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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/fuzzer_pass_add_dead_breaks.h"
#include "source/fuzz/fuzzer_util.h"
#include "source/fuzz/transformation_add_dead_break.h"
#include "source/opt/ir_context.h"
namespace spvtools {
namespace fuzz {
FuzzerPassAddDeadBreaks::FuzzerPassAddDeadBreaks(
opt::IRContext* ir_context, TransformationContext* transformation_context,
FuzzerContext* fuzzer_context,
protobufs::TransformationSequence* transformations)
: FuzzerPass(ir_context, transformation_context, fuzzer_context,
transformations) {}
void FuzzerPassAddDeadBreaks::Apply() {
// We first collect up lots of possibly-applicable transformations.
std::vector<TransformationAddDeadBreak> candidate_transformations;
// We consider each function separately.
for (auto& function : *GetIRContext()->module()) {
// For a given function, we find all the merge blocks in that function.
std::vector<opt::BasicBlock*> merge_blocks;
for (auto& block : function) {
auto maybe_merge_id = block.MergeBlockIdIfAny();
if (maybe_merge_id) {
auto merge_block =
fuzzerutil::MaybeFindBlock(GetIRContext(), maybe_merge_id);
assert(merge_block && "Merge block can't be null");
merge_blocks.push_back(merge_block);
}
}
// We rather aggressively consider the possibility of adding a break from
// every block in the function to every merge block. Many of these will be
// inapplicable as they would be illegal. That's OK - we later discard the
// ones that turn out to be no good.
for (auto& block : function) {
for (auto* merge_block : merge_blocks) {
// Populate this vector with ids that are available at the branch point
// of this basic block. We will use these ids to update OpPhi
// instructions later.
std::vector<uint32_t> phi_ids;
// Determine how we need to adjust OpPhi instructions' operands
// for this transformation to be valid.
//
// If |block| has a branch to |merge_block|, the latter must have all of
// its OpPhi instructions set up correctly - we don't need to adjust
// anything.
if (!block.IsSuccessor(merge_block)) {
merge_block->ForEachPhiInst([this, &phi_ids](opt::Instruction* phi) {
// Add an additional operand for OpPhi instruction. Use a constant
// if possible, and an undef otherwise.
if (fuzzerutil::CanCreateConstant(GetIRContext(), phi->type_id())) {
// We mark the constant as irrelevant so that we can replace it
// with a more interesting value later.
phi_ids.push_back(FindOrCreateZeroConstant(phi->type_id(), true));
} else {
phi_ids.push_back(FindOrCreateGlobalUndef(phi->type_id()));
}
});
}
// Make sure the module has a required boolean constant to be used in
// OpBranchConditional instruction.
auto break_condition = GetFuzzerContext()->ChooseEven();
FindOrCreateBoolConstant(break_condition, false);
auto candidate_transformation = TransformationAddDeadBreak(
block.id(), merge_block->id(), break_condition, std::move(phi_ids));
if (candidate_transformation.IsApplicable(
GetIRContext(), *GetTransformationContext())) {
// Only consider a transformation as a candidate if it is applicable.
candidate_transformations.push_back(
std::move(candidate_transformation));
}
}
}
}
// Go through the candidate transformations that were accumulated,
// probabilistically deciding whether to consider each one further and
// applying the still-applicable ones that are considered further.
//
// We iterate through the candidate transformations in a random order by
// repeatedly removing a random candidate transformation from the sequence
// until no candidate transformations remain. This is done because
// transformations can potentially disable one another, so that iterating
// through them in order would lead to a higher probability of
// transformations appearing early in the sequence being applied compared
// with later transformations.
while (!candidate_transformations.empty()) {
// Choose a random index into the sequence of remaining candidate
// transformations.
auto index = GetFuzzerContext()->RandomIndex(candidate_transformations);
// Remove the transformation at the chosen index from the sequence.
auto transformation = std::move(candidate_transformations[index]);
candidate_transformations.erase(candidate_transformations.begin() + index);
// Probabilistically decide whether to try to apply it vs. ignore it, in the
// case that it is applicable.
if (GetFuzzerContext()->ChoosePercentage(
GetFuzzerContext()->GetChanceOfAddingDeadBreak())) {
MaybeApplyTransformation(transformation);
}
}
}
} // namespace fuzz
} // namespace spvtools
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