On Tue, 9 Jan 2018, Richard Earnshaw (lists) wrote:
> Read condition 1 i) again.
>
>
> i) the conditional select instruction has a register data dependency on
> a load R1, that has been executed speculatively, for one of its input
> registers, and
Sorry - I don't know how I missed that. I understand the motivation for the
behavior of the new built-in now; the CSDB specification is surprising, but
that is off-topic I guess. Seems quite unfortunate that CSDB is constraining
the generic built-in like this, though.
Thank you for clearing this up.
I suggest that the user documentation in extend.texi should explicitly call out
that the load itself still may be speculatively executed - only its consumers
can be expected to be fenced off. For example, after
+but in addition target-specific code will be inserted to ensure that
+speculation using @code{*ptr} cannot occur when @var{cmpptr} lies outside of
+the specified bounds.
it can go on to say e.g.,
However note that the load of @code{*ptr} itself and the conditional branch
leading to it may still be subject to speculative execution (and the load may
have observable effects on the cache as a result).
Thanks.
Alexander
