Version N of the llvm_unreachable patch: VC++ doesn't recognize that abort()

doesn't return, so just go back to using the old runtime function instead
of trying to use abort() when __builtin_unreachable (or an equivalent) isn't
supported.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127629 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 11 insertions, 10 deletions

diff --git a/include/llvm/Support/Compiler.h b/include/llvm/Support/Compiler.h
index 0df154b5d6..e092157218 100644
--- a/include/llvm/Support/Compiler.h
+++ b/include/llvm/Support/Compiler.h
@@ -128,12 +128,10 @@
 
 // LLVM_BUILTIN_UNREACHABLE - On compilers which support it, expands
 // to an expression which states that it is undefined behavior for the
-// compiler to reach this point.
+// compiler to reach this point.  Otherwise is not defined.
 #if defined(__clang__) || (__GNUC__ > 4) \
  || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)
 # define LLVM_BUILTIN_UNREACHABLE __builtin_unreachable()
-#else
-# define LLVM_BUILTIN_UNREACHABLE abort()
 #endif
 
 #endif
diff --git a/include/llvm/Support/ErrorHandling.h b/include/llvm/Support/ErrorHandling.h
index fb0c88d8d8..95b01095c1 100644
--- a/include/llvm/Support/ErrorHandling.h
+++ b/include/llvm/Support/ErrorHandling.h
@@ -86,18 +86,21 @@ namespace llvm {
                                                          unsigned line=0);
 }
 
-/// Prints the message and location info to stderr in !NDEBUG builds.
-/// This is intended to be used for "impossible" situations that imply
-/// a bug in the compiler.
+/// Marks that the current location is not supposed to be reachable.
+/// In !NDEBUG builds, prints the message and location info to stderr.
+/// In NDEBUG builds, becomes an optimizer hint that the current location
+/// is not supposed to be reachable.  On compilers that don't support
+/// such hints, prints a reduced message instead.
 ///
-/// In NDEBUG mode it only prints "UNREACHABLE executed".
-/// Use this instead of assert(0), so that the compiler knows this path
-/// is not reachable even for NDEBUG builds.
+/// Use this instead of assert(0).  It conveys intent more clearly and
+/// allows compilers to omit some unnecessary code.
 #ifndef NDEBUG
 #define llvm_unreachable(msg) \
   ::llvm::llvm_unreachable_internal(msg, __FILE__, __LINE__)
-#else
+#elif defined(LLVM_BUILTIN_UNREACHABLE)
 #define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
+#else
+#define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()
 #endif
 
 #endif