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// 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include "source/opt/loop_unroller.h"
#include <limits>
#include <map>
#include <memory>
#include <unordered_map>
#include <utility>
#include <vector>
#include "source/opt/ir_builder.h"
#include "source/opt/loop_utils.h"
// Implements loop util unrolling functionality for fully and partially
// unrolling loops. Given a factor it will duplicate the loop that many times,
// appending each one to the end of the old loop and removing backedges, to
// create a new unrolled loop.
// 1 - User calls LoopUtils::FullyUnroll or LoopUtils::PartiallyUnroll with a
// loop they wish to unroll. LoopUtils::CanPerformUnroll is used to
// validate that a given loop can be unrolled. That method (along with the
// constructor of loop) checks that the IR is in the expected canonicalised
// format.
// 2 - The LoopUtils methods create a LoopUnrollerUtilsImpl object to actually
// perform the unrolling. This implements helper methods to copy the loop basic
// blocks and remap the ids of instructions used inside them.
// 3 - The core of LoopUnrollerUtilsImpl is the Unroll method, this method
// actually performs the loop duplication. It does this by creating a
// LoopUnrollState object and then copying the loop as given by the factor
// parameter. The LoopUnrollState object retains the state of the unroller
// between the loop body copies as each iteration needs information on the last
// to adjust the phi induction variable, adjust the OpLoopMerge instruction in
// the main loop header, and change the previous continue block to point to the
// new header and the new continue block to the main loop header.
// 4 - If the loop is to be fully unrolled then it is simply closed after step
// 3, with the OpLoopMerge being deleted, the backedge removed, and the
// condition blocks folded.
// 5 - If it is being partially unrolled: if the unrolling factor leaves the
// loop with an even number of bodies with respect to the number of loop
// iterations then step 3 is all that is needed. If it is uneven then we need to
// duplicate the loop completely and unroll the duplicated loop to cover the
// residual part and adjust the first loop to cover only the "even" part. For
// instance if you request an unroll factor of 3 on a loop with 10 iterations
// then copying the body three times would leave you with three bodies in the
// loop
// where the loop still iterates over each 4 times. So we make two loops one
// iterating once then a second loop of three iterating 3 times.
namespace spvtools {
namespace opt {
namespace {
// Loop control constant value for DontUnroll flag.
constexpr uint32_t kLoopControlDontUnrollIndex = 2;
// Operand index of the loop control parameter of the OpLoopMerge.
constexpr uint32_t kLoopControlIndex = 2;
// This utility class encapsulates some of the state we need to maintain between
// loop unrolls. Specifically it maintains key blocks and the induction variable
// in the current loop duplication step and the blocks from the previous one.
// This is because each step of the unroll needs to use data from both the
// preceding step and the original loop.
struct LoopUnrollState {
: previous_phi_(nullptr),
new_header_block(nullptr) {}
// Initialize from the loop descriptor class.
LoopUnrollState(Instruction* induction, BasicBlock* latch_block,
BasicBlock* condition, std::vector<Instruction*>&& phis)
: previous_phi_(induction),
new_header_block(nullptr) {
previous_phis_ = std::move(phis);
// Swap the state so that the new nodes are now the previous nodes.
void NextIterationState() {
previous_phi_ = new_phi;
previous_latch_block_ = new_latch_block;
previous_condition_block_ = new_condition_block;
previous_phis_ = std::move(new_phis_);
// Clear new nodes.
new_phi = nullptr;
new_continue_block = nullptr;
new_condition_block = nullptr;
new_header_block = nullptr;
new_latch_block = nullptr;
// Clear new block/instruction maps.
// The induction variable from the immediately preceding loop body.
Instruction* previous_phi_;
// All the phi nodes from the previous loop iteration.
std::vector<Instruction*> previous_phis_;
std::vector<Instruction*> new_phis_;
// The previous latch block. The backedge will be removed from this and
// added to the new latch block.
BasicBlock* previous_latch_block_;
// The previous condition block. This may be folded to flatten the loop.
BasicBlock* previous_condition_block_;
// The new induction variable.
Instruction* new_phi;
// The new continue block.
BasicBlock* new_continue_block;
// The new condition block.
BasicBlock* new_condition_block;
// The new header block.
BasicBlock* new_header_block;
// The new latch block.
