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

[ramana at gcc dot gnu.org]

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

--- Comment #35 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> ---
(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.  

In straight line code I can imagine this happening - In loopy code I would have
expected the constants to be hoisted - atleast that's what I remember seeing in
my analysis. You have seen -mcprelative-literal-loads haven't you ? 

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

That isn't really how this thing works. Well the constants are shared across
the program now as you say down thread. 

> 
> I'll investigate placing the constant after the function, as before, if the
> estimated function size allows for it.  I think that eliminating the ADRPs
> could potentially be more beneficial to code size than merging constants in
> a common literal pool (v. http://bit.ly/1Ptc8nh).

I was actually surprised by the amount of constant sharing that was happening
in what I looked at. 


Thanks,
Ramana