[Bug 109732] DiRT Rally has extreme graphical corruption and performance loss

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=109732

Kenneth Graunke <kenneth at whitecape.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|intel-3d-bugs at lists.freedes |kenneth at whitecape.org
                   |ktop.org                    |
             Status|NEW                         |ASSIGNED

--- Comment #4 from Kenneth Graunke <kenneth at whitecape.org> ---
Narrowed this down a lot, finally!  st is doing more aggressive peephole select
than i965, causing the NIR to be a fmov.sat of a b32csel.  The backend compiler
is creating a csel instruction for this, and dropping the saturate on the
floor.  This result is used to determine whether to discard pixels, resulting
in a lot of garbage pixels being drawn.  Disabling opt_peephole_csel in the
backend fixes it.

The shader containing 02fd04ad_088a284a_e477969e_48f676f2 in a comment (2626 in
shader-db) renders the text and exhibits the problem nicely.

Now to figure out *why* the compiler is doing that...

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