Are there SPARC [or other] aligned memory access requirements to avoid exceptions? [now that 11.0's armv6/v7 is allowing more unaligned accesses]

[Mark Millard]

On 2016-May-26, at 8:21 PM, Cedric Blancher <cedric.blancher at gmail.com> wrote:

> All pure RISC implementations enforce 'natural alignment' - a 32bit
> data type must be aligned 32bit, i.e. 4 bytes, a 64bit data type must
> be 8 byte aligned, a 128bit data type must be 16 byte aligned.
> Some RISC implementations are not pure, but still the misalignment
> comes with a (performance) penalty, either by issuing two loads or
> running through a whole trap handler (!!!!) function with hundreds of
> instructions.
> 
> Ced

Thanks for the notes.

Having once worked in a "micros" group in a logic analyzer product line for many years, working on the software tools that were used for that subject area, I'm very familiar with that general structure of alternatives --but not with SPARC specifics. In your terminology: I've no clue how pure of a RISC implementation is involved for any SPARC variation.

I'm looking for SPARC-specific information that suggests if the defect report originally for armv7-a/cortex-a7 as FreeBSD formerly configured things instead also likely applies to some SPARC variation/configuration that FreeBSD supports. (See later below.)

If FreeBSD has some other fairly strict alignment context that is not a SPARC that might also serve.

Unless upstream can be told that some specific FreeBSD variant is unsupported by their software because of presuming unaligned access is okay, I doubt that a report to upstream should be made based on FreeBSD as a context. (This presumes that the port passes a test under the new armv7-a/cortex-a7 and related alignment requirements. I'm not to that point yet.)

> On 27 May 2016 at 00:03, Mark Millard <markmi at dsl-only.net> wrote:
>> Is is safe to interpret that an rpi2 armv7/cortex-a7 unaligned access failure [from before -r300694] would (likely?) also be a failure on some forms of FreeBSD SPARC use?
>> 
>> 
>> Why I ask:
>> 
>> One of the ports that I had submitted a bug report for unaligned access problems on a rpi2 (armv7-a/cortex-a7 style handling) was:
>> 
>> archivers/lzo2
>> 
>> ( https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207096 ). I'd recently commented that the report might go away after testing what is now -r300694 (allowing more unaligned access on, for example, armv7-a/cortex-a7).
>> 
>> Matthias Andree has since asked in a comment:
>> 
>>> ISTR SPARC architectures also barf on unaligned access, so is it worth bothering the upstream author?
>> 
>> I have generally stuck to architectures for which I have examples to observe, if nothing else than to validate at least some of my understanding that is from reading materials. I normally only submit what I've observed in some form.
>> 
>> I've no such SPARC context nor do I have knowledge/reference material for SPARCs. Nor am I familiar with the choices FreeBSD may have made for SPARC configuration coverage.
>> 
>> As a matter of hear-say my impression is that some SPARCs can be configured to require some variation of strict alignment.
>> 
>> But I do not know how much I can infer from what I observed on a rpi2 (armv7-a/cortex-a7) to FreeBSD SPARC use getting similar results for at least come configurations. Nor do I have access to a test environment for SPARC.
>> 
>> So I wonder if my archivers/lzo2 submittal in question should survive because of SPARC even if the problem is validated to go away for the updated rpi2 like contexts (with armv7-a/cortex-a7 tailoring possibly involved). I have some other submittals that might face the same type of question.
>> 
>> ===
>> Mark Millard
>> markmi at dsl-only.net
>> 
>> _______________________________________________
>> freebsd-sparc64 at freebsd.org mailing list
>> https://lists.freebsd.org/mailman/listinfo/freebsd-sparc64
>> To unsubscribe, send any mail to "freebsd-sparc64-unsubscribe at freebsd.org"
> 
> 
> 
> -- 
> Cedric Blancher <cedric.blancher at gmail.com>
> Institute Pasteur


===
Mark Millard
markmi at dsl-only.net