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ed3e4c16dc
If there's only a single instruction in a basic block, then removing it would create an empty block. We seem to have trouble representing those as there are no instructions with an IP inside the block; several places mess up connections. While most blocks end in control flow instructions (which are rarely eliminated), ones preceding a DO instruction may end in an ordinary instruction. This makes such blocks tricky to merge with adjacent blocks - they may be between loops. Any optimization pass may may find such an instruction and want to eliminate it, and most of them are unprepared to perform such CFG link surgery. Nor do we want to make every pass aware of this issue. To work around this, we simply replace an instruction with a NOP when removing it from a block containing only that instruction, leaving the block in place. Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28971>
128 lines
4.3 KiB
C++
128 lines
4.3 KiB
C++
/*
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* Copyright © 2013 Intel Corporation
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*
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* Permission is hereby granted, free of charge, to any person obtaining a
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* copy of this software and associated documentation files (the "Software"),
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* to deal in the Software without restriction, including without limitation
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* the rights to use, copy, modify, merge, publish, distribute, sublicense,
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* and/or sell copies of the Software, and to permit persons to whom the
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* Software is furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice (including the next
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* paragraph) shall be included in all copies or substantial portions of the
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* Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
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* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
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* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
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* IN THE SOFTWARE.
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*/
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/** @file brw_dead_control_flow.cpp
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*
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* This file implements the dead control flow elimination optimization pass.
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*/
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#include "brw_fs.h"
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#include "brw_cfg.h"
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using namespace brw;
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/* Look for and eliminate dead control flow:
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*
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* - if/endif
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* - else in else/endif
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* - then in if/else/endif
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*/
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bool
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brw_fs_opt_dead_control_flow_eliminate(fs_visitor &s)
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{
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bool progress = false;
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foreach_block_safe (block, s.cfg) {
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bblock_t *prev_block = block->prev();
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if (!prev_block)
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continue;
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fs_inst *const inst = block->start();
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fs_inst *const prev_inst = prev_block->end();
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/* ENDIF instructions, by definition, can only be found at the start of
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* basic blocks.
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*/
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if (inst->opcode == BRW_OPCODE_ENDIF &&
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prev_inst->opcode == BRW_OPCODE_ELSE) {
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bblock_t *const else_block = prev_block;
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fs_inst *const else_inst = prev_inst;
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else_inst->remove(else_block);
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progress = true;
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} else if (inst->opcode == BRW_OPCODE_ENDIF &&
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prev_inst->opcode == BRW_OPCODE_IF) {
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bblock_t *const endif_block = block;
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bblock_t *const if_block = prev_block;
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fs_inst *const endif_inst = inst;
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fs_inst *const if_inst = prev_inst;
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bblock_t *earlier_block = NULL, *later_block = NULL;
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if (if_block->start_ip == if_block->end_ip) {
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earlier_block = if_block->prev();
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} else {
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earlier_block = if_block;
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}
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if_inst->remove(if_block);
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if (endif_block->start_ip == endif_block->end_ip) {
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later_block = endif_block->next();
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} else {
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later_block = endif_block;
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}
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endif_inst->remove(endif_block);
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assert((earlier_block == NULL) == (later_block == NULL));
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if (earlier_block && earlier_block->can_combine_with(later_block)) {
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earlier_block->combine_with(later_block);
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/* If ENDIF was in its own block, then we've now deleted it and
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* merged the two surrounding blocks, the latter of which the
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* __next block pointer was pointing to.
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*/
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if (endif_block != later_block) {
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__next = earlier_block->next();
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}
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}
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progress = true;
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} else if (inst->opcode == BRW_OPCODE_ELSE &&
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prev_inst->opcode == BRW_OPCODE_IF) {
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bblock_t *const else_block = block;
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fs_inst *const if_inst = prev_inst;
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fs_inst *const else_inst = inst;
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/* Since the else-branch is becoming the new then-branch, the
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* condition has to be inverted.
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*/
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if_inst->predicate_inverse = !if_inst->predicate_inverse;
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else_inst->remove(else_block);
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progress = true;
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} else if (inst->opcode == BRW_OPCODE_NOP &&
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prev_block->can_combine_with(block) &&
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exec_list_is_singular(&block->parents) &&
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exec_list_is_singular(&prev_block->children)) {
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prev_block->combine_with(block);
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inst->remove(prev_block);
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progress = true;
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}
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}
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if (progress)
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s.invalidate_analysis(DEPENDENCY_BLOCKS | DEPENDENCY_INSTRUCTIONS);
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return progress;
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}
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