[Bug 93840] [i965] Compiler backend uses too much stack with Alien: Isolation

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=93840

--- Comment #33 from Eero Tamminen <eero.t.tamminen at intel.com> ---
(In reply to Darius Spitznagel from comment #32)
> are there any news on this?

I've provided Matt with apitrace of the startup, so he can investigate the
compiler side.


> SKL and HSW are very different.
> Did you test mesa git with HSW instead of SKL?

Don't have a set up where I could try the real game on one, but apitrace replay
didn't compile instruction-count-wise as large CS shaders as SKL (before it
failed to run rest of the trace), this was the largest:
43621: message: shader compiler issue 653: CS SIMD16 shader: 5210 inst, 0
loops, 118772 cycles, 207:471 spills:fills, Promote
d 0 constants, compacted 83360 to 58672 bytes.


Btw. I tried Apitrace trace on BDW and as expected, it had the same issue as
SKL.  Perf says time is spent in intel backend:
-----------------------
Overhead  Symbol                                          
  47,44%  ra_allocate
  15,38%  fs_visitor::virtual_grf_interferes
  12,04%  fs_visitor::assign_regs
   8,95%  ra_add_node_adjacency
   2,98%  decrement_q.isra.2
   1,63%  ra_add_node_interference
-----------------------
Which seems very similar to bug 98455.


> Maybe Xorg does play a role here? Forgot to mention my current Xorg version
> in last post...
> 
> xserver-xorg-core 1.19.1

X version should have no impact on what gets compiled in compute shader.

On BDW+, SIMD32 is needed to satisfy compute workgroup requirements, but from
the Apitrace output it seems that on HSW SIMD16 is enough, and that requires a
lot less registers.

-> That explains why things works on HSW, but not on anything newer.  >3x
larger number of registers needs more stack to process and is *much* slower to
compile (it spills a lot).

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