For a very specific optimization, the optimizer may drop a volatile qualifier from a struct assignment. In some cases, the compiler may later perform an incorrect optimization, most commonly incorrectly reordering volatile assignments. This bug can only happen in a function compiled with optimization level 1 or higher which contains both of the following: 1) A struct assign where the destination is volatile, and the source is a known constant, and the struct contains a bit-field, and the struct is of size "int" or smaller. 2) Any bit-field access (read or write) other than as part of a struct assign or initialization expression. That is, the name of the bit-field is present in the access expression. Note that the optimizer can create this situation by inlining functions, so 1 and 2 might be in different functions in the source code. Consider a tree x = y where x and y are of type struct S. If the value of y is known at compile time (e.g. a const value), the optimizer will try to turn the struct constant into an integer constant (possibly combining bit-fields) and rewrite the tree to look like this: "(unsigned *)x = 32;" However, if y is volatile, that should be "*(volatile unsigned *)x = 32;" Because the access is not volatile, instruction scheduling could cause this instruction to drift past nearby volatile accesses.