BasicBlock* new_latch_block;
// A mapping of new block ids to the original blocks which they were copied
// from.
std::unordered_map<uint32_t, BasicBlock*> new_blocks;
// A mapping of the original instruction ids to the instruction ids to their
// copies.
std::unordered_map<uint32_t, uint32_t> new_inst;
std::unordered_map<uint32_t, Instruction*> ids_to_new_inst;
// This class implements the actual unrolling. It uses a LoopUnrollState to
// maintain the state of the unrolling in between steps.
class LoopUnrollerUtilsImpl {
using BasicBlockListTy = std::vector<std::unique_ptr<BasicBlock>>;
LoopUnrollerUtilsImpl(IRContext* c, Function* function)
: context_(c),
loop_init_value_(0) {}
// Unroll the |loop| by given |factor| by copying the whole body |factor|
// times. The resulting basicblock structure will remain a loop.
void PartiallyUnroll(Loop*, size_t factor);
// If partially unrolling the |loop| would leave the loop with too many bodies
// for its number of iterations then this method should be used. This method
// will duplicate the |loop| completely, making the duplicated loop the
// successor of the original's merge block. The original loop will have its
// condition changed to loop over the residual part and the duplicate will be
// partially unrolled. The resulting structure will be two loops.
void PartiallyUnrollResidualFactor(Loop* loop, size_t factor);
// Fully unroll the |loop| by copying the full body by the total number of
// loop iterations, folding all conditions, and removing the backedge from the
// continue block to the header.
void FullyUnroll(Loop* loop);
// Get the ID of the variable in the |phi| paired with |label|.
uint32_t GetPhiDefID(const Instruction* phi, uint32_t label) const;
// Close the loop by removing the OpLoopMerge from the |loop| header block and
// making the backedge point to the merge block.
void CloseUnrolledLoop(Loop* loop);
// Remove the OpConditionalBranch instruction inside |conditional_block| used
// to branch to either exit or continue the loop and replace it with an
// unconditional OpBranch to block |new_target|.
void FoldConditionBlock(BasicBlock* condtion_block, uint32_t new_target);
// Add all blocks_to_add_ to function_ at the |insert_point|.
void AddBlocksToFunction(const BasicBlock* insert_point);
// Duplicates the |old_loop|, cloning each body and remapping the ids without
// removing instructions or changing relative structure. Result will be stored
// in |new_loop|.
void DuplicateLoop(Loop* old_loop, Loop* new_loop);
inline size_t GetLoopIterationCount() const {
return number_of_loop_iterations_;
// Extracts the initial state information from the |loop|.
void Init(Loop* loop);
// Replace the uses of each induction variable outside the loop with the final
// value of the induction variable before the loop exit. To reflect the proper
// state of a fully unrolled loop.
void ReplaceInductionUseWithFinalValue(Loop* loop);
// Remove all the instructions in the invalidated_instructions_ vector.
void RemoveDeadInstructions();
// Replace any use of induction variables outwith the loop with the final
// value of the induction variable in the unrolled loop.
void ReplaceOutsideLoopUseWithFinalValue(Loop* loop);
// Set the LoopControl operand of the OpLoopMerge instruction to be
// DontUnroll.
void MarkLoopControlAsDontUnroll(Loop* loop) const;
// Remap all the in |basic_block| to new IDs and keep the mapping of new ids
// to old
// ids. |loop| is used to identify special loop blocks (header, continue,
// etc).
void AssignNewResultIds(BasicBlock* basic_block);
// Using the map built by AssignNewResultIds, replace the uses in |inst|
// by the id that the use maps to.
void RemapOperands(Instruction* inst);
// Using the map built by AssignNewResultIds, for each instruction in
// |basic_block| use
// that map to substitute the IDs used by instructions (in the operands) with
// the new ids.
void RemapOperands(BasicBlock* basic_block);
// Copy the whole body of the loop, all blocks dominated by the |loop| header
// and not dominated by the |loop| merge. The copied body will be linked to by
// the old |loop| continue block and the new body will link to the |loop|
// header via the new continue block. |eliminate_conditions| is used to decide
// whether or not to fold all the condition blocks other than the last one.
void CopyBody(Loop* loop, bool eliminate_conditions);
// Copy a given |block_to_copy| in the |loop| and record the mapping of the
// old/new ids. |preserve_instructions| determines whether or not the method
// will modify (other than result_id) instructions which are copied.
