[Mesa-dev] [PATCH] intel/fs: Restrict live intervals to the subset possibly reachable from any definition.
Connor Abbott
cwabbott0 at gmail.com
Fri Sep 8 15:17:16 UTC 2017
On Thu, Sep 7, 2017 at 9:30 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> On Thu, Sep 7, 2017 at 5:50 PM, Francisco Jerez <currojerez at riseup.net>
> wrote:
>>
>> Currently the liveness analysis pass would extend a live interval up
>> to the top of the program when no unconditional and complete
>> definition of the variable is found that dominates all of its uses.
>>
>> This can lead to a serious performance problem in shaders containing
>> many partial writes, like scalar arithmetic, FP64 and soon FP16
>> operations. The number of oversize live intervals in such workloads
>> can cause the compilation time of the shader to explode because of the
>> worse than quadratic behavior of the register allocator and scheduler
>> when running out of registers, and it can also cause the running time
>> of the shader to explode due to the amount of spilling it leads to,
>> which is orders of magnitude slower than GRF memory.
>>
>> This patch fixes it by computing the intersection of our current live
>> intervals with the subset of the program that can possibly be reached
>> from any definition of the variable. Extending the storage allocation
>> of the variable beyond that is pretty useless because its value is
>> guaranteed to be undefined at a point that cannot be reached from any
>> definition.
>>
>> No significant change in the running time of shader-db (with 5%
>> statistical significance).
>>
>> shader-db results on IVB:
>>
>> total cycles in shared programs: 61108780 -> 60932856 (-0.29%)
>> cycles in affected programs: 16335482 -> 16159558 (-1.08%)
>> helped: 5121
>> HURT: 4347
>>
>> total spills in shared programs: 1309 -> 1288 (-1.60%)
>> spills in affected programs: 249 -> 228 (-8.43%)
>> helped: 3
>> HURT: 0
>>
>> total fills in shared programs: 1652 -> 1597 (-3.33%)
>> fills in affected programs: 262 -> 207 (-20.99%)
>> helped: 4
>> HURT: 0
>>
>> LOST: 2
>> GAINED: 209
>>
>> shader-db results on BDW:
>>
>> total cycles in shared programs: 67617262 -> 67361220 (-0.38%)
>> cycles in affected programs: 23397142 -> 23141100 (-1.09%)
>> helped: 8045
>> HURT: 6488
>>
>> total spills in shared programs: 1456 -> 1252 (-14.01%)
>> spills in affected programs: 465 -> 261 (-43.87%)
>> helped: 3
>> HURT: 0
>>
>> total fills in shared programs: 1720 -> 1465 (-14.83%)
>> fills in affected programs: 471 -> 216 (-54.14%)
>> helped: 4
>> HURT: 0
>>
>> LOST: 2
>> GAINED: 162
>>
>> shader-db results on SKL:
>>
>> total cycles in shared programs: 65436248 -> 65245186 (-0.29%)
>> cycles in affected programs: 22560936 -> 22369874 (-0.85%)
>> helped: 8457
>> HURT: 6247
>
>
> I'm a but surprised at how many programs this affected. For something that
> doesn't do any partial writes before a full write, it should have no effect
> whatsoever.
Not quite. For example, in something like:
if (...) {
foo = ...;
}
// later, maybe in other control flow
... = foo;
originally, the live range of foo would extend to the beginning of the
program, but with this change it wouldn't since no definitions reach
there and foo is undefined anyways. This sort of happens automatically
in SSA with phi nodes and undef instructions, but not post-SSA.
Something like it probably explains most of the changes.
> Maybe this is because of constant combining or something?
>
> FYI, I applied this on top of my subgroups series and the dvec2 broadcast
> test went from around 1k fills per shader to 0 and the test run time (mostly
> compile time) goes from 130s to 7.5s. I'll let you immagine how long the
> dvec4 test takes to compile. :-)
>
> --Jason
>
>>
>> total spills in shared programs: 437 -> 437 (0.00%)
>> spills in affected programs: 0 -> 0
>> helped: 0
>> HURT: 0
>>
>> total fills in shared programs: 870 -> 854 (-1.84%)
>> fills in affected programs: 16 -> 0
>> helped: 1
>> HURT: 0
>>
>> LOST: 0
>> GAINED: 107
>> ---
>> src/intel/compiler/brw_fs_live_variables.cpp | 34
>> ++++++++++++++++++++++++----
>> src/intel/compiler/brw_fs_live_variables.h | 12 ++++++++++
>> 2 files changed, 42 insertions(+), 4 deletions(-)
>>
>> diff --git a/src/intel/compiler/brw_fs_live_variables.cpp
>> b/src/intel/compiler/brw_fs_live_variables.cpp
>> index c449672a519..059f076fa51 100644
>> --- a/src/intel/compiler/brw_fs_live_variables.cpp
>> +++ b/src/intel/compiler/brw_fs_live_variables.cpp
>> @@ -83,9 +83,11 @@ fs_live_variables::setup_one_write(struct block_data
>> *bd, fs_inst *inst,
>> /* The def[] bitset marks when an initialization in a block completely
>> * screens off previous updates of that variable (VGRF channel).
