I'm not sure if this is relevant to the i965 fs backend in particular, but Briggs in his thesis [1] recommended simply ignoring the liveness of any register read by a copy instruction immediately after said instruction when computing the interference graph in order to solve this exact problem (see section 2.2.2).
[1] http://www.cs.utexas.edu/~mckinley/380C/lecs/briggs-thesis-1992.pdf On Dec 19, 2013 4:40 PM, "Matt Turner" <matts...@gmail.com> wrote: > Previously we simply considered two registers whose live ranges > overlapped to interfere. Cases such as > > set A ------ > ... | > mov B, A -- | > ... | B | A > use B -- | > ... | > use A ------ > > would be considered to interfere, even though B is an unmodified copy of > A whose live range fit wholly inside that of A. > > If no writes to A or B occur between the mov B, A and the use of B then > we can safely coalesce them. > > Instead of removing MOV instructions, we make them NOPs and remove them > at once after the main pass is finished in order to avoid recomputing > live intervals (which are needed to perform the previous step). > > total instructions in shared programs: 1543768 -> 1513077 (-1.99%) > instructions in affected programs: 951563 -> 920872 (-3.23%) > GAINED: 46 > LOST: 22 > --- > src/mesa/drivers/dri/i965/brw_fs.cpp | 69 > ++++++++++++++++++++++++++++++++---- > 1 file changed, 62 insertions(+), 7 deletions(-) > > diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp > b/src/mesa/drivers/dri/i965/brw_fs.cpp > index e4ac0a5..ad56b87 100644 > --- a/src/mesa/drivers/dri/i965/brw_fs.cpp > +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp > @@ -2273,7 +2273,7 @@ fs_visitor::register_coalesce() > int last_use[MAX_SAMPLER_MESSAGE_SIZE]; > int next_ip = 0; > > - foreach_list_safe(node, &this->instructions) { > + foreach_list(node, &this->instructions) { > fs_inst *inst = (fs_inst *)node; > > int ip = next_ip; > @@ -2299,8 +2299,39 @@ fs_visitor::register_coalesce() > int var_to = live_intervals->var_from_reg(&inst->dst); > > if (live_intervals->vars_interfere(var_from, var_to) && > - !inst->dst.equals(inst->src[0])) > - continue; > + !inst->dst.equals(inst->src[0])) { > + > + if (live_intervals->end[var_to] > live_intervals->end[var_from]) > + continue; > + > + bool overwritten = false; > + int scan_ip = -1; > + > + foreach_list(n, &this->instructions) { > + fs_inst *scan_inst = (fs_inst *)n; > + scan_ip++; > + > + if (scan_inst->is_control_flow()) { > + overwritten = true; > + break; > + } > + > + if (scan_ip <= live_intervals->start[var_to]) > + continue; > + > + if (scan_ip > live_intervals->end[var_to]) > + break; > + > + if (scan_inst->dst.equals(inst->dst) || > + scan_inst->dst.equals(inst->src[0])) { > + overwritten = true; > + break; > + } > + } > + > + if (overwritten) > + continue; > + } > > if (reg_from != inst->src[0].reg) { > reg_from = inst->src[0].reg; > @@ -2342,9 +2373,18 @@ fs_visitor::register_coalesce() > if (live_channels_remaining) > continue; > > + bool removed = false; > for (int i = 0; i < src_size; i++) { > - if (mov[i]) > - mov[i]->remove(); > + if (mov[i]) { > + removed = true; > + > + mov[i]->opcode = BRW_OPCODE_NOP; > + mov[i]->conditional_mod = BRW_CONDITIONAL_NONE; > + mov[i]->dst = reg_undef; > + mov[i]->src[0] = reg_undef; > + mov[i]->src[1] = reg_undef; > + mov[i]->src[2] = reg_undef; > + } > } > > foreach_list(node, &this->instructions) { > @@ -2366,11 +2406,26 @@ fs_visitor::register_coalesce() > scan_inst->src[j].reg_offset = reg_to_offset[i]; > } > } > - > - progress = true; > } > } > } > + > + if (removed) { > + live_intervals->start[var_to] = > MIN2(live_intervals->start[var_to], > + > live_intervals->start[var_from]); > + live_intervals->end[var_to] = MAX2(live_intervals->end[var_to], > + > live_intervals->end[var_from]); > + reg_from = -1; > + } > + } > + > + foreach_list_safe(node, &this->instructions) { > + fs_inst *inst = (fs_inst *)node; > + > + if (inst->opcode == BRW_OPCODE_NOP) { > + inst->remove(); > + progress = true; > + } > } > > if (progress) > -- > 1.8.3.2 > > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > http://lists.freedesktop.org/mailman/listinfo/mesa-dev >
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