On Fri, 20 Sep 2019, Jeff Law wrote:
> > According to Bob Supnik (who is a very authoritative source on VAX),
> >
> >> Funny you should ask. The short answer is no. No VAX ever did
> >> speculative or out of order execution.
> >
> > As such, marking VAX as not needing speculation barriers.
> >
> >
> > PR target/86811
> > * config/vax/vax.c (TARGET_HAVE_SPECULATION_SAFE_VALUE):
> > Define to speculation_safe_value_not_needed.
> Installed on the trunk.
I don't think this is right. As I have just mentioned in a related
discussion elsewhere, the NVAX and NVAX+ implementations include a branch
predictor in their microarchitecture[1], so obviously they do execute
speculatively. I think this change would best be reverted and the issue
further investigated.
References:
[1] G. Michael Uhler et al, "The NVAX and NVAX+ High-performance VAX
Microprocessors", Digital Technical Journal Vol. 4 No. 3 Summer 1992
<ftp://ftp.linux-mips.org/pub/linux/mips/people/macro/DEC/DTJ/DTJ701/DTJ701PF.PDF>
Maciej
