[Bug 83658] Loop unrolling doesn't take into account whether it will decrease or increase performance

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=83658

--- Comment #3 from Tapani Pälli <lemody at gmail.com> ---
(In reply to Eero Tamminen from comment #2)
> (In reply to comment #1)
> > FWIW there exists code to detect expressions that are constant within a loop
> > (during loop analysis), I think this could be used as starting point for
> > such optimization.
> 
> Loop invariant code handling could also be the root cause making e.g.
> SynMark2 PSPhong 3x slower than it should be:
> - code doesn't get unrolled with all the loop invariant code inside the loop 
> - this resulting in cascade effect that causes also other problems. e.g.
> stuff which on Windows GL is handled with push & no sampler, changes to
> (unecessarily looped) pull and makes thing sampler bound with Mesa instead
> of it being PS ALU bound

For me it looks like invariant code in loop is not the problem within PSPhong
example, I will try to make a handwritten loop with invariant code to
investigate this optimization opportunity more.

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