[Mesa-dev] [PATCH] st/glsl_to_tgsi: drop the merge_registers() pass
Samuel Pitoiset
samuel.pitoiset at gmail.com
Mon Apr 24 21:18:13 UTC 2017
On 04/24/2017 11:12 PM, Rob Clark wrote:
> 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.
I can't say for these drivers, but it seems safer to add an option if
this can hurt them. How about PIPE_SHADER_CAP_TGSI_MERGE_REGISTERS?
>
> 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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