SPIRV-Tools/source/opt/propagator.cpp

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// Copyright (c) 2017 Google Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#include "source/opt/propagator.h"
namespace spvtools {
namespace opt {
void SSAPropagator::AddControlEdge(const Edge& edge) {
BasicBlock* dest_bb = edge.dest;
// Refuse to add the exit block to the work list.
if (dest_bb == ctx_->cfg()->pseudo_exit_block()) {
return;
}
// Try to mark the edge executable. If it was already in the set of
// executable edges, do nothing.
if (!MarkEdgeExecutable(edge)) {
return;
}
// If the edge had not already been marked executable, add the destination
// basic block to the work list.
blocks_.push(dest_bb);
}
void SSAPropagator::AddSSAEdges(Instruction* instr) {
// Ignore instructions that produce no result.
if (instr->result_id() == 0) {
return;
}
get_def_use_mgr()->ForEachUser(
instr->result_id(), [this](Instruction* use_instr) {
// If the basic block for |use_instr| has not been simulated yet, do
// nothing. The instruction |use_instr| will be simulated next time the
// block is scheduled.
if (!BlockHasBeenSimulated(ctx_->get_instr_block(use_instr))) {
return;
}
if (ShouldSimulateAgain(use_instr)) {
ssa_edge_uses_.push(use_instr);
}
});
}
bool SSAPropagator::IsPhiArgExecutable(Instruction* phi, uint32_t i) const {
BasicBlock* phi_bb = ctx_->get_instr_block(phi);
uint32_t in_label_id = phi->GetSingleWordOperand(i + 1);
Instruction* in_label_instr = get_def_use_mgr()->GetDef(in_label_id);
BasicBlock* in_bb = ctx_->get_instr_block(in_label_instr);
return IsEdgeExecutable(Edge(in_bb, phi_bb));
}
bool SSAPropagator::SetStatus(Instruction* inst, PropStatus status) {
bool has_old_status = false;
PropStatus old_status = kVarying;
if (HasStatus(inst)) {
has_old_status = true;
old_status = Status(inst);
}
assert((!has_old_status || old_status <= status) &&
"Invalid lattice transition");
bool status_changed = !has_old_status || (old_status != status);
if (status_changed) statuses_[inst] = status;
return status_changed;
}
bool SSAPropagator::Simulate(Instruction* instr) {
bool changed = false;
// Don't bother visiting instructions that should not be simulated again.
if (!ShouldSimulateAgain(instr)) {
return changed;
}
BasicBlock* dest_bb = nullptr;
PropStatus status = visit_fn_(instr, &dest_bb);
bool status_changed = SetStatus(instr, status);
if (status == kVarying) {
// The statement produces a varying result, add it to the list of statements
// not to simulate anymore and add its SSA def-use edges for simulation.
DontSimulateAgain(instr);
if (status_changed) {
AddSSAEdges(instr);
}
// If |instr| is a block terminator, add all the control edges out of its
// block.
if (instr->IsBlockTerminator()) {
BasicBlock* block = ctx_->get_instr_block(instr);
for (const auto& e : bb_succs_.at(block)) {
AddControlEdge(e);
}
}
return false;
} else if (status == kInteresting) {
// Add the SSA edges coming out of this instruction if the propagation
// status has changed.
if (status_changed) {
AddSSAEdges(instr);
}
// If there are multiple outgoing control flow edges and we know which one
// will be taken, add the destination block to the CFG work list.
if (dest_bb) {
AddControlEdge(Edge(ctx_->get_instr_block(instr), dest_bb));
}
changed = true;
}
// At this point, we are dealing with instructions that are in status
// kInteresting or kNotInteresting. To decide whether this instruction should
// be simulated again, we examine its operands. If at least one operand O is
// defined at an instruction D that should be simulated again, then the output
// of D might affect |instr|, so we should simulate |instr| again.
bool has_operands_to_simulate = false;
if (instr->opcode() == SpvOpPhi) {
// For Phi instructions, an operand causes the Phi to be simulated again if
// the operand comes from an edge that has not yet been traversed or if its
// definition should be simulated again.
for (uint32_t i = 2; i < instr->NumOperands(); i += 2) {
// Phi arguments come in pairs. Index 'i' contains the
// variable id, index 'i + 1' is the originating block id.
