[Mesa-dev] [PATCH 04/23] i965/fs: Don't emit dumb SEL

[Tapani Pälli]

On 03/20/2015 10:58 PM, Ian Romanick wrote:
> From: Ian Romanick <ian.d.romanick at intel.com>
>
> With previous changes to emit more ir_triop_csel, the i965 channel
> expressions pass can generate sequences like:
>
>      mov(8)          g45<1>F      1F
>      (+f0) sel(8)    g40<1>F      g45<8,8,1>F 1F
>
> There are no shader-db changes now, but this prevents a large number of
> regressions from a later commit (glsl: Optimize certain if-statements to
> ir_triop_csel).
>
> Signed-off-by: Ian Romanick <ian.d.romanick at intel.com>
> Cc: Tapani Pälli <tapani.palli at intel.com>
> ---
>   src/mesa/drivers/dri/i965/brw_fs_visitor.cpp | 9 +++++++++
>   1 file changed, 9 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> index e39dab3..1fbef5f 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> @@ -708,6 +708,15 @@ fs_visitor::visit(ir_expression *ir)
>         break;
>
>      case ir_triop_csel:
> +      /* After splitting an expression like 'v = csel(cond, vec4(a, b, c, 1),
> +       * vec4(d, e, f, 1))', there will be a 'v.w = csel(cond, 1, 1)'.  Detect
> +       * this, and avoid emitting the spurious SEL.
> +       */
> +      if (ir->operands[1]->equals(ir->operands[2])) {
> +         ir->operands[1]->accept(this);
> +         return;

The result is correct but why is this done in the backend? Couldn't the 
later commit mentioned for glsl already detect this or maybe additional 
optimization pass there?

> +      }
> +
>         if (try_opt_frontfacing_ternary(ir))
>            return;
>
>

// Tapani