>> */
>> - if (inst->dst.file == VGRF && !inst->is_partial_write()) {
>> - if (!BITSET_TEST(bd->use, var))
>> + if (inst->dst.file == VGRF) {
>> + if (!inst->is_partial_write() && !BITSET_TEST(bd->use, var))
>> BITSET_SET(bd->def, var);
>> +
>> + BITSET_SET(bd->defout, var);
>> }
>> }
>>
>> @@ -199,6 +201,28 @@ fs_live_variables::compute_live_variables()
>> }
>> }
>> }
>> +
>> + /* Propagate defin and defout down the CFG to calculate the union of
>> live
>> + * variables potentially defined along any possible control flow path.
>> + */
>> + do {
>> + cont = false;
>> +
>> + foreach_block (block, cfg) {
>> + const struct block_data *bd = &block_data[block->num];
>> +
>> + foreach_list_typed(bblock_link, child_link, link,
>> &block->children) {
>> + struct block_data *child_bd =
>> &block_data[child_link->block->num];
>> +
>> + for (int i = 0; i < bitset_words; i++) {
>> + const BITSET_WORD new_def = bd->defout[i] &
>> ~child_bd->defin[i];
>> + child_bd->defin[i] |= new_def;
>> + child_bd->defout[i] |= new_def;
>> + cont |= new_def;
>> + }
>> + }
>> + }
>> + } while (cont);
>> }
>>
>> /**
>> @@ -212,12 +236,12 @@ fs_live_variables::compute_start_end()
>> struct block_data *bd = &block_data[block->num];
>>
>> for (int i = 0; i < num_vars; i++) {
>> - if (BITSET_TEST(bd->livein, i)) {
>> + if (BITSET_TEST(bd->livein, i) && BITSET_TEST(bd->defin, i)) {
>> start[i] = MIN2(start[i], block->start_ip);
>> end[i] = MAX2(end[i], block->start_ip);
>> }
>>
>> - if (BITSET_TEST(bd->liveout, i)) {
>> + if (BITSET_TEST(bd->liveout, i) && BITSET_TEST(bd->defout, i)) {
>> start[i] = MIN2(start[i], block->end_ip);
>> end[i] = MAX2(end[i], block->end_ip);
>> }
>> @@ -260,6 +284,8 @@ fs_live_variables::fs_live_variables(fs_visitor *v,
>> const cfg_t *cfg)
>> block_data[i].use = rzalloc_array(mem_ctx, BITSET_WORD,
>> bitset_words);
>> block_data[i].livein = rzalloc_array(mem_ctx, BITSET_WORD,
>> bitset_words);
>> block_data[i].liveout = rzalloc_array(mem_ctx, BITSET_WORD,
>> bitset_words);
>> + block_data[i].defin = rzalloc_array(mem_ctx, BITSET_WORD,
>> bitset_words);
>> + block_data[i].defout = rzalloc_array(mem_ctx, BITSET_WORD,
>> bitset_words);
>>
>> block_data[i].flag_def[0] = 0;
>> block_data[i].flag_use[0] = 0;
>> diff --git a/src/intel/compiler/brw_fs_live_variables.h
>> b/src/intel/compiler/brw_fs_live_variables.h
>> index d2d5898ed1c..9e95e443170 100644
>> --- a/src/intel/compiler/brw_fs_live_variables.h
>> +++ b/src/intel/compiler/brw_fs_live_variables.h
>> @@ -55,6 +55,18 @@ struct block_data {
>> /** Which defs reach the exit point of the block. */
>> BITSET_WORD *liveout;
>>
>> + /**
>> + * Variables such that the entry point of the block may be reached
>> from any
>> + * of their definitions.
>> + */
>> + BITSET_WORD *defin;
>> +
>> + /**
>> + * Variables such that the exit point of the block may be reached from
>> any
>> + * of their definitions.
>> + */
>> + BITSET_WORD *defout;
>> +
>> BITSET_WORD flag_def[1];
>> BITSET_WORD flag_use[1];
>> BITSET_WORD flag_livein[1];
>> --
>> 2.14.1
>>
>
>
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