assert(i % 2 == 0 && i < instr->NumOperands() - 1 &&
"malformed Phi arguments");
uint32_t arg_id = instr->GetSingleWordOperand(i);
Instruction* arg_def_instr = get_def_use_mgr()->GetDef(arg_id);
if (!IsPhiArgExecutable(instr, i) || ShouldSimulateAgain(arg_def_instr)) {
has_operands_to_simulate = true;
break;
}
}
} else {
// For regular instructions, check if the defining instruction of each
// operand needs to be simulated again. If so, then this instruction should
// also be simulated again.
has_operands_to_simulate =
!instr->WhileEachInId([this](const uint32_t* use) {
Instruction* def_instr = get_def_use_mgr()->GetDef(*use);
if (ShouldSimulateAgain(def_instr)) {
return false;
}
return true;
});
}
if (!has_operands_to_simulate) {
DontSimulateAgain(instr);
}
return changed;
}
bool SSAPropagator::Simulate(BasicBlock* block) {
if (block == ctx_->cfg()->pseudo_exit_block()) {
return false;
}
// Always simulate Phi instructions, even if we have simulated this block
// before. We do this because Phi instructions receive their inputs from
// incoming edges. When those edges are marked executable, the corresponding
// operand can be simulated.
bool changed = false;
block->ForEachPhiInst(
[&changed, this](Instruction* instr) { changed |= Simulate(instr); });
// If this is the first time this block is being simulated, simulate every
// statement in it.
if (!BlockHasBeenSimulated(block)) {
block->ForEachInst([this, &changed](Instruction* instr) {
if (instr->opcode() != SpvOpPhi) {
changed |= Simulate(instr);
}
});
MarkBlockSimulated(block);
// If this block has exactly one successor, mark the edge to its successor
// as executable.
if (bb_succs_.at(block).size() == 1) {
AddControlEdge(bb_succs_.at(block).at(0));
}
}
return changed;
}
void SSAPropagator::Initialize(Function* fn) {
// Compute predecessor and successor blocks for every block in |fn|'s CFG.
// TODO(dnovillo): Move this to CFG and always build them. Alternately,
// move it to IRContext and build CFG preds/succs on-demand.
bb_succs_[ctx_->cfg()->pseudo_entry_block()].push_back(
Edge(ctx_->cfg()->pseudo_entry_block(), fn->entry().get()));
for (auto& block : *fn) {
const auto& const_block = block;
const_block.ForEachSuccessorLabel([this, &block](const uint32_t label_id) {
BasicBlock* succ_bb =
ctx_->get_instr_block(get_def_use_mgr()->GetDef(label_id));
bb_succs_[&block].push_back(Edge(&block, succ_bb));
bb_preds_[succ_bb].push_back(Edge(succ_bb, &block));
});
if (block.IsReturnOrAbort()) {
bb_succs_[&block].push_back(
Edge(&block, ctx_->cfg()->pseudo_exit_block()));
bb_preds_[ctx_->cfg()->pseudo_exit_block()].push_back(
Edge(ctx_->cfg()->pseudo_exit_block(), &block));
}
}
// Add the edges out of the entry block to seed the propagator.
const auto& entry_succs = bb_succs_[ctx_->cfg()->pseudo_entry_block()];
for (const auto& e : entry_succs) {
AddControlEdge(e);
}
}
bool SSAPropagator::Run(Function* fn) {
Initialize(fn);
bool changed = false;
while (!blocks_.empty() || !ssa_edge_uses_.empty()) {
// Simulate all blocks first. Simulating blocks will add SSA edges to
// follow after all the blocks have been simulated.
if (!blocks_.empty()) {
auto block = blocks_.front();
changed |= Simulate(block);
blocks_.pop();
continue;
}
// Simulate edges from the SSA queue.
if (!ssa_edge_uses_.empty()) {
Instruction* instr = ssa_edge_uses_.front();
changed |= Simulate(instr);
ssa_edge_uses_.pop();
}
}
#ifndef NDEBUG
// Verify all visited values have settled. No value that has been simulated
// should end on not interesting.
fn->ForEachInst([this](Instruction* inst) {
assert(
(!HasStatus(inst) || Status(inst) != SSAPropagator::kNotInteresting) &&
"Unsettled value");
});
#endif
return changed;
}
std::ostream& operator<<(std::ostream& str,
const SSAPropagator::PropStatus& status) {
switch (status) {
case SSAPropagator::kVarying:
str << "Varying";
break;
case SSAPropagator::kInteresting:
str << "Interesting";
break;
default:
str << "Not interesting";
break;
}
return str;
}
} // namespace opt
} // namespace spvtools