[Mesa-dev] [PATCH] st/glsl_to_tgsi: drop the merge_registers() pass

[Rob Clark]

so I guess this is likely to hurt pipe drivers that don't (yet?) have
a real compiler backend.  (Ie. etnaviv and freedreno/a2xx.)  So maybe
it should be optional.

Also I wonder about the pre-llvm radeon gen's, since sb uses the
actual instruction encoding for IR between tgsi->sb and backend opt
passes..  iirc they have had problems when the tgsi code uses too many
registers.

BR,
-R

On Mon, Apr 24, 2017 at 5:01 PM, Samuel Pitoiset
<samuel.pitoiset at gmail.com> wrote:
> The main goal of this pass to merge temporary registers in order
> to reduce the total number of registers and also to produce
> optimal TGSI code.
>
> In fact, compilers seem to be confused when temporary variables
> are already merged, maybe because it's done too early in the
> process.
>
> Removing the pass, reduce both the register pressure and the code
> size (TGSI is no longer optimized, but who cares?).
> shader-db results with RadeonSI and Nouveau are interesting.
>
> Nouveau:
>
> total instructions in shared programs : 3931608 -> 3929463 (-0.05%)
> total gprs used in shared programs    : 481255 -> 479014 (-0.47%)
> total local used in shared programs   : 27481 -> 27381 (-0.36%)
> total bytes used in shared programs   : 36031256 -> 36011120 (-0.06%)
>
>                 local        gpr       inst      bytes
>     helped          14        1471        1309        1309
>       hurt           1          88         384         384
>
> RadeonSI:
>
> PERCENTAGE DELTAS    Shaders     SGPRs     VGPRs SpillSGPR SpillVGPR  PrivVGPR   Scratch  CodeSize  MaxWaves     Waits
> ----------------------------------------------------------------------------------------------------------------------
> All affected            4906   -0.31 %   -0.40 %   -2.93 %  -20.00 %     .      -20.00 %   -0.18 %    0.19 %     .
> ----------------------------------------------------------------------------------------------------------------------
> Total                  47109   -0.04 %   -0.05 %   -1.97 %   -7.14 %     .       -0.30 %   -0.03 %    0.02 %     .
>
> Found by luck while fixing an issue in the TGSI dead code elimination
> pass which affects tex instructions with bindless samplers.
>
> Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
> ---
>  src/mesa/state_tracker/st_glsl_to_tgsi.cpp | 62 ------------------------------
>  1 file changed, 62 deletions(-)
>
> diff --git a/src/mesa/state_tracker/st_glsl_to_tgsi.cpp b/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
> index de7fe7837a..d033bdcc5a 100644
> --- a/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
> +++ b/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
> @@ -565,7 +565,6 @@ public:
>     int eliminate_dead_code(void);
>
>     void merge_two_dsts(void);
> -   void merge_registers(void);
>     void renumber_registers(void);
>
>     void emit_block_mov(ir_assignment *ir, const struct glsl_type *type,
> @@ -5262,66 +5261,6 @@ glsl_to_tgsi_visitor::merge_two_dsts(void)
>     }
>  }
>
> -/* Merges temporary registers together where possible to reduce the number of
> - * registers needed to run a program.
> - *
> - * Produces optimal code only after copy propagation and dead code elimination
> - * have been run. */
> -void
> -glsl_to_tgsi_visitor::merge_registers(void)
> -{
> -   int *last_reads = rzalloc_array(mem_ctx, int, this->next_temp);
> -   int *first_writes = rzalloc_array(mem_ctx, int, this->next_temp);
> -   struct rename_reg_pair *renames = rzalloc_array(mem_ctx, struct rename_reg_pair, this->next_temp);
> -   int i, j;
> -   int num_renames = 0;
> -
> -   /* Read the indices of the last read and first write to each temp register
> -    * into an array so that we don't have to traverse the instruction list as
> -    * much. */
> -   for (i = 0; i < this->next_temp; i++) {
> -      last_reads[i] = -1;
> -      first_writes[i] = -1;
> -   }
> -   get_last_temp_read_first_temp_write(last_reads, first_writes);
> -
> -   /* Start looking for registers with non-overlapping usages that can be
> -    * merged together. */
> -   for (i = 0; i < this->next_temp; i++) {
> -      /* Don't touch unused registers. */
> -      if (last_reads[i] < 0 || first_writes[i] < 0) continue;
> -
> -      for (j = 0; j < this->next_temp; j++) {
> -         /* Don't touch unused registers. */
> -         if (last_reads[j] < 0 || first_writes[j] < 0) continue;
> -
> -         /* We can merge the two registers if the first write to j is after or
> -          * in the same instruction as the last read from i.  Note that the
> -          * register at index i will always be used earlier or at the same time
> -          * as the register at index j. */
> -         if (first_writes[i] <= first_writes[j] &&
> -             last_reads[i] <= first_writes[j]) {
> -            renames[num_renames].old_reg = j;
> -            renames[num_renames].new_reg = i;
> -            num_renames++;
> -
> -            /* Update the first_writes and last_reads arrays with the new
> -             * values for the merged register index, and mark the newly unused
> -             * register index as such. */
> -            assert(last_reads[j] >= last_reads[i]);
> -            last_reads[i] = last_reads[j];
> -            first_writes[j] = -1;
> -            last_reads[j] = -1;
> -         }
> -      }
> -   }
> -
> -   rename_temp_registers(num_renames, renames);
> -   ralloc_free(renames);
> -   ralloc_free(last_reads);
> -   ralloc_free(first_writes);
> -}
> -
>  /* Reassign indices to temporary registers by reusing unused indices created
>   * by optimization passes. */
>  void
> @@ -6712,7 +6651,6 @@ get_mesa_program_tgsi(struct gl_context *ctx,
>     while (v->eliminate_dead_code());
>
>     v->merge_two_dsts();
> -   v->merge_registers();
>     v->renumber_registers();
>
>     /* Write the END instruction. */
> --
> 2.12.2
>
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