SPIRV-Tools/source/fuzz/fuzzer_pass_flatten_conditional_branches.cpp

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// Copyright (c) 2020 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_flatten_conditional_branches.h"
#include "source/fuzz/comparator_deep_blocks_first.h"
#include "source/fuzz/instruction_descriptor.h"
#include "source/fuzz/transformation_flatten_conditional_branch.h"
namespace spvtools {
namespace fuzz {
// A fuzzer pass that randomly selects conditional branches to flatten and
// flattens them, if possible.
FuzzerPassFlattenConditionalBranches::FuzzerPassFlattenConditionalBranches(
opt::IRContext* ir_context, TransformationContext* transformation_context,
FuzzerContext* fuzzer_context,
protobufs::TransformationSequence* transformations,
bool ignore_inapplicable_transformations)
: FuzzerPass(ir_context, transformation_context, fuzzer_context,
transformations, ignore_inapplicable_transformations) {}
void FuzzerPassFlattenConditionalBranches::Apply() {
for (auto& function : *GetIRContext()->module()) {
// Get all the selection headers that we want to flatten. We need to collect
// all of them first, because, since we are changing the structure of the
// module, it's not safe to modify them while iterating.
std::vector<opt::BasicBlock*> selection_headers;
for (auto& block : function) {
// Randomly decide whether to consider this block.
if (!GetFuzzerContext()->ChoosePercentage(
GetFuzzerContext()->GetChanceOfFlatteningConditionalBranch())) {
continue;
}
// Only consider this block if it is the header of a conditional, with a
// non-irrelevant condition.
if (block.GetMergeInst() &&
block.GetMergeInst()->opcode() == spv::Op::OpSelectionMerge &&
block.terminator()->opcode() == spv::Op::OpBranchConditional &&
!GetTransformationContext()->GetFactManager()->IdIsIrrelevant(
block.terminator()->GetSingleWordInOperand(0))) {
selection_headers.emplace_back(&block);
}
}
// Sort the headers so that those that are more deeply nested are considered
// first, possibly enabling outer conditionals to be flattened.
std::sort(selection_headers.begin(), selection_headers.end(),
ComparatorDeepBlocksFirst(GetIRContext()));
// Apply the transformation to the headers which can be flattened.
for (auto header : selection_headers) {
// Make a set to keep track of the instructions that need fresh ids.
std::set<opt::Instruction*> instructions_that_need_ids;
// Do not consider this header if the conditional cannot be flattened.
if (!TransformationFlattenConditionalBranch::
GetProblematicInstructionsIfConditionalCanBeFlattened(
GetIRContext(), header, *GetTransformationContext(),
&instructions_that_need_ids)) {
continue;
}
uint32_t convergence_block_id =
TransformationFlattenConditionalBranch::FindConvergenceBlock(
GetIRContext(), *header);
// If the SPIR-V version is restricted so that OpSelect can only work on
// scalar, pointer and vector types then we cannot apply this
// transformation to a header whose convergence block features OpPhi
// instructions on different types, as we cannot convert such instructions
// to OpSelect instructions.
if (TransformationFlattenConditionalBranch::
OpSelectArgumentsAreRestricted(GetIRContext())) {
if (!GetIRContext()
->cfg()
->block(convergence_block_id)
->WhileEachPhiInst(
[this](opt::Instruction* phi_instruction) -> bool {
switch (GetIRContext()
->get_def_use_mgr()
->GetDef(phi_instruction->type_id())
->opcode()) {
case spv::Op::OpTypeBool:
case spv::Op::OpTypeInt:
case spv::Op::OpTypeFloat:
case spv::Op::OpTypePointer:
case spv::Op::OpTypeVector:
return true;
default:
return false;
}
})) {
// An OpPhi is performed on a type not supported by OpSelect; we
// cannot flatten this selection.
continue;
}
}
// If the construct's convergence block features OpPhi instructions with
// vector result types then we may be *forced*, by the SPIR-V version, to
// turn these into component-wise OpSelect instructions, or we might wish
// to do so anyway. The following booleans capture whether we will opt
// to use a component-wise select even if we don't have to.
bool use_component_wise_2d_select_even_if_optional =
GetFuzzerContext()->ChooseEven();
bool use_component_wise_3d_select_even_if_optional =
GetFuzzerContext()->ChooseEven();
bool use_component_wise_4d_select_even_if_optional =
GetFuzzerContext()->ChooseEven();
// If we do need to perform any component-wise selections, we will need a
// fresh id for a boolean vector representing the selection's condition
// repeated N times, where N is the vector dimension.
