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. In b2c6ba0c4b21391dc35018e1c8c4f7f7d8952bea, we made this same change in the FS backend to great effect. Might as well keep it consistent and make the same change for vec4. --- .../drivers/dri/i965/brw_vec4_live_variables.cpp | 38 +++++++++++----------- 1 file changed, 19 insertions(+), 19 deletions(-) diff --git a/src/mesa/drivers/dri/i965/brw_vec4_live_variables.cpp b/src/mesa/drivers/dri/i965/brw_vec4_live_variables.cpp index 95b9d90..29b4a53 100644 --- a/src/mesa/drivers/dri/i965/brw_vec4_live_variables.cpp +++ b/src/mesa/drivers/dri/i965/brw_vec4_live_variables.cpp @@ -133,27 +133,9 @@ vec4_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]; @@ -173,6 +155,24 @@ vec4_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; + } } } } -- 2.4.3 _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev