Merge 81888 from mainline.

Change the marker byte for stubs from 0xcd to 0xce (another form of
interrupt instruction, which shouldn't arise any other way).  0xcd is
also used by JITMemoryManager to initialize the buffer to garbage,
which means it could appear following a noreturn call even when
that is not a stub, confusing X86CompilationCallback2.  PR 4929.

llvm-svn: 81981

Author: tlattner

llvm/lib/Target/X86/X86JITInfo.cpp

@@ -338,7 +338,7 @@ X86CompilationCallback2(intptr_t *StackPtr, intptr_t RetAddr) {
          "Could not find return address on the stack!");
 
   // It's a stub if there is an interrupt marker after the call.
-  bool isStub = ((unsigned char*)RetAddr)[0] == 0xCD;
+  bool isStub = ((unsigned char*)RetAddr)[0] == 0xCE;
 
   // The call instruction should have pushed the return value onto the stack...
 #if defined (X86_64_JIT)
@@ -377,7 +377,7 @@ X86CompilationCallback2(intptr_t *StackPtr, intptr_t RetAddr) {
     // If this is a stub, rewrite the call into an unconditional branch
     // instruction so that two return addresses are not pushed onto the stack
     // when the requested function finally gets called.  This also makes the
-    // 0xCD byte (interrupt) dead, so the marker doesn't effect anything.
+    // 0xCE byte (interrupt) dead, so the marker doesn't effect anything.
 #if defined (X86_64_JIT)
     // If the target address is within 32-bit range of the stub, use a
     // PC-relative branch instead of loading the actual address.  (This is
@@ -486,7 +486,10 @@ void *X86JITInfo::emitFunctionStub(const Function* F, void *Fn,
   JCE.emitWordLE((intptr_t)Fn-JCE.getCurrentPCValue()-4);
 #endif
 
-  JCE.emitByte(0xCD);   // Interrupt - Just a marker identifying the stub!
+  // This used to use 0xCD, but that value is used by JITMemoryManager to
+  // initialize the buffer with garbage, which means it may follow a
+  // noreturn function call, confusing X86CompilationCallback2.  PR 4929.
+  JCE.emitByte(0xCE);   // Interrupt - Just a marker identifying the stub!
   return JCE.finishGVStub(F);
 }