The "as if" rule -- which is stated in 5.1.2.3 [#3] in C99 -- is explicitly
defined to NOT allow optimizing away accesses to volatile objects.

 |        5.1.2.3  Program execution
 ...
 |        [#2]  Accessing  a  volatile  object,  modifying  an object,
 |        modifying a file, or calling a function  that  does  any  of
 |        those  operations are all side effects
 ...
 |        [#3] In the abstract machine, all expressions are  evaluated
 |        as  specified  by  the  semantics.  An actual implementation
 |        need not evaluate part of an expression  if  it  can  deduce
 |        that  its  value is not used and that no needed side effects
 |        are produced (including any caused by calling a function  or
 |        accessing a volatile object).
 ...
 |        [#5] The least requirements on a  conforming  implementation
 |        are:
 | 
 |          -- At  sequence points, volatile objects are stable in the
 |             sense  that  previous   accesses   are   complete   and
 |             subsequent accesses have not yet occurred.

-- 

This rule is worked around, so in simple instructions variables of 8 and 16
bits long performs as non-volatile ones. This feature can be disabled with
'-mno-volatile-workaround' gcc flag.