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/*
* 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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