void CopyBasicBlock(Loop* loop, const BasicBlock* block_to_copy,
bool preserve_instructions);
// The actual implementation of the unroll step. Unrolls |loop| by given
// |factor| by copying the body by |factor| times. Also propagates the
// induction variable value throughout the copies.
void Unroll(Loop* loop, size_t factor);
// Fills the loop_blocks_inorder_ field with the ordered list of basic blocks
// as computed by the method ComputeLoopOrderedBlocks.
void ComputeLoopOrderedBlocks(Loop* loop);
// Adds the blocks_to_add_ to both the |loop| and to the parent of |loop| if
// the parent exists.
void AddBlocksToLoop(Loop* loop) const;
// After the partially unroll step the phi instructions in the header block
// will be in an illegal format. This function makes the phis legal by making
// the edge from the latch block come from the new latch block and the value
// to be the actual value of the phi at that point.
void LinkLastPhisToStart(Loop* loop) const;
// Kill all debug declaration instructions from |bb|.
void KillDebugDeclares(BasicBlock* bb);
// A pointer to the IRContext. Used to add/remove instructions and for usedef
// chains.
IRContext* context_;
// A reference the function the loop is within.
Function& function_;
// A list of basic blocks to be added to the loop at the end of an unroll
// step.
BasicBlockListTy blocks_to_add_;
// List of instructions which are now dead and can be removed.
std::vector<Instruction*> invalidated_instructions_;
// Maintains the current state of the transform between calls to unroll.
LoopUnrollState state_;
// An ordered list containing the loop basic blocks.
std::vector<BasicBlock*> loop_blocks_inorder_;
// The block containing the condition check which contains a conditional
// branch to the merge and continue block.
BasicBlock* loop_condition_block_;
// The induction variable of the loop.
Instruction* loop_induction_variable_;
// Phis used in the loop need to be remapped to use the actual result values
// and then be remapped at the end.
std::vector<Instruction*> loop_phi_instructions_;
// The number of loop iterations that the loop would perform pre-unroll.
size_t number_of_loop_iterations_;
// The amount that the loop steps each iteration.
int64_t loop_step_value_;
// The value the loop starts stepping from.
int64_t loop_init_value_;
* Static helper functions.
// Retrieve the index of the OpPhi instruction |phi| which corresponds to the
// incoming |block| id.
uint32_t GetPhiIndexFromLabel(const BasicBlock* block, const Instruction* phi) {
for (uint32_t i = 1; i < phi->NumInOperands(); i += 2) {
if (block->id() == phi->GetSingleWordInOperand(i)) {
return i;
assert(false && "Could not find operand in instruction.");
return 0;
void LoopUnrollerUtilsImpl::Init(Loop* loop) {
loop_condition_block_ = loop->FindConditionBlock();
// When we reinit the second loop during PartiallyUnrollResidualFactor we need
// to use the cached value from the duplicate step as the dominator tree
// basded solution, loop->FindConditionBlock, requires all the nodes to be
// connected up with the correct branches. They won't be at this point.
if (!loop_condition_block_) {
loop_condition_block_ = state_.new_condition_block;
loop_induction_variable_ = loop->FindConditionVariable(loop_condition_block_);
bool found = loop->FindNumberOfIterations(
loop_induction_variable_, &*loop_condition_block_->ctail(),
&number_of_loop_iterations_, &loop_step_value_, &loop_init_value_);
(void)found; // To silence unused variable warning on release builds.
// Blocks are stored in an unordered set of ids in the loop class, we need to
// create the dominator ordered list.
// This function is used to partially unroll the loop when the factor provided
// would normally lead to an illegal optimization. Instead of just unrolling the
// loop it creates two loops and unrolls one and adjusts the condition on the
// other. The end result being that the new loop pair iterates over the correct
// number of bodies.
void LoopUnrollerUtilsImpl::PartiallyUnrollResidualFactor(Loop* loop,
size_t factor) {
// TODO(1841): Handle id overflow.
std::unique_ptr<Instruction> new_label{new Instruction(
context_, spv::Op::OpLabel, 0, context_->TakeNextId(), {})};
std::unique_ptr<BasicBlock> new_exit_bb{new BasicBlock(std::move(new_label))};
// Save the id of the block before we move it.
uint32_t new_merge_id = new_exit_bb->id();
// Add the block the list of blocks to add, we want this merge block to be
// right at the start of the new blocks.
