1 // Copyright (c) 2017 The Khronos Group Inc. 2 // Copyright (c) 2017 Valve Corporation 3 // Copyright (c) 2017 LunarG Inc. 4 // 5 // Licensed under the Apache License, Version 2.0 (the "License"); 6 // you may not use this file except in compliance with the License. 7 // You may obtain a copy of the License at 8 // 9 // http://www.apache.org/licenses/LICENSE-2.0 10 // 11 // Unless required by applicable law or agreed to in writing, software 12 // distributed under the License is distributed on an "AS IS" BASIS, 13 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 14 // See the License for the specific language governing permissions and 15 // limitations under the License. 16 17 #ifndef SOURCE_OPT_DEAD_BRANCH_ELIM_PASS_H_ 18 #define SOURCE_OPT_DEAD_BRANCH_ELIM_PASS_H_ 19 20 #include <algorithm> 21 #include <map> 22 #include <queue> 23 #include <unordered_map> 24 #include <unordered_set> 25 #include <utility> 26 #include <vector> 27 28 #include "source/opt/basic_block.h" 29 #include "source/opt/def_use_manager.h" 30 #include "source/opt/mem_pass.h" 31 #include "source/opt/module.h" 32 33 namespace spvtools { 34 namespace opt { 35 36 // See optimizer.hpp for documentation. 37 class DeadBranchElimPass : public MemPass { 38 using cbb_ptr = const BasicBlock*; 39 40 public: 41 DeadBranchElimPass() = default; 42 name()43 const char* name() const override { return "eliminate-dead-branches"; } 44 Status Process() override; 45 GetPreservedAnalyses()46 IRContext::Analysis GetPreservedAnalyses() override { 47 return IRContext::kAnalysisDefUse | 48 IRContext::kAnalysisInstrToBlockMapping | 49 IRContext::kAnalysisConstants | IRContext::kAnalysisTypes; 50 } 51 52 private: 53 // If |condId| is boolean constant, return conditional value in |condVal| and 54 // return true, otherwise return false. 55 bool GetConstCondition(uint32_t condId, bool* condVal); 56 57 // If |valId| is a 32-bit integer constant, return value via |value| and 58 // return true, otherwise return false. 59 bool GetConstInteger(uint32_t valId, uint32_t* value); 60 61 // Add branch to |labelId| to end of block |bp|. 62 void AddBranch(uint32_t labelId, BasicBlock* bp); 63 64 // For function |func|, look for BranchConditionals with constant condition 65 // and convert to a Branch to the indicated label. Delete resulting dead 66 // blocks. Note some such branches and blocks may be left to avoid creating 67 // invalid control flow. 68 // TODO(greg-lunarg): Remove remaining constant conditional branches and dead 69 // blocks. 70 bool EliminateDeadBranches(Function* func); 71 72 // Returns the basic block containing |id|. 73 // Note: this pass only requires correct instruction block mappings for the 74 // input. This pass does not preserve the block mapping, so it is not kept 75 // up-to-date during processing. 76 BasicBlock* GetParentBlock(uint32_t id); 77 78 // Marks live blocks reachable from the entry of |func|. Simplifies constant 79 // branches and switches as it proceeds, to limit the number of live blocks. 80 // It is careful not to eliminate backedges even if they are dead, but the 81 // header is live. Likewise, unreachable merge blocks named in live merge 82 // instruction must be retained (though they may be clobbered). 83 bool MarkLiveBlocks(Function* func, 84 std::unordered_set<BasicBlock*>* live_blocks); 85 86 // Checks for unreachable merge and continue blocks with live headers; those 87 // blocks must be retained. Continues are tracked separately so that a live 88 // phi can be updated to take an undef value from any of its predecessors 89 // that are unreachable continues. 