Force emulator to quit if it's built w/o global register variable support

Clang and llvm-gcc don't support global register variable (GRV) crucial
to emulator (where a register is reserved to point to target architecture
state for better code-gen).  Clang and llvm-gcc are provided in recent
Xcode to replace the original gcc with GRV support.  MacOSX developers
may accidentally install newer Xcode and break emulator.

This CL allows emulator (and the rest of Android tree) to build but forces
emulator to quit if it's built w/o GRV support.  Developers build
Android tree with clang or llvm-gcc can still get good system image,
but they have to use emulators built the other way or from SDKs to run it.

Related CL & bug entry:
  https://android-review.googlesource.com/#/c/33011
  http://code.google.com/p/android/issues/detail?id=32577

Change-Id: Ia585dd7bf9783e2ff4c114b4f0ec20b89684ab57

5 files changed, 28 insertions, 11 deletions

diff --git a/qemu-common.h b/qemu-common.h
index 79ac779..097e1fc 100644
--- a/qemu-common.h
+++ b/qemu-common.h
@@ -408,4 +408,20 @@ typedef enum DisplayType
 #define CLAMP_MAC_TIMEOUT(to) ((void)0)
 #endif  // _DARWIN_C_SOURCE
 
+#if defined(__clang__) || defined(__llvm__)
+/* Clang and llvm-gcc don't support global register variable (GRV).
+   Clang issues compile-time error for GRV.  llvm-gcc accepts GRV (because
+   its front-end is gcc) but ignores it in the llvm-based back-end.
+   Undefining GRV decl to allow external/qemu and the rest of Android
+   to compile.  But emulator built w/o GRV support will not function
+   correctly.  User will be greeted with an error message (issued
+   in tcg/tcg.c) when emulator built this way is launched.
+ */
+#define SUPPORT_GLOBAL_REGISTER_VARIABLE 0
+#define GLOBAL_REGISTER_VARIABLE_DECL
+#else
+#define SUPPORT_GLOBAL_REGISTER_VARIABLE 1
+#define GLOBAL_REGISTER_VARIABLE_DECL register
+#endif /* __clang__ || __llvm__ */
+
 #endif
diff --git a/target-arm/exec.h b/target-arm/exec.h
index 83571e6..43bfaa9 100644
--- a/target-arm/exec.h
+++ b/target-arm/exec.h
@@ -19,11 +19,7 @@
 #include "config.h"
 #include "dyngen-exec.h"
 
-/* Xcode 4.3 doesn't support global register variables */
-#if !defined(__APPLE_CC__) || __APPLE_CC__ < 5621
-	register
-#endif
-struct CPUARMState *env asm(AREG0);
+GLOBAL_REGISTER_VARIABLE_DECL struct CPUARMState *env asm(AREG0);
 
 #include "cpu.h"
 #include "exec-all.h"
diff --git a/target-i386/exec.h b/target-i386/exec.h
index d194f89..5d04b7f 100644
--- a/target-i386/exec.h
+++ b/target-i386/exec.h
@@ -29,11 +29,7 @@
 
 #include "cpu-defs.h"
 
-/* Xcode 4.3 doesn't support global register variables */
-#if !defined(__APPLE_CC__) || __APPLE_CC__ < 5621
-       register
-#endif
-struct CPUX86State *env asm(AREG0);
+GLOBAL_REGISTER_VARIABLE_DECL struct CPUX86State *env asm(AREG0);
 
 #include "qemu-common.h"
 #include "qemu-log.h"
diff --git a/target-mips/exec.h b/target-mips/exec.h
index 0720366..0d55790 100644
--- a/target-mips/exec.h
+++ b/target-mips/exec.h
@@ -8,7 +8,7 @@
 #include "dyngen-exec.h"
 #include "cpu-defs.h"
 
-register struct CPUMIPSState *env asm(AREG0);
+GLOBAL_REGISTER_VARIABLE_DECL struct CPUMIPSState *env asm(AREG0);
 
 #include "cpu.h"
 #include "exec-all.h"
diff --git a/tcg/tcg.c b/tcg/tcg.c
index 5882f00..66529f5 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -2030,6 +2030,15 @@ static inline int tcg_gen_code_common(TCGContext *s, uint8_t *gen_code_buf,
     unsigned int tpc2gpc_index = 0;
 #endif  // CONFIG_MEMCHECK
 
+#if !SUPPORT_GLOBAL_REGISTER_VARIABLE
+    printf("ERROR: This emulator is built by compiler without global register variable\n"
+           "support!  Emulator reserves a register to point to target architecture state\n"
+           "for better code generation.  LLVM-based compilers such as clang and llvm-gcc\n"
+           "currently don't support global register variable.  Please see\n"
+           "http://source.android.com/source/initializing.html for detail.\n\n");
+    tcg_abort();
+#endif
+
 #ifdef DEBUG_DISAS
     if (unlikely(qemu_loglevel_mask(CPU_LOG_TB_OP))) {
         qemu_log("OP:\n");