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// 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 {
typedef BasicBlock::RpoNumber RpoNumber;
#define TRACE(x) \
if (FLAG_trace_turbo_jt) PrintF x
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 JumpThreading::ComputeForwarding(Zone* local_zone,
ZoneVector<RpoNumber>& result,
InstructionSequence* code) {
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 RPO%d\n", static_cast<int>(stack.size()),
block->id().ToInt(), block->rpo_number().ToInt()));
bool fallthru = true;
RpoNumber fw = block->rpo_number();
for (int i = block->code_start(); i < block->code_end(); ++i) {
Instruction* instr = code->InstructionAt(i);
if (instr->IsGapMoves() && GapInstruction::cast(instr)->IsRedundant()) {
// skip redundant gap moves.
TRACE((" nop gap\n"));
continue;
} else if (instr->IsSourcePosition()) {
// skip source positions.
TRACE((" src pos\n"));
continue;
} 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"));
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) {
CHECK(num.IsValid());
}
#endif
if (FLAG_trace_turbo_jt) {
for (int i = 0; i < static_cast<int>(result.size()); i++) {
TRACE(("RPO%d B%d ", i,
code->InstructionBlockAt(RpoNumber::FromInt(i))->id().ToInt()));
int to = result[i].ToInt();
if (i != to) {
TRACE(("-> B%d\n",
code->InstructionBlockAt(RpoNumber::FromInt(to))->id().ToInt()));
} else {
TRACE(("\n"));
}
}
}
return state.forwarded;
}
void JumpThreading::ApplyForwarding(ZoneVector<RpoNumber>& result,
InstructionSequence* code) {
if (!FLAG_turbo_jt) return;
Zone local_zone;
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);
if (FlagsModeField::decode(instr->opcode()) == kFlags_branch) {
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++;
}
}
}
} // namespace compiler
} // namespace internal
} // namespace v8
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