[Bug 106390] Invalid imageAtomicExchange() writes.

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=106390

--- Comment #10 from Denis <denys.kostin at globallogic.com> ---
Created attachment 142211
  --> https://bugs.freedesktop.org/attachment.cgi?id=142211&action=edit
denys_expected

hi guys. I tested last attachment from Alexander on radeon and intel. Found
out, that on radeon test looks without visible squads (on the right part of the
picture). See attachment "denys_expected" with my "expected" output. 

Then I checked old mesa versions and found out, that the same picture I can see
on 13.0.6 mesa. Below there is a result of bisection (tested on SKL and KBL):


9919542f1cfff70524bc6117d19bf88e59159caa is the first bad commit
commit 9919542f1cfff70524bc6117d19bf88e59159caa
Author: Kenneth Graunke <kenneth at whitecape.org>
Date:   Sat Jan 14 23:32:12 2017 -0800

    i965: Make DCE set null destinations on messages with side effects.

    (Co-authored by Matt Turner.)

    Image atomics, for example, return a value - but the shader may not
    want to use it.  We assigned a useless VGRF destination.  This seemed
    harmless, but it can actually be quite harmful.  The register allocator
    has to assign that VGRF to a real register.  It may assign the same
    actual GRF to the destination of an instruction that follows soon after.

    This results in a write-after-write (WAW) dependency, and stall.

    A number of "Deus Ex: Mankind Divided" shaders use image atomics, but
    don't use the return value.  Several of these were hitting WAW stalls
    for nearly 14,000 (poorly estimated) cycles a pop.  Making dead code
    elimination null out the destination avoids this issue.

    This patch cuts one shader's estimated cycles by -98.39%!  Removing the
    message response should also help with data cluster bandwidth.

    On Skylake:

    (instruction counts remain identical)

    total cycles in shared programs: 255413890 -> 248081010 (-2.87%)
    cycles in affected programs: 12019948 -> 4687068 (-61.01%)
    helped: 24
    HURT: 10

    v2: Make can_omit_write independent of can_eliminate (Curro).

    Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
    Reviewed-by: Francisco Jerez <currojerez at riseup.net>
    Reviewed-by: Matt Turner <mattst88 at gmail.com>

:040000 040000 d3ac50f716bb55e2919ea2a8098656be67dffdc4
9763545331f08faf330b432997836bbcdce7e8c1 M    src

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