[Mesa-dev] [PATCH 5/5] glsl: Don't do constant propagation in opt_constant_folding.

[Ian Romanick]

This patch is

Reviewed-by: Ian Romanick <ian.d.romanick at intel.com>

On 05/09/2016 08:50 AM, Kenneth Graunke wrote:
> opt_constant_folding is supposed to fold trees of constants into a
> single constant.  Surprisingly, it was also propagating constant values
> from variables into expression trees - even when the result couldn't be
> folded together.  This is opt_constant_propagation's job.
> 
> The ir_dereference_variable::constant_expression_value() method returns
> a clone of var->constant_value.  So we would replace the dereference
> with a constant, propagating it into the tree.
> 
> Skip over ir_dereference_variable to avoid this surprising behavior.
> However, add code to explicitly continue doing it in the constant
> propagation pass, as it's useful to do so.
> 
> shader-db statistics on Broadwell:
> 
> total instructions in shared programs: 8905349 -> 8905126 (-0.00%)
> instructions in affected programs: 30100 -> 29877 (-0.74%)
> helped: 93
> HURT: 20
> 
> total cycles in shared programs: 71017030 -> 71015944 (-0.00%)
> cycles in affected programs: 132456 -> 131370 (-0.82%)
> helped: 54
> HURT: 45
> 
> The only hurt programs are by a single instruction, while the helped
> ones are helped by 1-4 instructions.
> 
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/compiler/glsl/opt_constant_folding.cpp     |  9 +++++++++
>  src/compiler/glsl/opt_constant_propagation.cpp | 12 ++++++++++++
>  2 files changed, 21 insertions(+)
> 
> diff --git a/src/compiler/glsl/opt_constant_folding.cpp b/src/compiler/glsl/opt_constant_folding.cpp
> index ee67420..97dcc7e 100644
> --- a/src/compiler/glsl/opt_constant_folding.cpp
> +++ b/src/compiler/glsl/opt_constant_folding.cpp
> @@ -91,6 +91,15 @@ ir_constant_fold(ir_rvalue **rvalue)
>                       !array_ref->array_index->as_constant()))
>        return false;
>  
> +   /* No constant folding can be performed on variable dereferences.  We need
> +    * to explicitly avoid them, as calling constant_expression_value() on a
> +    * variable dereference will return a clone of var->constant_value.  This
> +    * would make us propagate the value into the tree, which isn't our job.
> +    */
> +   ir_dereference_variable *var_ref = (*rvalue)->as_dereference_variable();
> +   if (var_ref)
> +      return false;
> +
>     ir_constant *constant = (*rvalue)->constant_expression_value();
>     if (constant) {
>        *rvalue = constant;
> diff --git a/src/compiler/glsl/opt_constant_propagation.cpp b/src/compiler/glsl/opt_constant_propagation.cpp
> index 4b82bd1..fbc22b0 100644
> --- a/src/compiler/glsl/opt_constant_propagation.cpp
> +++ b/src/compiler/glsl/opt_constant_propagation.cpp
> @@ -138,8 +138,20 @@ public:
>  void
>  ir_constant_propagation_visitor::constant_folding(ir_rvalue **rvalue)
>  {
> +   if (*rvalue == NULL)
> +      return;
> +
>     if (ir_constant_fold(rvalue))
>        this->progress = true;
> +
> +   ir_dereference_variable *var_ref = (*rvalue)->as_dereference_variable();
> +   if (var_ref) {
> +      ir_constant *constant = var_ref->constant_expression_value();
> +      if (constant) {
> +         *rvalue = constant;
> +         this->progress = true;
> +      }
> +   }
>  }
>  
>  void
>