uint32_t fresh_id_for_bvec2_selector = 0;
uint32_t fresh_id_for_bvec3_selector = 0;
uint32_t fresh_id_for_bvec4_selector = 0;
GetIRContext()
->cfg()
->block(convergence_block_id)
->ForEachPhiInst([this, &fresh_id_for_bvec2_selector,
&fresh_id_for_bvec3_selector,
&fresh_id_for_bvec4_selector,
use_component_wise_2d_select_even_if_optional,
use_component_wise_3d_select_even_if_optional,
use_component_wise_4d_select_even_if_optional](
opt::Instruction* phi_instruction) {
opt::Instruction* type_instruction =
GetIRContext()->get_def_use_mgr()->GetDef(
phi_instruction->type_id());
switch (type_instruction->opcode()) {
case spv::Op::OpTypeVector: {
uint32_t dimension =
type_instruction->GetSingleWordInOperand(1);
switch (dimension) {
case 2:
PrepareForOpPhiOnVectors(
dimension,
use_component_wise_2d_select_even_if_optional,
&fresh_id_for_bvec2_selector);
break;
case 3:
PrepareForOpPhiOnVectors(
dimension,
use_component_wise_3d_select_even_if_optional,
&fresh_id_for_bvec3_selector);
break;
case 4:
PrepareForOpPhiOnVectors(
dimension,
use_component_wise_4d_select_even_if_optional,
&fresh_id_for_bvec4_selector);
break;
default:
assert(false && "Invalid vector dimension.");
}
break;
}
default:
break;
}
});
// Some instructions will require to be enclosed inside conditionals
// because they have side effects (for example, loads and stores). Some of
// this have no result id, so we require instruction descriptors to
// identify them. Each of them is associated with the necessary ids for it
// via a SideEffectWrapperInfo message.
std::vector<protobufs::SideEffectWrapperInfo> wrappers_info;
for (auto instruction : instructions_that_need_ids) {
protobufs::SideEffectWrapperInfo wrapper_info;
*wrapper_info.mutable_instruction() =
MakeInstructionDescriptor(GetIRContext(), instruction);
wrapper_info.set_merge_block_id(GetFuzzerContext()->GetFreshId());
wrapper_info.set_execute_block_id(GetFuzzerContext()->GetFreshId());
// If the instruction has a non-void result id, we need to define more
// fresh ids and provide an id of the suitable type whose value can be
// copied in order to create a placeholder id.
if (TransformationFlattenConditionalBranch::InstructionNeedsPlaceholder(
GetIRContext(), *instruction)) {
wrapper_info.set_actual_result_id(GetFuzzerContext()->GetFreshId());
wrapper_info.set_alternative_block_id(
GetFuzzerContext()->GetFreshId());
wrapper_info.set_placeholder_result_id(
GetFuzzerContext()->GetFreshId());
// The id will be a zero constant if the type allows it, and an
// OpUndef otherwise. We want to avoid using OpUndef, if possible, to
// avoid undefined behaviour in the module as much as possible.
if (fuzzerutil::CanCreateConstant(GetIRContext(),
instruction->type_id())) {
wrapper_info.set_value_to_copy_id(
FindOrCreateZeroConstant(instruction->type_id(), true));
} else {
wrapper_info.set_value_to_copy_id(
FindOrCreateGlobalUndef(instruction->type_id()));
}
}
wrappers_info.push_back(std::move(wrapper_info));
}
// Apply the transformation, evenly choosing whether to lay out the true
// branch or the false branch first.
ApplyTransformation(TransformationFlattenConditionalBranch(
header->id(), GetFuzzerContext()->ChooseEven(),
fresh_id_for_bvec2_selector, fresh_id_for_bvec3_selector,
fresh_id_for_bvec4_selector, wrappers_info));
}
}
}
void FuzzerPassFlattenConditionalBranches::PrepareForOpPhiOnVectors(
uint32_t vector_dimension, bool use_vector_select_if_optional,
uint32_t* fresh_id_for_bvec_selector) {
if (*fresh_id_for_bvec_selector != 0) {
// We already have a fresh id for a component-wise OpSelect of this
// dimension
return;
}
if (TransformationFlattenConditionalBranch::OpSelectArgumentsAreRestricted(
GetIRContext()) ||
use_vector_select_if_optional) {
// We either have to, or have chosen to, perform a component-wise select, so
// we ensure that the right boolean vector type is available, and grab a
// fresh id.
FindOrCreateVectorType(FindOrCreateBoolType(), vector_dimension);
*fresh_id_for_bvec_selector = GetFuzzerContext()->GetFreshId();
}
}
} // namespace fuzz
} // namespace spvtools