AlexIndustrial/mesa
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

i965/fs_live_variables: Do liveness analysis bottom-to-top

Browse Source 
From Muchnick's Advanced Compiler Design and Implementation:

"To determine which variables are live at each point in a flowgraph, we
perform a backward data-flow analysis"

Previously, we were walking the blocks forwards and updating the livein and
then the liveout.  However, the livein calculation depends on the liveout
and the liveout depends on the successor blocks.  The net result is that it
takes one full iteration to go from liveout to livein and then another
full iteration to propagate to the predecessors.  This works out to an
O(n^2) computation where n is the number of blocks.  If we run things in
the other order, it's O(nl) where l is the maximum loop depth which is
practically bounded by 3.

On my HSW desktop, one particular shadertoy test gets a 20% improvement in
compile times:

N           Min           Max        Median           Avg        Stddev
x  10        15.965        16.884        16.026       16.1822    0.34736846
+  10        12.813        13.052        12.876       12.8891    0.06913666
Difference at 95.0% confidence
        -3.2931 +/- 0.235316
        -20.3501% +/- 1.45417%
        (Student's t, pooled s = 0.250444)

Reviewed-by: Matt Turner <mattst88@gmail.com>
This commit is contained in:
Jason Ekstrand
2015-06-08 16:03:19 -07:00
parent 104c8fc2c2
commit b2c6ba0c4b
1 changed files with 19 additions and 19 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/mesa/drivers/dri/i965/brw_fs_live_variables.cpp
+19 -19
View File
@@ -204,27 +204,9 @@ fs_live_variables::compute_live_variables()
while (cont) {
cont = false;
foreach_block (block, cfg) {
foreach_block_reverse (block, cfg) {
struct block_data *bd = &block_data[block->num];
/* Update livein */
for (int i = 0; i < bitset_words; i++) {
BITSET_WORD new_livein = (bd->use[i] |
(bd->liveout[i] &
~bd->def[i]));
if (new_livein & ~bd->livein[i]) {
bd->livein[i] |= new_livein;
cont = true;
}
}
BITSET_WORD new_livein = (bd->flag_use[0] |
(bd->flag_liveout[0] &
~bd->flag_def[0]));
if (new_livein & ~bd->flag_livein[0]) {
bd->flag_livein[0] |= new_livein;
cont = true;
}
/* Update liveout */
foreach_list_typed(bblock_link, child_link, link, &block->children) {
struct block_data *child_bd = &block_data[child_link->block->num];
@@ -244,6 +226,24 @@ fs_live_variables::compute_live_variables()
cont = true;
}
}
/* Update livein */
for (int i = 0; i < bitset_words; i++) {
BITSET_WORD new_livein = (bd->use[i] |
(bd->liveout[i] &
~bd->def[i]));
if (new_livein & ~bd->livein[i]) {
bd->livein[i] |= new_livein;
cont = true;
}
}
BITSET_WORD new_livein = (bd->flag_use[0] |
(bd->flag_liveout[0] &
~bd->flag_def[0]));
if (new_livein & ~bd->flag_livein[0]) {
bd->flag_livein[0] |= new_livein;
cont = true;
}
}
}
}