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

[Matt Turner]

On Sat, Aug 16, 2014 at 3:18 PM, 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)

I'd rather we made this brw->gen < 6. The internal BSpec says
Sandybridge behaves like Ivybridge, and my other series assumes that
it does. If we haven't seen bugs from this on Sandybridge, that seems
to confirm it.

With that,

Reviewed-by: Matt Turner <mattst88 at gmail.com>