BasicBlock* new_exit_bb_raw = blocks_to_add_[0].get();
Instruction& original_conditional_branch = *loop_condition_block_->tail();
// Duplicate the loop, providing access to the blocks of both loops.
// This is a naked new due to the VS2013 requirement of not having unique
// pointers in vectors, as it will be inserted into a vector with
// loop_descriptor.AddLoop.
std::unique_ptr<Loop> new_loop = MakeUnique<Loop>(*loop);
// Clear the basic blocks of the new loop.
DuplicateLoop(loop, new_loop.get());
// Add the blocks to the function.
// Create a new merge block for the first loop.
InstructionBuilder builder{context_, new_exit_bb_raw};
// Make the first loop branch to the second.
loop_condition_block_ = state_.new_condition_block;
loop_induction_variable_ = state_.new_phi;
// Unroll the new loop by the factor with the usual -1 to account for the
// existing block iteration.
Unroll(new_loop.get(), factor);
// Add the new merge block to the back of the list of blocks to be added. It
// needs to be the last block added to maintain dominator order in the binary.
// Add the blocks to the function.
// Reset the usedef analysis.
analysis::DefUseManager* def_use_manager = context_->get_def_use_mgr();
// The loop condition.
Instruction* condition_check = def_use_manager->GetDef(
// This should have been checked by the LoopUtils::CanPerformUnroll function
// before entering this.
// We need to account for the initial body when calculating the remainder.
int64_t remainder = Loop::GetResidualConditionValue(
condition_check->opcode(), loop_init_value_, loop_step_value_,
number_of_loop_iterations_, factor);
assert(remainder > std::numeric_limits<int32_t>::min() &&
remainder < std::numeric_limits<int32_t>::max());
Instruction* new_constant = nullptr;
// If the remainder is negative then we add a signed constant, otherwise just
// add an unsigned constant.
if (remainder < 0) {
new_constant = builder.GetSintConstant(static_cast<int32_t>(remainder));
} else {
new_constant = builder.GetUintConstant(static_cast<int32_t>(remainder));
uint32_t constant_id = new_constant->result_id();
// Update the condition check.
condition_check->SetInOperand(1, {constant_id});
// Update the next phi node. The phi will have a constant value coming in from
// the preheader block. For the duplicated loop we need to update the constant
// to be the amount of iterations covered by the first loop and the incoming
// block to be the first loops new merge block.
std::vector<Instruction*> new_inductions;
std::vector<Instruction*> old_inductions;
for (size_t index = 0; index < new_inductions.size(); ++index) {
Instruction* new_induction = new_inductions[index];
Instruction* old_induction = old_inductions[index];
// Get the index of the loop initalizer, the value coming in from the
// preheader.
uint32_t initalizer_index =
GetPhiIndexFromLabel(new_loop->GetPreHeaderBlock(), old_induction);
// Replace the second loop initalizer with the phi from the first
new_induction->SetInOperand(initalizer_index - 1,
new_induction->SetInOperand(initalizer_index, {new_merge_id});
// If the use of the first loop induction variable is outside of the loop
// then replace that use with the second loop induction variable.
uint32_t second_loop_induction = new_induction->result_id();
auto replace_use_outside_of_loop = [loop, second_loop_induction](
Instruction* user,
uint32_t operand_index) {
if (!loop->IsInsideLoop(user)) {
user->SetOperand(operand_index, {second_loop_induction});
context_->ReplaceAllUsesWith(loop->GetMergeBlock()->id(), new_merge_id);
LoopDescriptor& loop_descriptor = *context_->GetLoopDescriptor(&function_);
loop_descriptor.AddLoop(std::move(new_loop), loop->GetParent());
// Mark this loop as DontUnroll as it will already be unrolled and it may not
// be safe to unroll a previously partially unrolled loop.
void LoopUnrollerUtilsImpl::MarkLoopControlAsDontUnroll(Loop* loop) const {
Instruction* loop_merge_inst = loop->GetHeaderBlock()->GetLoopMergeInst();
assert(loop_merge_inst &&
"Loop merge instruction could not be found after entering unroller "
"(should have exited before this)");
// Duplicate the |loop| body |factor| - 1 number of times while keeping the loop
// backedge intact. This will leave the loop with |factor| number of bodies
// after accounting for the initial body.
void LoopUnrollerUtilsImpl::Unroll(Loop* loop, size_t factor) {
// If we unroll a loop partially it will not be safe to unroll it further.
