| // Copyright 2014 the V8 project authors. All rights reserved. | |
| // Use of this source code is governed by a BSD-style license that can be | |
| // found in the LICENSE file. | |
| #include "src/compiler/jump-threading.h" | |
| #include "src/compiler/code-generator-impl.h" | |
| namespace v8 { | |
| namespace internal { | |
| namespace compiler { | |
| #define TRACE(...) \ | |
| do { \ | |
| if (FLAG_trace_turbo_jt) PrintF(__VA_ARGS__); \ | |
| } while (false) | |
| namespace { | |
| struct JumpThreadingState { | |
| bool forwarded; | |
| ZoneVector<RpoNumber>& result; | |
| ZoneStack<RpoNumber>& stack; | |
| void Clear(size_t count) { result.assign(count, unvisited()); } | |
| void PushIfUnvisited(RpoNumber num) { | |
| if (result[num.ToInt()] == unvisited()) { | |
| stack.push(num); | |
| result[num.ToInt()] = onstack(); | |
| } | |
| } | |
| void Forward(RpoNumber to) { | |
| RpoNumber from = stack.top(); | |
| RpoNumber to_to = result[to.ToInt()]; | |
| bool pop = true; | |
| if (to == from) { | |
| TRACE(" xx %d\n", from.ToInt()); | |
| result[from.ToInt()] = from; | |
| } else if (to_to == unvisited()) { | |
| TRACE(" fw %d -> %d (recurse)\n", from.ToInt(), to.ToInt()); | |
| stack.push(to); | |
| result[to.ToInt()] = onstack(); | |
| pop = false; // recurse. | |
| } else if (to_to == onstack()) { | |
| TRACE(" fw %d -> %d (cycle)\n", from.ToInt(), to.ToInt()); | |
| result[from.ToInt()] = to; // break the cycle. | |
| forwarded = true; | |
| } else { | |
| TRACE(" fw %d -> %d (forward)\n", from.ToInt(), to.ToInt()); | |
| result[from.ToInt()] = to_to; // forward the block. | |
| forwarded = true; | |
| } | |
| if (pop) stack.pop(); | |
| } | |
| RpoNumber unvisited() { return RpoNumber::FromInt(-1); } | |
| RpoNumber onstack() { return RpoNumber::FromInt(-2); } | |
| }; | |
| bool IsBlockWithBranchPoisoning(InstructionSequence* code, | |
| InstructionBlock* block) { | |
| if (block->PredecessorCount() != 1) return false; | |
| RpoNumber pred_rpo = (block->predecessors())[0]; | |
| const InstructionBlock* pred = code->InstructionBlockAt(pred_rpo); | |
| if (pred->code_start() == pred->code_end()) return false; | |
| Instruction* instr = code->InstructionAt(pred->code_end() - 1); | |
| FlagsMode mode = FlagsModeField::decode(instr->opcode()); | |
| return mode == kFlags_branch_and_poison; | |
| } | |
| } // namespace | |
| bool JumpThreading::ComputeForwarding(Zone* local_zone, | |
| ZoneVector<RpoNumber>& result, | |
| InstructionSequence* code, | |
| bool frame_at_start) { | |
| ZoneStack<RpoNumber> stack(local_zone); | |
| JumpThreadingState state = {false, result, stack}; | |
| state.Clear(code->InstructionBlockCount()); | |
| // Iterate over the blocks forward, pushing the blocks onto the stack. | |
| for (auto const block : code->instruction_blocks()) { | |
| RpoNumber current = block->rpo_number(); | |
| state.PushIfUnvisited(current); | |
| // Process the stack, which implements DFS through empty blocks. | |
| while (!state.stack.empty()) { | |
| InstructionBlock* block = code->InstructionBlockAt(state.stack.top()); | |
| // Process the instructions in a block up to a non-empty instruction. | |
| TRACE("jt [%d] B%d\n", static_cast<int>(stack.size()), | |
| block->rpo_number().ToInt()); | |
| RpoNumber fw = block->rpo_number(); | |
| if (!IsBlockWithBranchPoisoning(code, block)) { | |
| bool fallthru = true; | |
| for (int i = block->code_start(); i < block->code_end(); ++i) { | |
| Instruction* instr = code->InstructionAt(i); | |
| if (!instr->AreMovesRedundant()) { | |
| // can't skip instructions with non redundant moves. | |
| TRACE(" parallel move\n"); | |
| fallthru = false; | |
| } else if (FlagsModeField::decode(instr->opcode()) != kFlags_none) { | |
| // can't skip instructions with flags continuations. | |
