[Bug target/63304] Aarch64 pc-relative load offset out of range

[wdijkstr at arm dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63304

--- Comment #33 from Wilco <wdijkstr at arm dot com> ---
(In reply to Evandro from comment #32)
> (In reply to Ramana Radhakrishnan from comment #31)
> > (In reply to Evandro from comment #30)
> > > The performance impact of always referring to constants as if they were far
> > > away is significant on targets which do not fuse ADRP and LDR together. 
> > 
> > What happens if you split them up and schedule them appropriately ? I didn't
> > see any significant impact in my benchmarking on implementations that did
> > not implement such fusion. Where people want performance in these cases they
> > can well use -mpc-relative-literal-loads or -mcmodel=tiny - it's in there
> > already.
> 
> Because of side effects of the Haiffa scheduler, the loads now pile up, and
> the ADRPs may affect the load issue rate rather badly if not fused.  At leas
> on our processor.  

ADRP latency to load-address should be zero on any OoO core - ADRP is basically
a move-immediate, so can execute early and hide any latency.

> Which brings another point, shouldn't there be just one ADRP per BB or,
> ideally, per function?  Or am I missing something?

That's not possible in this case as the section is mergeable. An alternative
implementation using anchors may be feasible, but GCC is extremely bad at using
anchors efficiently - functions using several global variables also end up with
a large number of ADRPs when you'd expect a single ADRP.