// This is due to the current method of calculating the number of loop
// iterations.
std::vector<Instruction*> inductions;
state_ = LoopUnrollState{loop_induction_variable_, loop->GetLatchBlock(),
loop_condition_block_, std::move(inductions)};
for (size_t i = 0; i < factor - 1; ++i) {
CopyBody(loop, true);
void LoopUnrollerUtilsImpl::RemoveDeadInstructions() {
// Remove the dead instructions.
for (Instruction* inst : invalidated_instructions_) {
void LoopUnrollerUtilsImpl::ReplaceInductionUseWithFinalValue(Loop* loop) {
IRContext::Analysis::kAnalysisLoopAnalysis |
IRContext::Analysis::kAnalysisDefUse |
std::vector<Instruction*> inductions;
for (size_t index = 0; index < inductions.size(); ++index) {
uint32_t trip_step_id = GetPhiDefID(state_.previous_phis_[index],
context_->ReplaceAllUsesWith(inductions[index]->result_id(), trip_step_id);
// Fully unroll the loop by partially unrolling it by the number of loop
// iterations minus one for the body already accounted for.
void LoopUnrollerUtilsImpl::FullyUnroll(Loop* loop) {
// We unroll the loop by number of iterations in the loop.
Unroll(loop, number_of_loop_iterations_);
// The first condition block is preserved until now so it can be copied.
FoldConditionBlock(loop_condition_block_, 1);
// Delete the OpLoopMerge and remove the backedge to the header.
// Mark the loop for later deletion. This allows us to preserve the loop
// iterators but still disregard dead loops.
// If the loop has a parent add the new blocks to the parent.
if (loop->GetParent()) {
// Add the blocks to the function.
// Invalidate all analyses.
IRContext::Analysis::kAnalysisLoopAnalysis |
void LoopUnrollerUtilsImpl::KillDebugDeclares(BasicBlock* bb) {
// We cannot kill an instruction inside BasicBlock::ForEachInst()
// because it will generate dangling pointers. We use |to_be_killed|
// to kill them after the loop.
std::vector<Instruction*> to_be_killed;
bb->ForEachInst([&to_be_killed, this](Instruction* inst) {
if (context_->get_debug_info_mgr()->IsDebugDeclare(inst)) {
for (auto* inst : to_be_killed) context_->KillInst(inst);
// Copy a given basic block, give it a new result_id, and store the new block
// and the id mapping in the state. |preserve_instructions| is used to determine
// whether or not this function should edit instructions other than the
// |result_id|.
void LoopUnrollerUtilsImpl::CopyBasicBlock(Loop* loop, const BasicBlock* itr,
bool preserve_instructions) {
// Clone the block exactly, including the IDs.
BasicBlock* basic_block = itr->Clone(context_);
// We do not want to duplicate DebugDeclare.
// Assign each result a new unique ID and keep a mapping of the old ids to
// the new ones.
// If this is the continue block we are copying.
if (itr == loop->GetContinueBlock()) {
// Make the OpLoopMerge point to this block for the continue.
if (!preserve_instructions) {
Instruction* merge_inst = loop->GetHeaderBlock()->GetLoopMergeInst();
merge_inst->SetInOperand(1, {basic_block->id()});
state_.new_continue_block = basic_block;
// If this is the header block we are copying.
if (itr == loop->GetHeaderBlock()) {
state_.new_header_block = basic_block;
if (!preserve_instructions) {
// Remove the loop merge instruction if it exists.
Instruction* merge_inst = basic_block->GetLoopMergeInst();
if (merge_inst) invalidated_instructions_.push_back(merge_inst);
// If this is the latch block being copied, record it in the state.
if (itr == loop->GetLatchBlock()) state_.new_latch_block = basic_block;
// If this is the condition block we are copying.
if (itr == loop_condition_block_) {
state_.new_condition_block = basic_block;
// Add this block to the list of blocks to add to the function at the end of
// the unrolling process.
// Keep tracking the old block via a map.
state_.new_blocks[itr->id()] = basic_block;
void LoopUnrollerUtilsImpl::CopyBody(Loop* loop, bool eliminate_conditions) {
// Copy each basic block in the loop, give them new ids, and save state
// information.
for (const BasicBlock* itr : loop_blocks_inorder_) {
CopyBasicBlock(loop, itr, false);
// Set the previous latch block to point to the new header.
