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 | IRContext::kAnalysisInstrToBlockMapping; 48 } 49 50 private: 51 // If |condId| is boolean constant, return conditional value in |condVal| and 52 // return true, otherwise return false. 53 bool GetConstCondition(uint32_t condId, bool* condVal); 54 55 // If |valId| is a 32-bit integer constant, return value via |value| and 56 // return true, otherwise return false. 57 bool GetConstInteger(uint32_t valId, uint32_t* value); 58 59 // Add branch to |labelId| to end of block |bp|. 60 void AddBranch(uint32_t labelId, BasicBlock* bp); 61 62 // For function |func|, look for BranchConditionals with constant condition 63 // and convert to a Branch to the indicated label. Delete resulting dead 64 // blocks. Note some such branches and blocks may be left to avoid creating 65 // invalid control flow. 66 // TODO(greg-lunarg): Remove remaining constant conditional branches and dead 67 // blocks. 68 bool EliminateDeadBranches(Function* func); 69 70 // Returns the basic block containing |id|. 71 // Note: this pass only requires correct instruction block mappings for the 72 // input. This pass does not preserve the block mapping, so it is not kept 73 // up-to-date during processing. 74 BasicBlock* GetParentBlock(uint32_t id); 75 76 // Marks live blocks reachable from the entry of |func|. Simplifies constant 77 // branches and switches as it proceeds, to limit the number of live blocks. 78 // It is careful not to eliminate backedges even if they are dead, but the 79 // header is live. Likewise, unreachable merge blocks named in live merge 80 // instruction must be retained (though they may be clobbered). 81 bool MarkLiveBlocks(Function* func, 82 std::unordered_set<BasicBlock*>* live_blocks); 83 84 // Checks for unreachable merge and continue blocks with live headers; those 85 // blocks must be retained. Continues are tracked separately so that a live 86 // phi can be updated to take an undef value from any of its predecessors 87 // that are unreachable continues. 88 // 89 // |unreachable_continues| maps the id of an unreachable continue target to 90 // the header block that declares it. 91 void MarkUnreachableStructuredTargets( 92 const std::unordered_set<BasicBlock*>& live_blocks, 93 std::unordered_set<BasicBlock*>* unreachable_merges, 94 std::unordered_map<BasicBlock*, BasicBlock*>* unreachable_continues); 95 96 // Fix phis in reachable blocks so that only live (or unremovable) incoming 97 // edges are present. If the block now only has a single live incoming edge, 98 // remove the phi and replace its uses with its data input. If the single 99 // remaining incoming edge is from the phi itself, the the phi is in an 100 // unreachable single block loop. Either the block is dead and will be 101 // removed, or it's reachable from an unreachable continue target. In the 102 // latter case that continue target block will be collapsed into a block that 103 // only branches back to its header and we'll eliminate the block with the 104 // phi. 105 // 106 // |unreachable_continues| maps continue targets that cannot be reached to 107 // merge instruction that declares them. 108 bool FixPhiNodesInLiveBlocks( 109 Function* func, const std::unordered_set<BasicBlock*>& live_blocks, 110 const std::unordered_map<BasicBlock*, BasicBlock*>& 111 unreachable_continues); 112 113 // Erases dead blocks. Any block captured in |unreachable_merges| or 114 // |unreachable_continues| is a dead block that is required to remain due to 115 // a live merge instruction in the corresponding header. These blocks will 116 // have their instructions clobbered and will become a label and terminator. 117 // Unreachable merge blocks are terminated by OpUnreachable, while 118 // unreachable continue blocks are terminated by an unconditional branch to 119 // the header. Otherwise, blocks are dead if not explicitly captured in 120 // |live_blocks| and are totally removed. 121 // 122 // |unreachable_continues| maps continue targets that cannot be reached to 123 // corresponding header block that declares them. 124 bool EraseDeadBlocks( 125 Function* func, const std::unordered_set<BasicBlock*>& live_blocks, 126 const std::unordered_set<BasicBlock*>& unreachable_merges, 127 const std::unordered_map<BasicBlock*, BasicBlock*>& 128 unreachable_continues); 129 130 // Reorders blocks in reachable functions so that they satisfy dominator 131 // block ordering rules. 132 void FixBlockOrder(); 133 134 // Return the first branch instruction that is a conditional branch to 135 // |merge_block_id|. Returns |nullptr| if no such branch exists. If there are 136 // multiple such branches, the first one is the one that would be executed 137 // first when running the code. That is, the one that dominates all of the 138 // others. 139 // 140 // |start_block_id| must be a block whose innermost containing merge construct 141 // has |merge_block_id| as the merge block. 142 // 143 // |loop_merge_id| and |loop_continue_id| are the merge and continue block ids 144 // of the innermost loop containing |start_block_id|. 145 Instruction* FindFirstExitFromSelectionMerge(uint32_t start_block_id, 146 uint32_t merge_block_id, 147 uint32_t loop_merge_id, 148 uint32_t loop_continue_id); 149 }; 150 151 } // namespace opt 152 } // namespace spvtools 153 154 #endif // SOURCE_OPT_DEAD_BRANCH_ELIM_PASS_H_ 155