Hi,
On 20.12.2017 16:29, Eero Tamminen wrote:
I got unexpected results with this, when testing it on BXT & SKL GT2.
While performance in GpuTest Volplosion and GfxBench v4 Tessellation
test improved slightly, performance in SynMark v7 CSDof and GpuTest v0.7
Piano dropped clearly.
Piano dropped only on SKL, but there the drop was >10%.
When looking at the generated shader assembly before and after, there's
one huge shader for which Mesa is able to generated only SIMD8 assembly.
Before the patch:
SIMD8 shader: 2727 instructions. 5 loops. 470840 cycles. 0:0
spills:fills. Promoted 58 constants. Compacted 43632 to 32560 bytes (25%)
After the patch:
SIMD8 shader: 4784 instructions. 5 loops. 808006 cycles. 212:367
spills:fills. Promoted 9 constants. Compacted 76544 to 46192 bytes (40%)
Because shader spills i.e. there are no free regs, there are also more
register bank conflicts.
- Eero
> There was also
> a peculiar change in its CPU vs. GPU utilization at the start of the
> test. With this patch, there's 5-10s of 100% CPU usage before the test
> starts, which extra CPU usage happens only with this patch.
>
> Attached is Callgrind output of where the extra time at startup goes.
>
(I didn't see that issue with the other tests which perf changed.)
- Eero
On 20.12.2017 04:19, Francisco Jerez wrote:
Previously the dataflow propagation algorithm would calculate the ACP
live-in and -out sets in a two-pass fixed-point algorithm. The first
pass would update the live-out sets of all basic blocks of the program
based on their live-in sets, while the second pass would update the
live-in sets based on the live-out sets. This is incredibly
inefficient in the typical case where the CFG of the program is
approximately acyclic, because it can take up to 2*n passes for an ACP
entry introduced at the top of the program to reach the bottom (where
n is the number of basic blocks in the program), until which point the
algorithm won't be able to reach a fixed point.
The same effect can be achieved in a single pass by computing the
live-in and -out sets in lock-step, because that makes sure that
processing of any basic block will pick up the updated live-out sets
of the lexically preceding blocks. This gives the dataflow
propagation algorithm effectively O(n) run-time instead of O(n^2) in
the acyclic case.
The time spent in dataflow propagation is reduced by 30x in the
GLES31.functional.ssbo.layout.random.all_shared_buffer.5 dEQP
test-case on my CHV system (the improvement is likely to be of the
same order of magnitude on other platforms). This more than reverses
an apparent run-time regression in this test-case from my previous
copy-propagation undefined-value handling patch, which was ultimately
caused by the additional work introduced in that commit to account for
undefined values being multiplied by a huge quadratic factor.
According to Chad this test was failing on CHV due to a 30s time-out
imposed by the Android CTS (this was the case regardless of my
undefined-value handling patch, even though my patch substantially
exacerbated the issue). On my CHV system this patch reduces the
overall run-time of the test by approximately 12x, getting us to
around 13s, well below the time-out.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104271
Reported-by: Chad Versace <chadvers...@chromium.org>
---
src/intel/compiler/brw_fs_copy_propagation.cpp | 30
++++++++------------------
1 file changed, 9 insertions(+), 21 deletions(-)
diff --git a/src/intel/compiler/brw_fs_copy_propagation.cpp
b/src/intel/compiler/brw_fs_copy_propagation.cpp
index af5635eacef..8d6ab18cd13 100644
--- a/src/intel/compiler/brw_fs_copy_propagation.cpp
+++ b/src/intel/compiler/brw_fs_copy_propagation.cpp
@@ -228,34 +228,17 @@ fs_copy_prop_dataflow::run()
do {
progress = false;
- /* Update liveout for all blocks. */
foreach_block (block, cfg) {
if (block->parents.is_empty())
continue;
for (int i = 0; i < bitset_words; i++) {
const BITSET_WORD old_liveout = bd[block->num].liveout[i];
-
- bd[block->num].liveout[i] =
- bd[block->num].copy[i] | (bd[block->num].livein[i] &
- ~bd[block->num].kill[i]);
-
- if (old_liveout != bd[block->num].liveout[i])
- progress = true;
- }
- }
-
- /* Update livein for all blocks. If a copy is live out of all
parent
- * blocks, it's live coming in to this block.
- */
- foreach_block (block, cfg) {
- if (block->parents.is_empty())
- continue;
-
- for (int i = 0; i < bitset_words; i++) {
- const BITSET_WORD old_livein = bd[block->num].livein[i];
BITSET_WORD livein_from_any_block = 0;
+ /* Update livein for this block. If a copy is live out
of all
+ * parent blocks, it's live coming in to this block.
+ */
bd[block->num].livein[i] = ~0u;
foreach_list_typed(bblock_link, parent_link, link,
&block->parents) {
bblock_t *parent = parent_link->block;
@@ -278,7 +261,12 @@ fs_copy_prop_dataflow::run()
*/
bd[block->num].livein[i] &= livein_from_any_block;
- if (old_livein != bd[block->num].livein[i])
+ /* Update liveout for this block. */
+ bd[block->num].liveout[i] =
+ bd[block->num].copy[i] | (bd[block->num].livein[i] &
+ ~bd[block->num].kill[i]);
+
+ if (old_liveout != bd[block->num].liveout[i])
progress = true;
}
}
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