Instruction* latch_branch = state_.previous_latch_block_->terminator();
latch_branch->SetInOperand(0, {state_.new_header_block->id()});
// As the algorithm copies the original loop blocks exactly, the tail of the
// latch block on iterations after the first one will be a branch to the new
// header and not the actual loop header. The last continue block in the loop
// should always be a backedge to the global header.
Instruction* new_latch_branch = state_.new_latch_block->terminator();
new_latch_branch->SetInOperand(0, {loop->GetHeaderBlock()->id()});
std::vector<Instruction*> inductions;
for (size_t index = 0; index < inductions.size(); ++index) {
Instruction* primary_copy = inductions[index];
assert(primary_copy->result_id() != 0);
Instruction* induction_clone =
assert(induction_clone->result_id() != 0);
if (!state_.previous_phis_.empty()) {
state_.new_inst[primary_copy->result_id()] = GetPhiDefID(
state_.previous_phis_[index], state_.previous_latch_block_->id());
} else {
// Do not replace the first phi block ids.
state_.new_inst[primary_copy->result_id()] = primary_copy->result_id();
if (eliminate_conditions &&
state_.new_condition_block != loop_condition_block_) {
FoldConditionBlock(state_.new_condition_block, 1);
// Only reference to the header block is the backedge in the latch block,
// don't change this.
state_.new_inst[loop->GetHeaderBlock()->id()] = loop->GetHeaderBlock()->id();
for (auto& pair : state_.new_blocks) {
for (Instruction* dead_phi : state_.new_phis_)
// Swap the state so the new is now the previous.
uint32_t LoopUnrollerUtilsImpl::GetPhiDefID(const Instruction* phi,
uint32_t label) const {
for (uint32_t operand = 3; operand < phi->NumOperands(); operand += 2) {
if (phi->GetSingleWordOperand(operand) == label) {
return phi->GetSingleWordOperand(operand - 1);
assert(false && "Could not find a phi index matching the provided label");
return 0;
void LoopUnrollerUtilsImpl::FoldConditionBlock(BasicBlock* condition_block,
uint32_t operand_label) {
// Remove the old conditional branch to the merge and continue blocks.
Instruction& old_branch = *condition_block->tail();
uint32_t new_target = old_branch.GetSingleWordOperand(operand_label);
DebugScope scope = old_branch.GetDebugScope();
const std::vector<Instruction> lines = old_branch.dbg_line_insts();
// Add the new unconditional branch to the merge block.
InstructionBuilder builder(
context_, condition_block,
IRContext::Analysis::kAnalysisDefUse |
Instruction* new_branch = builder.AddBranch(new_target);
if (!lines.empty()) new_branch->AddDebugLine(&lines.back());
void LoopUnrollerUtilsImpl::CloseUnrolledLoop(Loop* loop) {
// Remove the OpLoopMerge instruction from the function.
Instruction* merge_inst = loop->GetHeaderBlock()->GetLoopMergeInst();
// Remove the final backedge to the header and make it point instead to the
// merge block.
Instruction* latch_instruction = state_.previous_latch_block_->terminator();
latch_instruction->SetInOperand(0, {loop->GetMergeBlock()->id()});
// Remove all induction variables as the phis will now be invalid. Replace all
// uses with the constant initializer value (all uses of phis will be in
// the first iteration with the subsequent phis already having been removed).
std::vector<Instruction*> inductions;
// We can use the state instruction mechanism to replace all internal loop
// values within the first loop trip (as the subsequent ones will be updated
// by the copy function) with the value coming in from the preheader and then
// use context ReplaceAllUsesWith for the uses outside the loop with the final
// trip phi value.
for (Instruction* induction : inductions) {
uint32_t initalizer_id =
GetPhiDefID(induction, loop->GetPreHeaderBlock()->id());
state_.new_inst[induction->result_id()] = initalizer_id;
for (BasicBlock* block : loop_blocks_inorder_) {
for (auto& block_itr : blocks_to_add_) {
// Rewrite the last phis, since they may still reference the original phi.
for (Instruction* last_phi : state_.previous_phis_) {
// Uses the first loop to create a copy of the loop with new IDs.
void LoopUnrollerUtilsImpl::DuplicateLoop(Loop* old_loop, Loop* new_loop) {
std::vector<BasicBlock*> new_block_order;
// Copy every block in the old loop.
for (const BasicBlock* itr : loop_blocks_inorder_) {
CopyBasicBlock(old_loop, itr, true);
// Clone the merge block, give it a new id and record it in the state.
