[Info-vax] x86-64 data aligment / faulting

[Bob Gezelter]

On Friday, February 25, 2022 at 7:12:55 PM UTC-5, Arne Vajhøj wrote:
> On 2/25/2022 6:57 PM, Mark Daniel wrote: 
> > On 26/2/22 8:23 am, Mark Daniel wrote: 
> >> Alpha and Itanium had data alignment requirements with penalties for 
> >> faulting.  Does x86-64?  Is sys$start_align_fault_report() et al. 
> >> still relevant? 
> > 
> > Hmmm.  Using an alignment fault generator and reporter I'm seeing plenty 
> > on Alpha and Itanium; zero on x86-64.
> I had an old Fortran program testing alignment overhead and 
> I just ran it on Windows x86-64 and it showed absolutely 
> no overhead for bad alignment of REAL*8 arrays (and there is 
> a lot of overhead on VMS Alpha and Itanium). 
> 
> I guess we can say welcome back to CISC. :-) 
> 
> Arne
Arne,

With all due respect, the performance penalty for non-aligned references is still very real, speaking as one who did a lot of work on non-faulting IBM System/370 processors back in the day. The same was true with VAX CPUs. They did not fault, but they paid a performance penalty.

There is a difference in context from the days of the System/370 and the VAX: multi-level large caches.

The caches close to the processing core are very fast. This obscures the loss of performance due to non-aligned references. Second, all loads/stores to/from a cache are, almost by definition, aligned.

A program designed to produce alignment faults is also very likely to not abuse the memory system in a way to detect the mis-aligned data penalty. Faults, which are synchronous interrupts, have overhead orders of magnitude more than a double memory fetch, particularly when sequential elements are referenced (sequential elements may well be in the same cache line, even if not aligned on the proper boundary).

If I had the spare time to play with it, I would write a program to randomly address a storage area beyond total cache size, so that every memory reference is a cache miss. Run aligned and unaligned data references and compare the result. 

It is easy for a benchmark to measure the incorrect phenomenon.

- Bob Gezelter, http://www.rlgsc.com