[Mesa-dev] [PATCH 11/12] i965/vec4: Eliminate writes that are never read.

[Eric Anholt]

Matt Turner <mattst88 at gmail.com> writes:

> With an awful O(n^2) algorithm that searches previous instructions for
> dead writes.
>
> total instructions in shared programs: 805582 -> 788074 (-2.17%)
> instructions in affected programs:     144561 -> 127053 (-12.11%)
> ---
>  src/mesa/drivers/dri/i965/brw_vec4.cpp | 46 ++++++++++++++++++++++++++++++++++
>  1 file changed, 46 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4.cpp b/src/mesa/drivers/dri/i965/brw_vec4.cpp
> index 4ad398a..e9219a9 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4.cpp
> @@ -369,6 +369,7 @@ bool
>  vec4_visitor::dead_code_eliminate()
>  {
>     bool progress = false;
> +   bool seen_control_flow = false;
>     int pc = -1;
>  
>     calculate_live_intervals();
> @@ -378,6 +379,8 @@ vec4_visitor::dead_code_eliminate()
>  
>        pc++;
>  
> +      seen_control_flow = inst->is_control_flow() || seen_control_flow;
> +

So, once there's control flow in the program, this piece of optimization
doesn't happen ever after it?  Seems like in the walk backwards you
could just stop the walk when you find a control flow instruction.

>        if (inst->dst.file != GRF || inst->has_side_effects())
>           continue;
>  
> @@ -393,6 +396,49 @@ vec4_visitor::dead_code_eliminate()
>        }
>  
>        progress = try_eliminate_instruction(inst, write_mask) || progress;
> +
> +      if (seen_control_flow || inst->predicate || inst->prev == NULL)
> +         continue;
> +
> +      int dead_channels = inst->dst.writemask;
> +
> +      for (int i = 0; i < 3; i++) {
> +         if (inst->src[i].file != GRF ||
> +             inst->src[i].reg != inst->dst.reg)
> +               continue;
> +
> +         for (int j = 0; j < 4; j++) {
> +            int swiz = BRW_GET_SWZ(inst->src[i].swizzle, j);
> +            dead_channels &= ~(1 << swiz);
> +         }
> +      }
> +
> +      for (exec_node *node = inst->prev, *prev = node->prev;
> +           prev != NULL && dead_channels != 0;
> +           node = prev, prev = prev->prev) {

You could potentially terminate the loop when you're out of the live
range of the dst, which would reduce the pain of n^2.

> +         vec4_instruction *scan_inst = (vec4_instruction  *)node;
> +
> +         if (scan_inst->dst.file != GRF || scan_inst->has_side_effects())
> +            continue;
> +
> +         if (inst->dst.reg == scan_inst->dst.reg) {
> +            int new_writemask = scan_inst->dst.writemask & ~dead_channels;
> +
> +            progress = try_eliminate_instruction(scan_inst, new_writemask) ||
> +                       progress;
> +         }
> +
> +         for (int i = 0; i < 3; i++) {
> +            if (scan_inst->src[i].file != GRF ||
> +                scan_inst->src[i].reg != inst->dst.reg)
> +               continue;
> +
> +            for (int j = 0; j < 4; j++) {
> +               int swiz = BRW_GET_SWZ(scan_inst->src[i].swizzle, j);
> +               dead_channels &= ~(1 << swiz);
> +            }
> +         }
> +      }
>     }
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