BasicBlock* new_merge = old_loop->GetMergeBlock()->Clone(context_);
state_.new_blocks[old_loop->GetMergeBlock()->id()] = new_merge;
// Remap the operands of every instruction in the loop to point to the new
// copies.
for (auto& pair : state_.new_blocks) {
loop_blocks_inorder_ = std::move(new_block_order);
// Whenever the utility copies a block it stores it in a temporary buffer, this
// function adds the buffer into the Function. The blocks will be inserted
// after the block |insert_point|.
void LoopUnrollerUtilsImpl::AddBlocksToFunction(
const BasicBlock* insert_point) {
for (auto basic_block_iterator = function_.begin();
basic_block_iterator != function_.end(); ++basic_block_iterator) {
if (basic_block_iterator->id() == insert_point->id()) {
false &&
"Could not add basic blocks to function as insert point was not found.");
// Assign all result_ids in |basic_block| instructions to new IDs and preserve
// the mapping of new ids to old ones.
void LoopUnrollerUtilsImpl::AssignNewResultIds(BasicBlock* basic_block) {
analysis::DefUseManager* def_use_mgr = context_->get_def_use_mgr();
// Label instructions aren't covered by normal traversal of the
// instructions.
// TODO(1841): Handle id overflow.
uint32_t new_label_id = context_->TakeNextId();
// Assign a new id to the label.
state_.new_inst[basic_block->GetLabelInst()->result_id()] = new_label_id;
for (Instruction& inst : *basic_block) {
// Do def/use analysis on new lines
for (auto& line : inst.dbg_line_insts())
uint32_t old_id = inst.result_id();
// Ignore stores etc.
if (old_id == 0) {
// Give the instruction a new id.
// TODO(1841): Handle id overflow.
// Save the mapping of old_id -> new_id.
state_.new_inst[old_id] = inst.result_id();
// Check if this instruction is the induction variable.
if (loop_induction_variable_->result_id() == old_id) {
// Save a pointer to the new copy of it.
state_.new_phi = &inst;
state_.ids_to_new_inst[inst.result_id()] = &inst;
void LoopUnrollerUtilsImpl::RemapOperands(Instruction* inst) {
auto remap_operands_to_new_ids = [this](uint32_t* id) {
auto itr = state_.new_inst.find(*id);
if (itr != state_.new_inst.end()) {
*id = itr->second;
void LoopUnrollerUtilsImpl::RemapOperands(BasicBlock* basic_block) {
for (Instruction& inst : *basic_block) {
// Generate the ordered list of basic blocks in the |loop| and cache it for
// later use.
void LoopUnrollerUtilsImpl::ComputeLoopOrderedBlocks(Loop* loop) {
// Adds the blocks_to_add_ to both the loop and to the parent.
void LoopUnrollerUtilsImpl::AddBlocksToLoop(Loop* loop) const {
// Add the blocks to this loop.
for (auto& block_itr : blocks_to_add_) {
// Add the blocks to the parent as well.
if (loop->GetParent()) AddBlocksToLoop(loop->GetParent());
void LoopUnrollerUtilsImpl::LinkLastPhisToStart(Loop* loop) const {
std::vector<Instruction*> inductions;
for (size_t i = 0; i < inductions.size(); ++i) {
Instruction* last_phi_in_block = state_.previous_phis_[i];
uint32_t phi_index =
GetPhiIndexFromLabel(state_.previous_latch_block_, last_phi_in_block);
uint32_t phi_variable =
last_phi_in_block->GetSingleWordInOperand(phi_index - 1);
uint32_t phi_label = last_phi_in_block->GetSingleWordInOperand(phi_index);
Instruction* phi = inductions[i];
phi->SetInOperand(phi_index - 1, {phi_variable});
phi->SetInOperand(phi_index, {phi_label});
// Duplicate the |loop| body |factor| number of times while keeping the loop
// backedge intact.
void LoopUnrollerUtilsImpl::PartiallyUnroll(Loop* loop, size_t factor) {
Unroll(loop, factor);
* End LoopUtilsImpl.
