[Mesa-dev] [PATCH] glsl: Speed up constant folding for swizzles.

[Jordan Justen]

Reviewed-by: Jordan Justen <jordan.l.justen at intel.com>

On Fri, Sep 12, 2014 at 3:16 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> ir_rvalue::constant_expression_value() recursively walks down an IR
> tree, attempting to reduce it to a single constant value.  This is
> useful when you want to know whether a variable has a constant
> expression value at all, and if so, what it is.
>
> The constant folding optimization pass attempts to replace rvalues with
> their constant expression value from the bottom up.  That way, we can
> optimize subexpressions, and ideally stop as soon as we find a
> non-constant subexpression.
>
> In order to obtain the actual value of an expression, the optimization
> pass calls constant_expression_value().  But it should only do so if it
> knows the value can be combined into a constant.  Otherwise, at each
> step of walking back up the tree, it will walk down the tree again, only
> to discover what it already knew: it isn't constant.
>
> We properly avoided this call for ir_expression nodes, but not for
> ir_swizzle nodes.  This patch fixes that, drastically reducing compile
> times on certain shaders where tree grafting has given us huge
> expression trees.  It also fixes SuperTuxKart.
>
> Thanks to Iago and Mike for help in tracking this down.
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=78468
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> Cc: mesa-stable at lists.freedesktop.org
> ---
>  src/glsl/opt_constant_folding.cpp | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/src/glsl/opt_constant_folding.cpp b/src/glsl/opt_constant_folding.cpp
> index d0e5754..74b855e 100644
> --- a/src/glsl/opt_constant_folding.cpp
> +++ b/src/glsl/opt_constant_folding.cpp
> @@ -79,6 +79,11 @@ ir_constant_folding_visitor::handle_rvalue(ir_rvalue **rvalue)
>        }
>     }
>
> +   /* Ditto for swizzles. */
> +   ir_swizzle *swiz = (*rvalue)->as_swizzle();
> +   if (swiz && !swiz->val->as_constant())
> +      return;
> +
>     ir_constant *constant = (*rvalue)->constant_expression_value();
>     if (constant) {
>        *rvalue = constant;
> --
> 2.1.0
>
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