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// Copyright (c) 2019 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/fuzz/transformation_add_dead_block.h"
#include "source/fuzz/fuzzer_util.h"
namespace spvtools {
namespace fuzz {
TransformationAddDeadBlock::TransformationAddDeadBlock(
protobufs::TransformationAddDeadBlock message)
: message_(std::move(message)) {}
TransformationAddDeadBlock::TransformationAddDeadBlock(uint32_t fresh_id,
uint32_t existing_block,
bool condition_value) {
message_.set_fresh_id(fresh_id);
message_.set_existing_block(existing_block);
message_.set_condition_value(condition_value);
}
bool TransformationAddDeadBlock::IsApplicable(
opt::IRContext* ir_context,
const TransformationContext& transformation_context) const {
// The new block's id must be fresh.
if (!fuzzerutil::IsFreshId(ir_context, message_.fresh_id())) {
return false;
}
// First, we check that a constant with the same value as
// |message_.condition_value| is present.
if (!fuzzerutil::MaybeGetBoolConstant(ir_context, transformation_context,
message_.condition_value(), false)) {
// The required constant is not present, so the transformation cannot be
// applied.
return false;
}
// The existing block must indeed exist.
auto existing_block =
fuzzerutil::MaybeFindBlock(ir_context, message_.existing_block());
if (!existing_block) {
return false;
}
// It must not head a loop.
if (existing_block->IsLoopHeader()) {
return false;
}
// It must end with OpBranch.
if (existing_block->terminator()->opcode() != SpvOpBranch) {
return false;
}
// Its successor must not be a merge block nor continue target.
auto successor_block_id =
existing_block->terminator()->GetSingleWordInOperand(0);
if (fuzzerutil::IsMergeOrContinue(ir_context, successor_block_id)) {
return false;
}
// The successor must not be a loop header (i.e., |message_.existing_block|
// must not be a back-edge block.
if (ir_context->cfg()->block(successor_block_id)->IsLoopHeader()) {
return false;
}
// |existing_block| must be reachable.
if (!ir_context->IsReachable(*existing_block)) {
return false;
}
assert(existing_block->id() != successor_block_id &&
"|existing_block| must be different from |successor_block_id|");
// Even though we know |successor_block_id| is not a merge block, it might
// still have multiple predecessors because divergent control flow is allowed
// to converge early (before the merge block). In this case, when we create
// the selection construct, its header |existing_block| will not dominate the
// merge block |successor_block_id|, which is invalid. Thus, |existing_block|
// must dominate |successor_block_id|.
opt::DominatorAnalysis* dominator_analysis =
ir_context->GetDominatorAnalysis(existing_block->GetParent());
if (!dominator_analysis->Dominates(existing_block->id(),
successor_block_id)) {
return false;
}
return true;
}
void TransformationAddDeadBlock::Apply(
opt::IRContext* ir_context,
TransformationContext* transformation_context) const {
// Update the module id bound so that it is at least the id of the new block.
fuzzerutil::UpdateModuleIdBound(ir_context, message_.fresh_id());
// Get the existing block and its successor.
auto existing_block = ir_context->cfg()->block(message_.existing_block());
auto successor_block_id =
existing_block->terminator()->GetSingleWordInOperand(0);
// Get the id of the boolean value that will be used as the branch condition.
auto bool_id = fuzzerutil::MaybeGetBoolConstant(
ir_context, *transformation_context, message_.condition_value(), false);
// Make a new block that unconditionally branches to the original successor
// block.
auto enclosing_function = existing_block->GetParent();
std::unique_ptr<opt::BasicBlock> new_block =
MakeUnique<opt::BasicBlock>(MakeUnique<opt::Instruction>(
ir_context, SpvOpLabel, 0, message_.fresh_id(),
opt::Instruction::OperandList()));
new_block->AddInstruction(MakeUnique<opt::Instruction>(
ir_context, SpvOpBranch, 0, 0,
opt::Instruction::OperandList(
{{SPV_OPERAND_TYPE_ID, {successor_block_id}}})));
// Turn the original block into a selection merge, with its original successor
// as the merge block.
existing_block->terminator()->InsertBefore(MakeUnique<opt::Instruction>(
ir_context, SpvOpSelectionMerge, 0, 0,
opt::Instruction::OperandList(
{{SPV_OPERAND_TYPE_ID, {successor_block_id}},
{SPV_OPERAND_TYPE_SELECTION_CONTROL,
{SpvSelectionControlMaskNone}}})));
// Change the original block's terminator to be a conditional branch on the
// given boolean, with the original successor and the new successor as branch
// targets, and such that at runtime control will always transfer to the
// original successor.
existing_block->terminator()->SetOpcode(SpvOpBranchConditional);
existing_block->terminator()->SetInOperands(
{{SPV_OPERAND_TYPE_ID, {bool_id}},
{SPV_OPERAND_TYPE_ID,
{message_.condition_value() ? successor_block_id
: message_.fresh_id()}},
{SPV_OPERAND_TYPE_ID,
{message_.condition_value() ? message_.fresh_id()
: successor_block_id}}});
// Add the new block to the enclosing function.
enclosing_function->InsertBasicBlockAfter(std::move(new_block),
existing_block);
// Fix up OpPhi instructions in the successor block, so that the values they
// yield when control has transferred from the new block are the same as if
// control had transferred from |message_.existing_block|. This is guaranteed
// to be valid since |message_.existing_block| dominates the new block by
// construction. Other transformations can change these phi operands to more
// interesting values.
ir_context->cfg()
->block(successor_block_id)
->ForEachPhiInst([this](opt::Instruction* phi_inst) {
// Copy the operand that provides the phi value for the first of any
// existing predecessors.
opt::Operand copy_of_existing_operand = phi_inst->GetInOperand(0);
// Use this as the value associated with the new predecessor.
phi_inst->AddOperand(std::move(copy_of_existing_operand));
phi_inst->AddOperand({SPV_OPERAND_TYPE_ID, {message_.fresh_id()}});
});
// Do not rely on any existing analysis results since the control flow graph
// of the module has changed.
ir_context->InvalidateAnalysesExceptFor(opt::IRContext::kAnalysisNone);
// Record the fact that the new block is dead.
transformation_context->GetFactManager()->AddFactBlockIsDead(
message_.fresh_id());
}
protobufs::Transformation TransformationAddDeadBlock::ToMessage() const {
protobufs::Transformation result;
*result.mutable_add_dead_block() = message_;
return result;
}
std::unordered_set<uint32_t> TransformationAddDeadBlock::GetFreshIds() const {
return {message_.fresh_id()};
}
} // namespace fuzz
} // namespace spvtools