} // namespace
* Begin Utils.
* */
bool LoopUtils::CanPerformUnroll() {
// The loop is expected to be in structured order.
if (!loop_->GetHeaderBlock()->GetMergeInst()) {
return false;
// Find check the loop has a condition we can find and evaluate.
const BasicBlock* condition = loop_->FindConditionBlock();
if (!condition) return false;
// Check that we can find and process the induction variable.
const Instruction* induction = loop_->FindConditionVariable(condition);
if (!induction || induction->opcode() != spv::Op::OpPhi) return false;
// Check that we can find the number of loop iterations.
if (!loop_->FindNumberOfIterations(induction, &*condition->ctail(), nullptr))
return false;
// ClusterFuzz/OSS-Fuzz is likely to yield examples with very high loop
// iteration counts. This can cause timeouts and memouts during fuzzing that
// are not classed as bugs. To avoid this noise, loop unrolling is not applied
// to loops with large iteration counts when fuzzing.
constexpr size_t kFuzzerIterationLimit = 100;
size_t num_iterations;
loop_->FindNumberOfIterations(induction, &*condition->ctail(),
if (num_iterations > kFuzzerIterationLimit) {
return false;
// Make sure the latch block is a unconditional branch to the header
// block.
const Instruction& branch = *loop_->GetLatchBlock()->ctail();
bool branching_assumption =
branch.opcode() == spv::Op::OpBranch &&
branch.GetSingleWordInOperand(0) == loop_->GetHeaderBlock()->id();
if (!branching_assumption) {
return false;
std::vector<Instruction*> inductions;
// Ban breaks within the loop.
const std::vector<uint32_t>& merge_block_preds =
if (merge_block_preds.size() != 1) {
return false;
// Ban continues within the loop.
const std::vector<uint32_t>& continue_block_preds =
if (continue_block_preds.size() != 1) {
return false;
// Ban returns in the loop.
// Iterate over all the blocks within the loop and check that none of them
// exit the loop.
for (uint32_t label_id : loop_->GetBlocks()) {
const BasicBlock* block = context_->cfg()->block(label_id);
if (block->ctail()->opcode() == spv::Op::OpKill ||
block->ctail()->opcode() == spv::Op::OpReturn ||
block->ctail()->opcode() == spv::Op::OpReturnValue ||
block->ctail()->opcode() == spv::Op::OpTerminateInvocation) {
return false;
// Can only unroll inner loops.
if (!loop_->AreAllChildrenMarkedForRemoval()) {
return false;
return true;
bool LoopUtils::PartiallyUnroll(size_t factor) {
if (factor == 1 || !CanPerformUnroll()) return false;
// Create the unroller utility.
LoopUnrollerUtilsImpl unroller{context_,
// If the unrolling factor is larger than or the same size as the loop just
// fully unroll the loop.
if (factor >= unroller.GetLoopIterationCount()) {
return true;
// If the loop unrolling factor is an residual number of iterations we need to
// let run the loop for the residual part then let it branch into the unrolled
// remaining part. We add one when calucating the remainder to take into
// account the one iteration already in the loop.
if (unroller.GetLoopIterationCount() % factor != 0) {
unroller.PartiallyUnrollResidualFactor(loop_, factor);
} else {
unroller.PartiallyUnroll(loop_, factor);
return true;
bool LoopUtils::FullyUnroll() {
if (!CanPerformUnroll()) return false;
std::vector<Instruction*> inductions;
LoopUnrollerUtilsImpl unroller{context_,
return true;
void LoopUtils::Finalize() {
// Clean up the loop descriptor to preserve the analysis.
LoopDescriptor* LD = context_->GetLoopDescriptor(&function_);
* Begin Pass.
Pass::Status LoopUnroller::Process() {
bool changed = false;
for (Function& f : *context()->module()) {
if (f.IsDeclaration()) {
LoopDescriptor* LD = context()->GetLoopDescriptor(&f);
for (Loop& loop : *LD) {
LoopUtils loop_utils{context(), &loop};
if (!loop.HasUnrollLoopControl() || !loop_utils.CanPerformUnroll()) {
if (fully_unroll_) {
} else {
changed = true;
return changed ? Status::SuccessWithChange : Status::SuccessWithoutChange;
} // namespace opt
} // namespace spvtools