Fix finding entry point unwind record for a method with fragmented un…

src/vm/codeman.cpp

@@ -894,6 +894,35 @@ BOOL IsFunctionFragment(TADDR baseAddress, PTR_RUNTIME_FUNCTION pFunctionEntry)
 #endif
 }
 
+// When we have fragmented unwind we usually want to refer to the
+// unwind record that includes the prolog. We can find it by searching
+// back in the sequence of unwind records.
+PTR_RUNTIME_FUNCTION FindRootEntry(PTR_RUNTIME_FUNCTION pFunctionEntry, TADDR baseAddress)
+{
+    LIMITED_METHOD_DAC_CONTRACT;
+
+    PTR_RUNTIME_FUNCTION pRootEntry = pFunctionEntry;
+
+    if (pRootEntry != NULL)
+    {
+        // Walk backwards in the RUNTIME_FUNCTION array until we find a non-fragment.
+        // We're guaranteed to find one, because we require that a fragment live in a function or funclet
+        // that has a prolog, which will have non-fragment .xdata.
+        for (;;)
+        {
+            if (!IsFunctionFragment(baseAddress, pRootEntry))
+            {
+                // This is not a fragment; we're done
+                break;
+            }
+
+            --pRootEntry;
+        }
+    }
+
+    return pRootEntry;
+}
+
 #endif // EXCEPTION_DATA_SUPPORTS_FUNCTION_FRAGMENTS
 
 
@@ -1078,31 +1107,13 @@ TADDR IJitManager::GetFuncletStartAddress(EECodeInfo * pCodeInfo)
 #endif
 
     TADDR baseAddress = pCodeInfo->GetModuleBase();
-    TADDR funcletStartAddress = baseAddress + RUNTIME_FUNCTION__BeginAddress(pFunctionEntry);
 
 #if defined(EXCEPTION_DATA_SUPPORTS_FUNCTION_FRAGMENTS)
-    // Is the RUNTIME_FUNCTION a fragment? If so, we need to walk backwards until we find the first
-    // non-fragment RUNTIME_FUNCTION, and use that one. This happens when we have very large functions
-    // and multiple RUNTIME_FUNCTION entries per function or funclet. However, all but the first will
-    // have the "F" bit set in the unwind data, indicating a fragment (with phantom prolog unwind codes).
-
-    for (;;)
-    {
-        if (!IsFunctionFragment(baseAddress, pFunctionEntry))
-        {
-            // This is not a fragment; we're done
-            break;
-        }
-
-        // We found a fragment. Walk backwards in the RUNTIME_FUNCTION array until we find a non-fragment.
-        // We're guaranteed to find one, because we require that a fragment live in a function or funclet
-        // that has a prolog, which will have non-fragment .xdata.
-        --pFunctionEntry;
-
-        funcletStartAddress = baseAddress + RUNTIME_FUNCTION__BeginAddress(pFunctionEntry);
-    }
+    pFunctionEntry = FindRootEntry(pFunctionEntry, baseAddress);
 #endif // EXCEPTION_DATA_SUPPORTS_FUNCTION_FRAGMENTS
 
+    TADDR funcletStartAddress = baseAddress + RUNTIME_FUNCTION__BeginAddress(pFunctionEntry);
+
     return funcletStartAddress;
 }
 
@@ -3878,6 +3889,8 @@ void EEJitManager::NibbleMapSet(HeapList * pHp, TADDR pCode, BOOL bSet)
 #endif // !DACCESS_COMPILE
 
 #if defined(WIN64EXCEPTIONS)
+// Note: This returns the root unwind record (the one that describes the prolog)
+// in cases where there is fragmented unwind.
 PTR_RUNTIME_FUNCTION EEJitManager::LazyGetFunctionEntry(EECodeInfo * pCodeInfo)
 {
     CONTRACTL {
@@ -3907,6 +3920,12 @@ PTR_RUNTIME_FUNCTION EEJitManager::LazyGetFunctionEntry(EECodeInfo * pCodeInfo)
 
         if (RUNTIME_FUNCTION__BeginAddress(pFunctionEntry) <= address && address < RUNTIME_FUNCTION__EndAddress(pFunctionEntry, baseAddress))
         {
+
+#if defined(EXCEPTION_DATA_SUPPORTS_FUNCTION_FRAGMENTS)
+            // If we may have fragmented unwind make sure we're returning the root record
+            pFunctionEntry = FindRootEntry(pFunctionEntry, baseAddress);
+#endif
+
             return pFunctionEntry;
         }
     }