On 03/16/2015 01:25 PM, Andrew Haley wrote:
On 03/16/2015 11:36 AM, David Holmes wrote:
On 16/03/2015 7:29 PM, Andrew Haley wrote:
On 16/03/15 00:40, David Holmes wrote:
Experimental options are supposed to be opt-in only via
UnlockExperimentalVMOptions. You presently have the experimental
UseUnalignedAccesses being turned on unconditionally on those
architectures that support it.
Well, it works. I guess this could be changed to be a product option,
but it's only really needed for testing. And aren't product options
supposed to be properly documented for all users?
In other words: please don't say "don't do that." Please tell me what
I should do instead. All suggestions are welcome, really because I'm
rather stuck.
Either it should be an experimental option, in which case the logic that
sets the flag on should check that UnlockExperimentalVMOptions is also
on (else do nothing).
But UnlockExperimentalVMOptions is handled by the option processing
already.
(Though this would still be a unusual use-case for
an experimental option as the intent is that the user chooses to turn
them on directly.)
Or it should not be an experimental option.
We need a flag which defaults to whatever is right for the platform
(so it must be set in the back ends) but can be overridden by testing.
What we have right now fits that requirement. I can see no way to do
what I think you're asking for with the current option machinery. I
certainly don't think this should be a product flag.
Andrew.
Hi Andrew,
I think the confusion is caused by UseUnalignedAccesses flag (part of a
public interface unless it is experimental - the command line options)
that is used internally to communicate the cpu default to the
implementation of Unsafe.unalignedAccess() method. What if:
- you define an internal constant SUPPORTS_UNALIGNED_ACCESSES (I don't
know how to do that correctly) in per-cpu vm_version_cpu.hpp files.
- rename UseUnalignedAccesses to EmulateUnalignedAccesses (reverse the
meaning, so that default can be false which must be for experimental
options)
- implement Unsafe.unalignedAccess() to return:
SUPPORTS_UNALIGNED_ACCESSES && !EmulateUnalignedAccesses ...
Regards, Peter
