// Copyright (c) 2018 Google LLC
//
// 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/if_conversion.h"
#include <memory>
#include <vector>
#include "source/opt/value_number_table.h"
namespace spvtools {
namespace opt {
Pass::Status IfConversion::Process() {
if (!context()->get_feature_mgr()->HasCapability(SpvCapabilityShader)) {
return Status::SuccessWithoutChange;
}
const ValueNumberTable& vn_table = *context()->GetValueNumberTable();
bool modified = false;
std::vector<Instruction*> to_kill;
for (auto& func : *get_module()) {
DominatorAnalysis* dominators = context()->GetDominatorAnalysis(&func);
for (auto& block : func) {
// Check if it is possible for |block| to have phis that can be
// transformed.
BasicBlock* common = nullptr;
if (!CheckBlock(&block, dominators, &common)) continue;
// Get an insertion point.
auto iter = block.begin();
while (iter != block.end() && iter->opcode() == SpvOpPhi) {
++iter;
}
InstructionBuilder builder(
context(), &*iter,
IRContext::kAnalysisDefUse | IRContext::kAnalysisInstrToBlockMapping);
block.ForEachPhiInst([this, &builder, &modified, &common, &to_kill,
dominators, &block, &vn_table](Instruction* phi) {
// This phi is not compatible, but subsequent phis might be.
if (!CheckType(phi->type_id())) return;
// We cannot transform cases where the phi is used by another phi in the
// same block due to instruction ordering restrictions.
// TODO(alan-baker): If all inappropriate uses could also be
// transformed, we could still remove this phi.
if (!CheckPhiUsers(phi, &block)) return;
// Identify the incoming values associated with the true and false
// branches. If |then_block| dominates |inc0| or if the true edge
// branches straight to this block and |common| is |inc0|, then |inc0|
// is on the true branch. Otherwise the |inc1| is on the true branch.
BasicBlock* inc0 = GetIncomingBlock(phi, 0u);
Instruction* branch = common->terminator();
uint32_t condition = branch->GetSingleWordInOperand(0u);
BasicBlock* then_block = GetBlock(branch->GetSingleWordInOperand(1u));
Instruction* true_value = nullptr;
Instruction* false_value = nullptr;
if ((then_block == &block && inc0 == common) ||
dominators->Dominates(then_block, inc0)) {
true_value = GetIncomingValue(phi, 0u);
false_value = GetIncomingValue(phi, 1u);
} else {
true_value = GetIncomingValue(phi, 1u);
false_value = GetIncomingValue(phi, 0u);
}
BasicBlock* true_def_block = context()->get_instr_block(true_value);
BasicBlock* false_def_block = context()->get_instr_block(false_value);
uint32_t true_vn = vn_table.GetValueNumber(true_value);
uint32_t false_vn = vn_table.GetValueNumber(false_value);
if (true_vn != 0 && true_vn == false_vn) {
Instruction* inst_to_use = nullptr;
// Try to pick an instruction that is not in a side node. If we can't
// pick either the true for false branch as long as they can be
// legally moved.
if (!true_def_block ||
dominators->Dominates(true_def_block, &block)) {
inst_to_use = true_value;
} else if (!false_def_block ||
dominators->Dominates(false_def_block, &block)) {
inst_to_use = false_value;
} else if (CanHoistInstruction(true_value, common, dominators)) {
inst_to_use = true_value;
} else if (CanHoistInstruction(false_value, common, dominators)) {
inst_to_use = false_value;
}
if (inst_to_use != nullptr) {
modified = true;
HoistInstruction(inst_to_use, common, dominators);
context()->KillNamesAndDecorates(phi);
context()->ReplaceAllUsesWith(phi->result_id(),
inst_to_use->result_id());
}
return;
}
// If either incoming value is defined in a block that does not dominate
// this phi, then we cannot eliminate the phi with a select.
// TODO(alan-baker): Perform code motion where it makes sense to enable
// the transform in this case.
if (true_def_block && !dominators->Dominates(true_def_block, &block))
return;
if (false_def_block && !dominators->Dominates(false_def_block, &block))
return;
analysis::Type* data_ty =
context()->get_type_mgr()->GetType(true_value->type_id());
if (analysis::Vector* vec_data_ty = data_ty->AsVector()) {
condition = SplatCondition(vec_data_ty, condition, &builder);
}
Instruction* select = builder.AddSelect(phi->type_id(), condition,
true_value->result_id(),
false_value->result_id());
context()->ReplaceAllUsesWith(phi->result_id(), select->result_id());
to_kill.push_back(phi);
modified = true;
return;
});
}
}
for (auto inst : to_kill) {
context()->KillInst(inst);
}
return modified ? Status::SuccessWithChange : Status::SuccessWithoutChange;
}
bool IfConversion::CheckBlock(BasicBlock* block, DominatorAnalysis* dominators,
BasicBlock** common) {
const std::vector<uint32_t>& preds = cfg()->preds(block->id());
// TODO(alan-baker): Extend to more than two predecessors
if (preds.size() != 2) return false;
BasicBlock* inc0 = context()->get_instr_block(preds[0]);
if (dominators->Dominates(block, inc0)) return false;
BasicBlock* inc1 = context()->get_instr_block(preds[1]);
if (dominators->Dominates(block, inc1)) return false;
// All phis will have the same common dominator, so cache the result
// for this block. If there is no common dominator, then we cannot transform
// any phi in this basic block.
