Allow ILCodeVersion to fallback to default IL (#18322)

For compat with profilers that used our APIs in unexpected ways we can allow
the ILCodeVersion to fallback to the default IL code when no IL was explicitly
given.

1 files changed, 23 insertions, 8 deletions

diff --git a/src/vm/codeversion.cpp b/src/vm/codeversion.cpp
index 7406d19109..cf756e14f9 100644
--- a/src/vm/codeversion.cpp
+++ b/src/vm/codeversion.cpp
@@ -773,23 +773,38 @@ PTR_COR_ILMETHOD ILCodeVersion::GetIL() const
     }
     CONTRACTL_END
 
+    PTR_COR_ILMETHOD pIL = NULL;
     if (m_storageKind == StorageKind::Explicit)
     {
-        return AsNode()->GetIL();
+        pIL = AsNode()->GetIL();
     }
-    else
+    
+    // For the default code version we always fetch the globally stored default IL for a method
+    //
+    // In the non-default code version we assume NULL is the equivalent of explicitly requesting to
+    // re-use the default IL. Ideally there would be no reason to create a new version that re-uses
+    // the default IL (just use the default code version for that) but we do it here for compat. We've 
+    // got some profilers that use ReJIT to create a new code version and then instead of calling
+    // ICorProfilerFunctionControl::SetILFunctionBody they call ICorProfilerInfo::SetILFunctionBody. 
+    // This mutates the default IL so that it is now correct for their new code version. Of course this
+    // also overwrote the previous default IL so now the default code version GetIL() is out of sync
+    // with the jitted code. In the majority of cases we never re-read the IL after the initial
+    // jitting so this issue goes unnoticed.
+    //
+    // If changing the default IL after it is in use becomes more problematic in the future we would
+    // need to add enforcement that prevents profilers from using ICorProfilerInfo::SetILFunctionBody
+    // that way + coordinate with them because it is a breaking change for any profiler currently doing it.
+    if(pIL == NULL)
     {
         PTR_Module pModule = GetModule();
         PTR_MethodDesc pMethodDesc = dac_cast<PTR_MethodDesc>(pModule->LookupMethodDef(GetMethodDef()));
-        if (pMethodDesc == NULL)
+        if (pMethodDesc != NULL)
         {
-            return NULL;
-        }
-        else
-        {
-            return dac_cast<PTR_COR_ILMETHOD>(pMethodDesc->GetILHeader(TRUE));
+            pIL = dac_cast<PTR_COR_ILMETHOD>(pMethodDesc->GetILHeader(TRUE));
         }
     }
+
+    return pIL;
 }
 
 PTR_COR_ILMETHOD ILCodeVersion::GetILNoThrow() const