 05:09:40]

I'm not sure why, but doing that instead doesn't fix the warning:

../../third_party/WebKit/Source\wtf/CheckedArithmetic.h(80,5) :  error: function
declared 'noreturn' should not return [-Werror,-Winvalid-noreturn]
    }
    ^

[Nico, 2015/07/02 05:22:36]

Hm, strange. Seems to work in a reduced example:

Nicos-MacBook-Pro-2:llvm-build thakis$ cat test.cc
__declspec(noreturn) void f() {
}

__declspec(noreturn) void g() {
  __builtin_trap();
}
Nicos-MacBook-Pro-2:llvm-build thakis$ bin/clang-cl -c test.cc
test.cc(2,1) :  warning: function declared 'noreturn' should not return
[-Winvalid-noreturn]
}
^
1 warning generated.

I wonder if a header in a different project defines IMMEDIATE_CRASH to something
else…but it looks like blink is the only project defining this:
https://code.google.com/p/chromium/codesearch#search/&q=%5Cbimmediate_crash%5...

Can you build that one translation unit with `ninja ... -v -d keeprsp`, then
copy the compile line and paste it with `/P /Fiout.txt` appended (cd into
out\Debug first) and then look at out.txt to see what the macros expand to?

[Sam McNally, 2015/07/02 06:01:12]

It turns out that CRASH is defined as __debugbreak on windows
(https://code.google.com/p/chromium/codesearch/#chromium/src/third_party/WebKi...)
so IMMEDIATE_CRASH() isn't involved here.

[Sam McNally, 2015/07/03 01:16:05]

I've changed this to add a __builtin_unreachable() to the MSVC version of
CRASH() when COMPILER(CLANG) and moved it into Assertion.h instead of doing it
in the gn/gyp. WDYT?

 #define IMMEDIATE_CRASH() __builtin_trap()
 #else
 #define IMMEDIATE_CRASH() ((void)(*(volatile char*)0 = 0))
 #endif
 #endif
 
 /* CRASH() - Raises a fatal error resulting in program termination and triggering either the debugger or the crash reporter.
 
    Use CRASH() in response to known, unrecoverable errors like out-of-memory.
    Macro is enabled in both debug and release mode.
    To test for unknown errors and verify assumptions, use ASSERT instead, to avoid impacting performance in release builds.
 
    Signals are ignored by the crash reporter on OS X so we must do better.
 */
 #ifndef CRASH
 #define CRASH() \
     (WTFReportBacktrace(), \
      (*(int*)0xfbadbeef = 0), \
      IMMEDIATE_CRASH())
 #endif
 
-#if COMPILER(CLANG)
+#if COMPILER(CLANG) && COMPILER(GCC)
 #define NO_RETURN_DUE_TO_CRASH NO_RETURN
 #else
 #define NO_RETURN_DUE_TO_CRASH
 #endif
 
 /* BACKTRACE
 
   Print a backtrace to the same location as ASSERT messages.
 */
 #if BACKTRACE_DISABLED
@@ skipping 230 matching lines @@
 } \
 inline const thisType& to##thisType(const argumentType& argumentName) \
 { \
     ASSERT_WITH_SECURITY_IMPLICATION(referencePredicate); \
     return static_cast<const thisType&>(argumentName); \
 } \
 void to##thisType(const thisType*); \
 void to##thisType(const thisType&)
 
 #endif /* WTF_Assertions_h */