[Mesa-dev] [PATCH] i965: Disable try_emit_b2f_of_compare on Gen4-6.

[Chris Forbes]

Reviewed-by: Chris Forbes <chrisf at ijw.co.nz>
On Aug 17, 2014 10:14 AM, "Kenneth Graunke" <kenneth at whitecape.org> wrote:

> The optimization relies on CMP setting the destination to 0, which is
> equivalent to 0.0f.  However, early platforms only set the least
> significant byte, leaving the other bits undefined.  So, we must disable
> the optimization on those platforms.
>
> Oddly, Sandybridge wasn't reported as broken, even though it's
> documented to only set the LSB.  It's unclear whether it follows ILK on
> IVB behavior.
>
> +32 piglits on Ironlake.
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?=79963
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
> b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
> index f22d38d..33cb9a9 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
> @@ -1135,6 +1135,13 @@ vec4_visitor::try_emit_mad(ir_expression *ir)
>  bool
>  vec4_visitor::try_emit_b2f_of_compare(ir_expression *ir)
>  {
> +   /* This optimization relies on CMP setting the destination to 0 when
> +    * false.  Early hardware only sets the least significant bit, and
> +    * leaves the other bits undefined.  So we can't use it.
> +    */
> +   if (brw->gen < 7)
> +      return false;
> +
>     ir_expression *const cmp = ir->operands[0]->as_expression();
>
>     if (cmp == NULL)
> --
> 1.9.2
>
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