--- /dev/null
+/*
+ * Copyright © 2016 Intel Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+#include "nir.h"
+
+/**
+ * \file nir_opt_move_comparisons.c
+ *
+ * This pass moves ALU comparison operations just before their first use.
+ *
+ * It only moves instructions within a single basic block; cross-block
+ * movement is left to global code motion.
+ *
+ * Many GPUs generate condition codes for comparisons, and use predication
+ * for conditional selects and control flow. In a sequence such as:
+ *
+ * vec1 32 ssa_1 = flt a b
+ * <some other operations>
+ * vec1 32 ssa_2 = bcsel ssa_1 c d
+ *
+ * the backend would likely do the comparison, producing condition codes,
+ * then save those to a boolean value. The intervening operations might
+ * trash the condition codes. Then, in order to do the bcsel, it would
+ * need to re-populate the condition code register based on the boolean.
+ *
+ * By moving the comparison just before the bcsel, the condition codes could
+ * be used directly. This eliminates the need to reload them from the boolean
+ * (generally eliminating an instruction). It may also eliminate the need to
+ * create a boolean value altogether (unless it's used elsewhere), which could
+ * lower register pressure.
+ */
+
+static bool
+is_comparison(nir_op op)
+{
+ switch (op) {
+ case nir_op_flt:
+ case nir_op_fge:
+ case nir_op_feq:
+ case nir_op_fne:
+ case nir_op_ilt:
+ case nir_op_ult:
+ case nir_op_ige:
+ case nir_op_uge:
+ case nir_op_ieq:
+ case nir_op_ine:
+ case nir_op_i2b:
+ case nir_op_f2b:
+ case nir_op_inot:
+ case nir_op_fnot:
+ return true;
+ default:
+ return false;
+ }
+}
+
+static bool
+move_comparison_source(nir_src *src, nir_block *block, nir_instr *before)
+{
+ if (!src->is_ssa)
+ return false;
+
+ nir_instr *src_instr = src->ssa->parent_instr;
+
+ if (src_instr->block == block &&
+ src_instr->type == nir_instr_type_alu &&
+ is_comparison(nir_instr_as_alu(src_instr)->op)) {
+
+ exec_node_remove(&src_instr->node);
+
+ if (before)
+ exec_node_insert_node_before(&before->node, &src_instr->node);
+ else
+ exec_list_push_tail(&block->instr_list, &src_instr->node);
+
+ return true;
+ }
+
+ return false;
+}
+
+static bool
+move_comparison_source_cb(nir_src *src, void *data)
+{
+ bool *progress = data;
+
+ nir_instr *instr = src->parent_instr;
+ if (move_comparison_source(src, instr->block, instr))
+ *progress = true;
+
+ return true; /* nir_foreach_src should keep going */
+}
+
+static bool
+move_comparisons(nir_block *block)
+{
+ bool progress = false;
+
+ /* We use a simple approach: walk instructions backwards.
+ *
+ * If the instruction's source is a comparison from the same block,
+ * simply move it here. This may break SSA if it's used earlier in
+ * the block as well. However, as we walk backwards, we'll find the
+ * earlier use and move it again, further up. It eventually ends up
+ * dominating all uses again, restoring SSA form.
+ *
+ * Before walking instructions, we consider the if-condition at the
+ * end of the block, if one exists. It's effectively a use at the
+ * bottom of the block.
+ */
+ nir_if *iff = nir_block_get_following_if(block);
+ if (iff) {
+ progress |= move_comparison_source(&iff->condition, block, NULL);
+ }
+
+ nir_foreach_instr_reverse(instr, block) {
+ /* The sources of phi instructions happen after the predecessor block
+ * but before this block. (Yes, that's between blocks). This means
+ * that we don't need to move them in order for them to be correct.
+ * We could move them to encourage comparisons that are used in a phi to
+ * the end of the block, doing so correctly would make the pass
+ * substantially more complicated and wouldn't gain us anything since
+ * the phi can't use a flag value anyway.
+ */
+ if (instr->type == nir_instr_type_phi) {
+ /* We're going backwards so everything else is a phi too */
+ break;
+ } else if (instr->type == nir_instr_type_alu) {
+ /* Walk ALU instruction sources backwards so that bcsel's boolean
+ * condition is processed last.
+ */
+ nir_alu_instr *alu = nir_instr_as_alu(instr);
+ for (int i = nir_op_infos[alu->op].num_inputs - 1; i >= 0; i--) {
+ progress |= move_comparison_source(&alu->src[i].src,
+ block, instr);
+ }
+ } else {
+ nir_foreach_src(instr, move_comparison_source_cb, &progress);
+ }
+ }
+
+ return progress;
+}
+
+bool
+nir_opt_move_comparisons(nir_shader *shader)
+{
+ bool progress = false;
+
+ nir_foreach_function(func, shader) {
+ if (!func->impl)
+ continue;
+
+ nir_foreach_block(block, func->impl) {
+ if (move_comparisons(block)) {
+ nir_metadata_preserve(func->impl, nir_metadata_block_index |
+ nir_metadata_dominance |
+ nir_metadata_live_ssa_defs);
+ progress = true;
+ }
+ }
+ }
+
+ return progress;
+}
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