This patch is
Reviewed-by: Ian Romanick <ian.d.roman...@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 <kenn...@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
>
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