90 // 91 // |unreachable_continues| maps the id of an unreachable continue target to 92 // the header block that declares it. 93 void MarkUnreachableStructuredTargets( 94 const std::unordered_set<BasicBlock*>& live_blocks, 95 std::unordered_set<BasicBlock*>* unreachable_merges, 96 std::unordered_map<BasicBlock*, BasicBlock*>* unreachable_continues); 97 98 // Fix phis in reachable blocks so that only live (or unremovable) incoming 99 // edges are present. If the block now only has a single live incoming edge, 100 // remove the phi and replace its uses with its data input. If the single 101 // remaining incoming edge is from the phi itself, the phi is in an 102 // unreachable single block loop. Either the block is dead and will be 103 // removed, or it's reachable from an unreachable continue target. In the 104 // latter case that continue target block will be collapsed into a block that 105 // only branches back to its header and we'll eliminate the block with the 106 // phi. 107 // 108 // |unreachable_continues| maps continue targets that cannot be reached to 109 // merge instruction that declares them. 110 bool FixPhiNodesInLiveBlocks( 111 Function* func, const std::unordered_set<BasicBlock*>& live_blocks, 112 const std::unordered_map<BasicBlock*, BasicBlock*>& 113 unreachable_continues); 114 115 // Erases dead blocks. Any block captured in |unreachable_merges| or 116 // |unreachable_continues| is a dead block that is required to remain due to 117 // a live merge instruction in the corresponding header. These blocks will 118 // have their instructions clobbered and will become a label and terminator. 119 // Unreachable merge blocks are terminated by OpUnreachable, while 120 // unreachable continue blocks are terminated by an unconditional branch to 121 // the header. Otherwise, blocks are dead if not explicitly captured in 122 // |live_blocks| and are totally removed. 123 // 124 // |unreachable_continues| maps continue targets that cannot be reached to 125 // corresponding header block that declares them. 126 bool EraseDeadBlocks( 127 Function* func, const std::unordered_set<BasicBlock*>& live_blocks, 128 const std::unordered_set<BasicBlock*>& unreachable_merges, 129 const std::unordered_map<BasicBlock*, BasicBlock*>& 130 unreachable_continues); 131 132 // Reorders blocks in reachable functions so that they satisfy dominator 133 // block ordering rules. 134 void FixBlockOrder(); 135 136 // Return the first branch instruction that is a conditional branch to 137 // |merge_block_id|. Returns |nullptr| if no such branch exists. If there are 138 // multiple such branches, the first one is the one that would be executed 139 // first when running the code. That is, the one that dominates all of the 140 // others. 141 // 142 // |start_block_id| must be a block whose innermost containing merge construct 143 // has |merge_block_id| as the merge block. 144 // 145 // |loop_merge_id| and |loop_continue_id| are the merge and continue block ids 146 // of the innermost loop containing |start_block_id|. 147 Instruction* FindFirstExitFromSelectionMerge(uint32_t start_block_id, 148 uint32_t merge_block_id, 149 uint32_t loop_merge_id, 150 uint32_t loop_continue_id, 151 uint32_t switch_merge_id); 152 153 // Adds to |blocks_with_back_edges| all of the blocks on the path from the 154 // basic block |cont_id| to |header_id| and |merge_id|. The intention is that 155 // |cond_id| is a the continue target of a loop, |header_id| is the header of 156 // the loop, and |merge_id| is the merge block of the loop. 157 void AddBlocksWithBackEdge( 158 uint32_t cont_id, uint32_t header_id, uint32_t merge_id, 159 std::unordered_set<BasicBlock*>* blocks_with_back_edges); 160 161 // Returns true if there is a branch to the merge node of the selection 162 // construct |switch_header_id| that is inside a nested selection construct or 163 // in the header of the nested selection construct. 164 bool SwitchHasNestedBreak(uint32_t switch_header_id); 165 166 // Return true of the terminator of |block| is successfully replaced with a 167 // branch to |live_lab_id|. The merge instruction is deleted or moved as 168 // needed to maintain structured control flow. Assumes that the 169 // StructuredCFGAnalysis is valid for the constructs containing |block|. 170 bool SimplifyBranch(BasicBlock* block, uint32_t live_lab_id); 171 }; 172 173 } // namespace opt 174 } // namespace spvtools 175 176 #endif // SOURCE_OPT_DEAD_BRANCH_ELIM_PASS_H_ 177