[Bug 107212] Dual-Core CPU E5500 / G45: RetroArch with reicast core results in corrupted graphics

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=107212

--- Comment #41 from Diego Viola <diego.viola at gmail.com> ---
commit 08a5c395abdafd0d7556060596f78c238b4a989f
Author: Kenneth Graunke <kenneth at whitecape.org>
Date:   Thu Aug 2 15:02:18 2018 -0700

    intel: Fix SIMD16 unaligned payload GRF reads on Gen4-5.

    When the SIMD16 Gen4-5 fragment shader payload contains source depth
    (g2-3), destination stencil (g4), and destination depth (g5-6), the
    single register of stencil makes the destination depth unaligned.

    We were generating this instruction in the RT write payload setup:

       mov(16)   m14<1>F   g5<8,8,1>F   { align1 compr };

    which is illegal, instructions with a source region spanning more than
    one register need to be aligned to even registers.  This is because the
    hardware implicitly does (nr | 1) instead of (nr + 1) when splitting the
    compressed instruction into two mov(8)'s.

    I believe this would cause the hardware to load g5 twice, replicating
    subspan 0-1's destination depth to subspan 2-3.  This showed up as 2x2
    artifact blocks in both TIS-100 and Reicast.

    Normally, we rely on the register allocator to even-align our virtual
    GRFs.  But we don't control the payload, so we need to lower SIMD widths
    to make it work.  To fix this, we teach lower_simd_width about the
    restriction, and then call it again after lower_load_payload (which is
    what generates the offending MOV).

    Fixes: 8aee87fe4cce0a883867df3546db0e0a36908086 (i965: Use SIMD16 instead
of SIMD8 on Gen4 when possible.)
    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107212
    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=13728
    Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>
    Tested-by: Diego Viola <diego.viola at gmail.com>

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