DCL17-C. Beware of miscompiled volatile-qualified variables

As described in-depth in rule [DCL34-C. Use volatile for data that cannot be cached], a volatile-qualified variable "shall be evaluated strictly according to the rules of the abstract machine" [ISO/IEC 9899:1999]. In other words, the volatile qualifier is used to instruct the compiler to not make caching optimizations about a variable.

However, as demonstrated in [Eide and Regehr], all tested compilers generated some percentage of incorrect compiled code with regards to volatile accesses. Therefore, it is necessary to know how your compiler behaves when the standard volatile behavior is required. There is also a workaround that eliminates some or all of these errors [Eide and Regehr].

Noncompliant Code Example

As demonstrated in [Eide and Regehr], the following code example compiles incorrectly using GCC version 4.3.0 for IA32 and the -Os optimization flag:

const volatile int x;
volatile int y;
void foo(void) {
  for(y = 0; y < 10; y++) {
    int z = x;
  }
}

Because the variable x is volatile-qualified, it should be accessed ten times in this program. However, as shown in the compiled object code, it is only accessed once due to a loop-hoisting optimization [Eide and Regehr]:

foo:
  movl $0, y
  movl x, %eax
  jmp .L2
.L3:
  movl y, %eax
  incl %eax
  movl %eax, y
.L2:
  movl y, %eax
  cmpl $10, %eax
  jg .L3
  ret

Should x represent a hardware register or some other memory-mapped device that has side effects when accessed, the previous miscompiled code example may produce unexpected behavior.

Compliant Solution

Eide and Regehr tested a workaround by wrapping volatile accesses with function calls. They describe it with the intuition that "we can replace an action that compilers empirically get wrong by a different action—a function call—that compilers can get right" [Eide and Regehr]. For example, the workaround for the noncompliant code example would be

int vol_read_int(volatile int *vp) {
  return *vp;
}
volatile int *vol_id_int(volatile int *vp) {
  return vp;
}

const volatile int x;
volatile int y;
void foo(void) {
  for(*vol_id_int(&y) = 0; vol_read_int(&y) < 10; *vol_id_int(&y) = vol_read_int(&y) + 1) {
    int z = vol_read_int(&x);
  }
}

The workarounds proposed in [Eide and Regehr] fix many of the volatile-access bugs in the tested compilers. However, compilers are always changing so critical sections of code should be compiled as if for deployment and the compiled object code should be inspected for the correct behavior.

Risk Assesment

The volatile qualifier should be used with caution in mission-critical situations. Always make sure that code that assumes certain behavior when using the volatile qualifier is inspected at the object code level for compiler bugs.

Related Guidelines

ISO/IEC 9899:1999 Section 6.7.3, "Type qualifiers"

Bibliography

[Eide and Regehr] "Volatiles are miscompiled, and what to do about it"

DCL16-C. Use 'L', not 'l', to indicate a long value      02. Declarations and Initialization (DCL)