[Mesa-dev] [PATCH 2/4] i965: Fix multiplication of immediates on Cherryview/Broxton.

[Jordan Justen]

On 2016-06-10 14:19:44, Kenneth Graunke wrote:
> Cherryview and Broxton don't support DW x DW multiplication.  We have
> piles of code to handle this, but apparently weren't retyping in the
> immediate case.
> 
> For example,
> tests/spec/arb_tessellation_shader/execution/dvec3-vs-tcs-tes
> makes the simulator angry about instructions such as:
> 
>    mul(8) r18<1>:D r10.0<8;8,1>:D 0x00000003:D
> 
> Just retype to UW.  It should be safe everywhere.
> 
> Cc: "12.0" <mesa-stable at lists.freedesktop.org>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=95462
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/mesa/drivers/dri/i965/brw_fs.cpp | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
> index 4b29ee5..13246c2 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
> @@ -3564,7 +3564,10 @@ fs_visitor::lower_integer_multiplication()
>                 ibld.MOV(imm, inst->src[1]);
>                 ibld.MUL(inst->dst, imm, inst->src[0]);
>              } else {
> -               ibld.MUL(inst->dst, inst->src[0], inst->src[1]);
> +               const bool ud = (inst->src[1].type == BRW_REGISTER_TYPE_UD);
> +               ibld.MUL(inst->dst, inst->src[0],
> +                        ud ? brw_imm_uw(inst->src[1].ud)
> +                           : brw_imm_w(inst->src[1].d));

This change looks fine, but will it actually be possible to hit this
code path for negative numbers? Above, we have:

   if (inst->src[1].file == IMM &&
       inst->src[1].ud < (1 << 16)) {

Bit 31 would be set if inst->src[1].d has a negative number.

-Jordan

>              }
>           } else {
>              /* Gen < 8 (and some Gen8+ low-power parts like Cherryview) cannot
> -- 
> 2.8.3
> 
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