ExInterlockedAddLargeStatistic

This function adds a 32-bit integer to a 64-bit integer in a way that is in some sense safe for multi-processing.

Declaration

VOID 
FASTCALL 
ExInterlockedAddLargeStatistic (
    PLARGE_INTEGER Addend, 
    ULONG Increment);

Parameters

The Addend argument addresses the 64-bit integer, i.e., the large statistic, that is to be added to.

The Increment argument is the amount to add.

Availability

The ExInterlockedAddLargeStatistic function is exported by name from x86 builds of the kernel in version 3.50 and higher. Though the function continues to be exported, it was in effect retired by the Device Driver Kit (DDK) for Windows Server 2003. While defining the function for other architectures by way of macros, this DDK redefined the x86 export in terms of the compiler intrinsic _InterlockedAddLargeStatistic.

That said, the compiler intrinsic is used liberally in the Windows kernel, even as recently as version 10.0, and plausibly still orginates in the source code as macro interpretation of the ExInterlockedAddLargeStatistic function.

Documentation Status

The ExInterlockedAddLargeStatistic function has long been documented but was not documented immediately. The first documentation known to this study is from the DDK for Windows 2000. The function has, however, always had a C-language declaration or macro definition in NTDDK.H and/or WDM.H, at least as far back as the DDK for Windows NT 3.51.

Behaviour

The coding, in assembly language, has not changed since version 3.50. The Increment is added to the low 32 bits at Addend. If this overflows, the carry is added to the high 32 bits. Both additions have the lock prefix.

Note that the safety for multi-processing is partial. The 64-bit statistic is updated in halves not as a whole. Overlapping calls to the function from different processors can interfere with each other when either (or both) causes an overflow of the lower half. How the function is safe for multi-processing is that it does not lose anything of what’s to be added. All increments are certain to accumulate correctly in the large statistic eventually. If, however, the statistic is read while additions might be attempted, then a concurrent addition need not yet be complete. This is true even if using the cmpxchg8b instruction to be sure that all 64 bits of what’s read truly were in memory at one time. What’s read of any statistic that is updated by this function is not reliable, except if it is known separately that updating is paused. Successive reads can even show the statistic as having gone backwards.

The point to the function is presumably in its name. The statistic in the name will be a counter of some sort. The aim is to add to the counter in the fewest instructions possible without risk that concurrent additions get lost. That an addition might not yet be fully accumulated in the statistic when next read is less important than knowing it will be eventually. This will have been an easy compromise before processors were sure to have the cmpxchg8b instruction, for the reading and writing back of a 64-bit integer without interruption could not be done without external synchronisation, the acquisition and release of which would typically disturb execution far more than is tolerable for maintaining a counter.

That the function, albeit converted to a compiler intrinsic, continues not just to be defined but used, e.g., in the Windows 10 kernel, suggests that the compromise is still worthwhile even now that unavailability of cmpxchg8b is a consideration only for software that may find itself running on an older version than Windows XP. Though the two writes in

        lock    add dword ptr [ecx],edx
        jnc     done
        lock    adc dword ptr [ecx+4],0
done:

(which is what’s coded) allow that the statistic is momentarily incomplete, the code is very much more efficient than

        push    esi
        push    edi
        mov     esi,ecx
        mov     edi,edx
retry:
        mov     eax,[esi]
        mov     edx,[esi+4]
        mov     ecx,eax
        mov     ebx,edx
        add     ecx,edi
        adc     ebx,0
        lock    cmpxchg8b qword ptr [esi]
        jne     retry
        pop     edi
        pop     esi

Even though this unwieldy code ensures that the statistic is only ever written as a whole, the advantage hardly matters unless the consumer of the statistic is willing to do something similar to be sure of reading the statistic without interruption. For these reasons, if not just for backwards compatibility, ExInterlockedAddLargeStatistic likely isn’t ever going away.