Cleanup fixme comments

Author: kg

src/coreclr/vm/interpexec.cpp

@@ -2001,12 +2001,14 @@ void InterpExecMethod(InterpreterFrame *pInterpreterFrame, InterpMethodContextFr
                     if (isTailcall)
                     {
                         // Move args from callArgsOffset to start of stack frame.
-                        // Interpreter-FIXME: GC refs
-                        // Interpreter-FIXME: Validate return address offset
                         InterpMethod* pTargetMethod = targetIp->Method;
                         assert(pTargetMethod->CheckIntegrity());
-                        memcpyNoGCRefs(pFrame->pStack, stack + callArgsOffset, pTargetMethod->argsSize);
-                        // Reuse current stack frame.
+                        // It is safe to use memcpy because the source and destination are both on the interp stack, not in the GC heap.
+                        // We need to use the target method's argsSize, not our argsSize, because tail calls (unlike CEE_JMP) can have a
+                        //  different signature from the caller.
+                        memcpy(pFrame->pStack, stack + callArgsOffset, pTargetMethod->argsSize);
+                        // Reuse current stack frame. We discard the call insn's returnOffset because it's not important and tail calls are
+                        //  required to be followed by a ret, so we know nothing is going to read from stack[returnOffset] after the call.
                         pFrame->ReInit(pFrame->pParent, targetIp, pFrame->pRetVal, pFrame->pStack);
                     }
                     else