[PATCH target/86811] Mark VAX as not needing speculation barriers

[Maciej W. Rozycki]

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