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[Bug target/63304] Aarch64 pc-relative load offset out of range
- From: "wdijkstr at arm dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Fri, 06 Nov 2015 20:41:33 +0000
- Subject: [Bug target/63304] Aarch64 pc-relative load offset out of range
- Auto-submitted: auto-generated
- References: <bug-63304-4 at http dot gcc dot gnu dot org/bugzilla/>
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.