alignment requirements for atomic x86 instructions

Question

Microsoft offers the InterlockedCompareExchange function for performing atomic compare-and-swap operations. There is also an _InterlockedCompareExchange intrinsic.

On x86 these are implemented using the cmpxchg instruction.

However, reading through the documentation on these three approaches, they don't seem to agree on the alignment requirements.

Intel's reference manual says nothing about alignment (other than that if alignment checking is enabled and an unaligned memory reference is made, an exception is generated)

I also looked up the lock prefix, which specifically states that

The integrity of the LOCK prefix is not affected by the alignment of the memory field.

(emphasis mine)

So Intel seems to say that alignment is irrelevant. The operation will be atomic no matter what.

The _InterlockedCompareExchange intrinsic documentation also says nothing about alignment, however the InterlockedCompareExchange function states that

The parameters for this function must be aligned on a 32-bit boundary; otherwise, the function will behave unpredictably on multiprocessor x86 systems and any non-x86 systems.

So what gives? Are the alignment requirements for InterlockedCompareExchange just to make sure the function will work even on pre-486 CPU's where the cmpxchg instruction isn't available? That seems likely based on the above information, but I'd like to be sure before I rely on it. :)

Or is alignment required by the ISA to guarantee atomicity, and I'm just looking the wrong places in Intel's reference manuals?

Answer 1

See this SO question: natural alignment is important for performance, and is required on the x64 architecture (so it's not just PRE-x86 systems, but POST-x86 ones too -- x64 may still be a bit of a niche case but it's growing in popularity after all;-); that may be why Microsoft documents it as required (hard to find docs on whether MS has decided to FORCE the alignment issue by enabling alignment checking -- that may vary by Windows version; by claiming in the docs that alignment is required, MS keeps the freedom to force it in some version of Windows even if they did not force it on others).

Answer 2

Microsoft's Interlocked APIs also applied to ia64 (while it still existed). There was no lock prefix on ia64, only the cmpxchg.acq and cmpxchg.rel instructions (or fetchadd and other similar beasties), and these all required alignment if I recall correctly.