| TRACE(" flags\n"); | |
| fallthru = false; | |
| } else if (instr->IsNop()) { | |
| // skip nops. | |
| TRACE(" nop\n"); | |
| continue; | |
| } else if (instr->arch_opcode() == kArchJmp) { | |
| // try to forward the jump instruction. | |
| TRACE(" jmp\n"); | |
| // if this block deconstructs the frame, we can't forward it. | |
| // TODO(mtrofin): we can still forward if we end up building | |
| // the frame at start. So we should move the decision of whether | |
| // to build a frame or not in the register allocator, and trickle it | |
| // here and to the code generator. | |
| if (frame_at_start || !(block->must_deconstruct_frame() || | |
| block->must_construct_frame())) { | |
| fw = code->InputRpo(instr, 0); | |
| } | |
| fallthru = false; | |
| } else { | |
| // can't skip other instructions. | |
| TRACE(" other\n"); | |
| fallthru = false; | |
| } | |
| break; | |
| } | |
| if (fallthru) { | |
| int next = 1 + block->rpo_number().ToInt(); | |
| if (next < code->InstructionBlockCount()) | |
| fw = RpoNumber::FromInt(next); | |
| } | |
| } | |
| state.Forward(fw); | |
| } | |
| } | |
| #ifdef DEBUG | |
| for (RpoNumber num : result) { | |
| DCHECK(num.IsValid()); | |
| } | |
| #endif | |
| if (FLAG_trace_turbo_jt) { | |
| for (int i = 0; i < static_cast<int>(result.size()); i++) { | |
| TRACE("B%d ", i); | |
| int to = result[i].ToInt(); | |
| if (i != to) { | |
| TRACE("-> B%d\n", to); | |
| } else { | |
| TRACE("\n"); | |
| } | |
| } | |
| } | |
| return state.forwarded; | |
| } | |
| void JumpThreading::ApplyForwarding(Zone* local_zone, | |
| ZoneVector<RpoNumber>& result, | |
| InstructionSequence* code) { | |
| if (!FLAG_turbo_jt) return; | |
| ZoneVector<bool> skip(static_cast<int>(result.size()), false, local_zone); | |
| // Skip empty blocks when the previous block doesn't fall through. | |
| bool prev_fallthru = true; | |
| for (auto const block : code->instruction_blocks()) { | |
| int block_num = block->rpo_number().ToInt(); | |
| skip[block_num] = !prev_fallthru && result[block_num].ToInt() != block_num; | |
| bool fallthru = true; | |
| for (int i = block->code_start(); i < block->code_end(); ++i) { | |
| Instruction* instr = code->InstructionAt(i); | |
| FlagsMode mode = FlagsModeField::decode(instr->opcode()); | |
| if (mode == kFlags_branch || mode == kFlags_branch_and_poison) { | |
| fallthru = false; // branches don't fall through to the next block. | |
| } else if (instr->arch_opcode() == kArchJmp) { | |
| if (skip[block_num]) { | |
| // Overwrite a redundant jump with a nop. | |
| TRACE("jt-fw nop @%d\n", i); | |
| instr->OverwriteWithNop(); | |
| } | |
| fallthru = false; // jumps don't fall through to the next block. | |
| } | |
| } | |
| prev_fallthru = fallthru; | |
| } | |
| // Patch RPO immediates. | |
| InstructionSequence::Immediates& immediates = code->immediates(); | |
| for (size_t i = 0; i < immediates.size(); i++) { | |
| Constant constant = immediates[i]; | |
| if (constant.type() == Constant::kRpoNumber) { | |
| RpoNumber rpo = constant.ToRpoNumber(); | |
| RpoNumber fw = result[rpo.ToInt()]; | |
| if (!(fw == rpo)) immediates[i] = Constant(fw); | |
| } | |
| } | |
| // Recompute assembly order numbers. | |
| int ao = 0; | |
| for (auto const block : code->instruction_blocks()) { | |
| if (!block->IsDeferred()) { | |
| block->set_ao_number(RpoNumber::FromInt(ao)); | |
| if (!skip[block->rpo_number().ToInt()]) ao++; | |
| } | |
| } | |
| for (auto const block : code->instruction_blocks()) { | |
| if (block->IsDeferred()) { | |
| block->set_ao_number(RpoNumber::FromInt(ao)); | |
| if (!skip[block->rpo_number().ToInt()]) ao++; | |
| } | |
| } | |
| } | |
| #undef TRACE | |
| } // namespace compiler | |
| } // namespace internal | |
| } // namespace v8 |