*common = dominators->CommonDominator(inc0, inc1);
if (!*common || cfg()->IsPseudoEntryBlock(*common)) return false;
Instruction* branch = (*common)->terminator();
if (branch->opcode() != SpvOpBranchConditional) return false;
auto merge = (*common)->GetMergeInst();
if (!merge || merge->opcode() != SpvOpSelectionMerge) return false;
if ((*common)->MergeBlockIdIfAny() != block->id()) return false;
return true;
}
bool IfConversion::CheckPhiUsers(Instruction* phi, BasicBlock* block) {
return get_def_use_mgr()->WhileEachUser(phi, [block,
this](Instruction* user) {
if (user->opcode() == SpvOpPhi && context()->get_instr_block(user) == block)
return false;
return true;
});
}
uint32_t IfConversion::SplatCondition(analysis::Vector* vec_data_ty,
uint32_t cond,
InstructionBuilder* builder) {
// If the data inputs to OpSelect are vectors, the condition for
// OpSelect must be a boolean vector with the same number of
// components. So splat the condition for the branch into a vector
// type.
analysis::Bool bool_ty;
analysis::Vector bool_vec_ty(&bool_ty, vec_data_ty->element_count());
uint32_t bool_vec_id =
context()->get_type_mgr()->GetTypeInstruction(&bool_vec_ty);
std::vector<uint32_t> ids(vec_data_ty->element_count(), cond);
return builder->AddCompositeConstruct(bool_vec_id, ids)->result_id();
}
bool IfConversion::CheckType(uint32_t id) {
Instruction* type = get_def_use_mgr()->GetDef(id);
SpvOp op = type->opcode();
if (spvOpcodeIsScalarType(op) || op == SpvOpTypePointer ||
op == SpvOpTypeVector)
return true;
return false;
}
BasicBlock* IfConversion::GetBlock(uint32_t id) {
return context()->get_instr_block(get_def_use_mgr()->GetDef(id));
}
BasicBlock* IfConversion::GetIncomingBlock(Instruction* phi,
uint32_t predecessor) {
uint32_t in_index = 2 * predecessor + 1;
return GetBlock(phi->GetSingleWordInOperand(in_index));
}
Instruction* IfConversion::GetIncomingValue(Instruction* phi,
uint32_t predecessor) {
uint32_t in_index = 2 * predecessor;
return get_def_use_mgr()->GetDef(phi->GetSingleWordInOperand(in_index));
}
void IfConversion::HoistInstruction(Instruction* inst, BasicBlock* target_block,
DominatorAnalysis* dominators) {
BasicBlock* inst_block = context()->get_instr_block(inst);
if (!inst_block) {
// This is in the header, and dominates everything.
return;
}
if (dominators->Dominates(inst_block, target_block)) {
// Already in position. No work to do.
return;
}
assert(inst->IsOpcodeCodeMotionSafe() &&
"Trying to move an instruction that is not safe to move.");
// First hoist all instructions it depends on.
analysis::DefUseManager* def_use_mgr = context()->get_def_use_mgr();
inst->ForEachInId(
[this, target_block, def_use_mgr, dominators](uint32_t* id) {
Instruction* operand_inst = def_use_mgr->GetDef(*id);
HoistInstruction(operand_inst, target_block, dominators);
});
Instruction* insertion_pos = target_block->terminator();
if ((insertion_pos)->PreviousNode()->opcode() == SpvOpSelectionMerge) {
insertion_pos = insertion_pos->PreviousNode();
}
inst->RemoveFromList();
insertion_pos->InsertBefore(std::unique_ptr<Instruction>(inst));
context()->set_instr_block(inst, target_block);
}
bool IfConversion::CanHoistInstruction(Instruction* inst,
BasicBlock* target_block,
DominatorAnalysis* dominators) {
BasicBlock* inst_block = context()->get_instr_block(inst);
if (!inst_block) {
// This is in the header, and dominates everything.
return true;
}
if (dominators->Dominates(inst_block, target_block)) {
// Already in position. No work to do.
return true;
}
if (!inst->IsOpcodeCodeMotionSafe()) {
return false;
}
// Check all instruction |inst| depends on.
analysis::DefUseManager* def_use_mgr = context()->get_def_use_mgr();
return inst->WhileEachInId(
[this, target_block, def_use_mgr, dominators](uint32_t* id) {
Instruction* operand_inst = def_use_mgr->GetDef(*id);
return CanHoistInstruction(operand_inst, target_block, dominators);
});
}
} // namespace opt
} // namespace spvtools