Set TARGET_OS_* from GN, falling back to HOST_OS_*.

runtime/platform/globals.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef RUNTIME_PLATFORM_GLOBALS_H_
 #define RUNTIME_PLATFORM_GLOBALS_H_
 
 // __STDC_FORMAT_MACROS has to be defined before including <inttypes.h> to
 // enable platform independent printf format specifiers.
 #ifndef __STDC_FORMAT_MACROS
@@ skipping 84 matching lines @@
 #define HOST_OS_LINUX 1
 
 #elif defined(__APPLE__)
 
 // Define the flavor of Mac OS we are running on.
 #include <TargetConditionals.h>
 // TODO(iposva): Rename HOST_OS_MACOS to HOST_OS_MAC to inherit
 // the value defined in TargetConditionals.h
 #define HOST_OS_MACOS 1
 #if TARGET_OS_IPHONE
-// Test for this #define by saying '#if TARGET_OS_IOS' rather than the usual
-// '#if defined(TARGET_OS_IOS)'. TARGET_OS_IOS is defined to be 0 in
-// XCode >= 7.0. See Issue #24453.
 #define HOST_OS_IOS 1
 #endif
 
 #elif defined(_WIN32)
 
 // Windows, both 32- and 64-bit, regardless of the check for _WIN32.
 #define HOST_OS_WINDOWS 1
 
 #elif defined(__Fuchsia__)
 #define HOST_OS_FUCHSIA
@@ skipping 259 matching lines @@
 #error Unknown architecture.
 #endif
 
 // Disable background threads by default on armv5te. The relevant
 // implementations are uniprocessors.
 #if !defined(TARGET_ARCH_ARM_5TE)
 #define ARCH_IS_MULTI_CORE 1
 #endif
 
 
-#if defined(TARGET_ARCH_ARM)
-#if defined(TARGET_ABI_IOS) && defined(TARGET_ABI_EABI)
-#error Both TARGET_ABI_IOS and TARGET_ABI_EABI defined.
-#elif !defined(TARGET_ABI_IOS) && !defined(TARGET_ABI_EABI)
-#if defined(HOST_OS_MAC)
-#define TARGET_ABI_IOS 1
+#if !defined(TARGET_OS_ANDROID)

[zra, 2017/03/17 17:24:24]

Why not:
#if !defined(...) && \
     !defined(...) && \
     ...

[rmacnak, 2017/03/17 18:22:16]

Copied idiom from the equivalent section for TARGET_ARCH. Updated both to &&.

+#if !defined(TARGET_OS_FUCHSIA)
+#if !defined(TARGET_OS_IOS)
+#if !defined(TARGET_OS_LINUX)
+#if !defined(TARGET_OS_MACOS)
+#if !defined(TARGET_OS_WINDOWS)
+// No target OS specified; pick the one matching the host OS.
+#if defined(HOST_OS_ANDROID)
+#define TARGET_OS_ANDROID 1
+#elif defined(HOST_OS_FUCHSIA)
+#define TARGET_OS_FUCHSIA 1
+#elif defined(HOST_OS_IOS)
+#define TARGET_OS_MACOS 1
+// Test for this #define by saying '#if TARGET_OS_IOS' rather than the usual
+// '#if defined(TARGET_OS_IOS)'. TARGET_OS_IOS is defined to be 0 in
+// XCode >= 7.0. See Issue #24453.

[zra, 2017/03/17 17:24:24]

Can we use a different symbol? Maybe TARGET_OS_MACOS_IOS? TARGET_OS_IPHONE_OS?

[rmacnak, 2017/03/17 18:22:16]

Sure. TARGET_OS_MACOS_IOS seems like a good choice because it reflects our
treatment of iOS as a subtype of Mac in the platform-specific files.

+#define TARGET_OS_IOS 1
+#elif defined(HOST_OS_LINUX)
+#define TARGET_OS_LINUX 1
+#elif defined(HOST_OS_MACOS)
+#define TARGET_OS_MACOS 1
+#elif defined(HOST_OS_WINDOWS)
+#define TARGET_OS_WINDOWS 1
 #else
-#define TARGET_ABI_EABI 1
+#error Automatic target OS detection failed.
 #endif
 #endif
-#endif  // TARGET_ARCH_ARM
+#endif
+#endif
+#endif
+#endif
+#endif
+
+
+// TARGET_OS_IOS is defined as 0 on macOS. Always define it so we can test
+// uniformly.
+#if !defined(TARGET_OS_IOS)
+#define TARGET_OS_IOS 0
+#endif
 
 
 // Short form printf format specifiers
 #define Pd PRIdPTR
 #define Pu PRIuPTR
 #define Px PRIxPTR
 #define Pd64 PRId64
 #define Pu64 PRIu64
 #define Px64 PRIx64
 
@@ skipping 302 matching lines @@
 // tag in the ICData and check it when recreating the flow graph in
 // optimizing compiler. Enable it for other modes (product, release) if needed
 // for debugging.
 #if defined(DEBUG)
 #define TAG_IC_DATA
 #endif
 
 }  // namespace dart
 
 #endif  // RUNTIME_PLATFORM_GLOBALS_H_