Make LoggingTest.CheckCausesDistinctBreakpoints work on clang win

base/logging.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_LOGGING_H_
 #define BASE_LOGGING_H_
 
 #include <stddef.h>
 
 #include <cassert>
@@ skipping 482 matching lines @@
   std::string* message() { return message_; }
 
  private:
   std::string* message_;
 };
 
 // Crashes in the fastest, simplest possible way with no attempt at logging.
 #if defined(COMPILER_GCC)
 #define IMMEDIATE_CRASH() __builtin_trap()
 #elif defined(COMPILER_MSVC)
+
+// Clang is cleverer about coalescing int3s, so we need to add a unique-ish
+// instruction following the __debugbreak() to have it emit distinct locations
+// for CHECKs rather than collapsing them all together. It would be nice to use
+// a short intrinsic to do this (and perhaps have only one implementation for
+// both clang and MSVC), however clang-cl currently does not support intrinsics
+// here. Adding the nullptr store to the MSVC path adds unnecessary bloat.
+// TODO(scottmg): Reinvestigate a short sequence that will work on both
+// compilers once clang supports more intrinsics. See https://crbug.com/693713.
+#if defined(__clang__)
+#define IMMEDIATE_CRASH() \
+  (__debugbreak(),        \
+   (void)(*reinterpret_cast<volatile unsigned char*>(0) = __COUNTER__))

[Primiano Tucci (use gerrit), 2017/02/21 11:02:07]

I arrived late on this.  I had the same exact problem on Android.

Assuming that:
1. my https://codereview.chromium.org/2705053002 sticks
2. clang on windows supports statement expressions like it does on Linux

I think you can just reuse my 
#define TRAP_SEQUENCE()
 asm volatile("int3; ud2; push %0;" ::"i"(static_cast<unsigned
char>(__COUNTER__)))

Rationale:
1) It takes 4 bytes instead of 8
int3 is 1 byte long
ud2 is 1 byte long
push byte immediate: is 2 bytes long

while *nullptr = __COUNTER__ is 8 bytes long. From objdump:
   1:   c6 04 25 00 00 00 00    movb   $0x0,0x0
   8:   00

2) We can reduce the number of different black magics on logging.h. My
TRAP_SRQUENCE there is just few lines above. I think you can just nicely
re-route the #ifdefs there.

+#else
 #define IMMEDIATE_CRASH() __debugbreak()
+#endif  // __clang__
+
 #else
 #error Port
 #endif
 
 // CHECK dies with a fatal error if condition is not true.  It is *not*
 // controlled by NDEBUG, so the check will be executed regardless of
 // compilation mode.
 //
 // We make sure CHECK et al. always evaluates their arguments, as
 // doing CHECK(FunctionWithSideEffect()) is a common idiom.
@@ skipping 543 matching lines @@
 #elif NOTIMPLEMENTED_POLICY == 5
 #define NOTIMPLEMENTED() do {\
   static bool logged_once = false;\
   LOG_IF(ERROR, !logged_once) << NOTIMPLEMENTED_MSG;\
   logged_once = true;\
 } while(0);\
 EAT_STREAM_PARAMETERS
 #endif
 
 #endif  // BASE_LOGGING_H_