[Bug 86469] Unreal Engine demo doesn't run

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=86469

--- Comment #12 from Iago Toral <itoral at igalia.com> ---
(In reply to Tapani Pälli from comment #2)
> (In reply to Kenneth Graunke from comment #1)
> > This should actually be pretty easy to fix.  Sandybridge has more than 16
> > MRFs, so we could just adjust the spilling code to use the higher numbered
> > ones which aren't used for anything else.
> > 
> > The check also seems a bit heavy handed...it may not actually conflict...
> 
> I've checked that Effects works on SNB by simply setting spill_base_mrf 15
> (as 14 is used). I did not find documentation that would indicate more than
> 16 MRFs available though. Docs indicate there are 16 from which 1 .. 15 can
> be used.

Even if we keep 16 as the limit for now, why not compute spill_base_mrf based
on the actual data in mrf_used? Something like this:

for (int i = BRW_MAX_MRF - 1; !mrf_used[i]; i--);
spill_base_mrf = i + 1;
if (spill_base_mrf >= BRW_MAX_MRF) {
   fail("Register spilling not supported with m%d used", i);
   return;
}

We would have to do this in spill_reg and emit_spill, but that would make sure
that we don't fail if there enough unused MRFs available within that limit,
which is what is happening here.

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