From b2c6ba0c4b21391dc35018e1c8c4f7f7d8952bea Mon Sep 17 00:00:00 2001 From: Jason Ekstrand Date: Mon, 8 Jun 2015 16:03:19 -0700 Subject: [PATCH] i965/fs_live_variables: Do liveness analysis bottom-to-top 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 --- .../dri/i965/brw_fs_live_variables.cpp | 38 +++++++++---------- 1 file changed, 19 insertions(+), 19 deletions(-) diff --git a/src/mesa/drivers/dri/i965/brw_fs_live_variables.cpp b/src/mesa/drivers/dri/i965/brw_fs_live_variables.cpp index 502161d5128..19aec92fad1 100644 --- a/src/mesa/drivers/dri/i965/brw_fs_live_variables.cpp +++ b/src/mesa/drivers/dri/i965/brw_fs_live_variables.cpp @@ -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; + } } } } -- 2.30.2