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SPIRV-Tools/source/opt/loop_unswitch_pass.cpp
Victor Lomuller 3497a94460 Add loop unswitch pass. 
It moves all conditional branching and switch whose conditions are loop
invariant and uniform. Before performing the loop unswitch we check that
the loop does not contain any instruction that would prevent it
(barriers, group instructions etc.).
2018-02-27 08:52:46 -05:00

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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
//
// 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 "loop_unswitch_pass.h"
#include <functional>
#include <list>
#include <memory>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#include "basic_block.h"
#include "dominator_tree.h"
#include "fold.h"
#include "function.h"
#include "instruction.h"
#include "ir_builder.h"
#include "ir_context.h"
#include "loop_descriptor.h"
#include "loop_utils.h"
namespace spvtools {
namespace opt {
namespace {
static const uint32_t kTypePointerStorageClassInIdx = 0;
static const uint32_t kBranchCondTrueLabIdInIdx = 1;
static const uint32_t kBranchCondFalseLabIdInIdx = 2;
} // anonymous namespace
namespace {
// This class handle the unswitch procedure for a given loop.
// The unswitch will not happen if:
// - The loop has any instruction that will prevent it;
// - The loop invariant condition is not uniform.
class LoopUnswitch {
public:
LoopUnswitch(ir::IRContext* context, ir::Function* function, ir::Loop* loop,
ir::LoopDescriptor* loop_desc)
: function_(function),
loop_(loop),
loop_desc_(*loop_desc),
context_(context),
switch_block_(nullptr) {}
// Returns true if the loop can be unswitched.
// Can be unswitch if:
// - The loop has no instructions that prevents it (such as barrier);
// - The loop has one conditional branch or switch that do not depends on the
// loop;
// - The loop invariant condition is uniform;
bool CanUnswitchLoop() {
if (switch_block_) return true;
if (loop_->IsSafeToClone()) return false;
ir::CFG& cfg = *context_->cfg();
for (uint32_t bb_id : loop_->GetBlocks()) {
ir::BasicBlock* bb = cfg.block(bb_id);
if (bb->terminator()->IsBranch() &&
bb->terminator()->opcode() != SpvOpBranch) {
if (IsConditionLoopInvariant(bb->terminator())) {
switch_block_ = bb;
break;
}
}
}
return switch_block_;
}
// Return the iterator to the basic block |bb|.
ir::Function::iterator FindBasicBlockPosition(ir::BasicBlock* bb_to_find) {
ir::Function::iterator it = std::find_if(
function_->begin(), function_->end(),
[bb_to_find](const ir::BasicBlock& bb) { return bb_to_find == &bb; });
assert(it != function_->end() && "Basic Block not found");
return it;
}
// Creates a new basic block and insert it into the function |fn| at the
// position |ip|. This function preserves the def/use and instr to block
// managers.
ir::BasicBlock* CreateBasicBlock(ir::Function::iterator ip) {
analysis::DefUseManager* def_use_mgr = context_->get_def_use_mgr();
ir::BasicBlock* bb = &*ip.InsertBefore(std::unique_ptr<ir::BasicBlock>(
new ir::BasicBlock(std::unique_ptr<ir::Instruction>(new ir::Instruction(
context_, SpvOpLabel, 0, context_->TakeNextId(), {})))));
bb->SetParent(function_);
def_use_mgr->AnalyzeInstDef(bb->GetLabelInst());
context_->set_instr_block(bb->GetLabelInst(), bb);
return bb;
}
// Unswitches |loop_|.
void PerformUnswitch() {
assert(CanUnswitchLoop() &&
"Cannot unswitch if there is not constant condition");
assert(loop_->GetPreHeaderBlock() && "This loop has no pre-header block");
assert(loop_->IsLCSSA() && "This loop is not in LCSSA form");
ir::CFG& cfg = *context_->cfg();
DominatorTree* dom_tree =
&context_->GetDominatorAnalysis(function_, *context_->cfg())
->GetDomTree();
analysis::DefUseManager* def_use_mgr = context_->get_def_use_mgr();
LoopUtils loop_utils(context_, loop_);
//////////////////////////////////////////////////////////////////////////////
// Step 1: Create the if merge block for structured modules.
// To do so, the |loop_| merge block will become the if's one and we
// create a merge for the loop. This will limit the amount of duplicated
// code the structured control flow imposes.
// For non structured program, the new loop will be connected to
// the old loop's exit blocks.
//////////////////////////////////////////////////////////////////////////////
// Get the merge block if it exists.
ir::BasicBlock* if_merge_block = loop_->GetMergeBlock();
// The merge block is only created if the loop has a unique exit block. We
// have this guarantee for structured loops, for compute loop it will
// trivially help maintain both a structured-like form and LCSAA.
ir::BasicBlock* loop_merge_block =
if_merge_block
? CreateBasicBlock(FindBasicBlockPosition(if_merge_block))
: nullptr;
if (loop_merge_block) {
// Add the instruction and update managers.
opt::InstructionBuilder builder(
context_, loop_merge_block,
ir::IRContext::kAnalysisDefUse |
ir::IRContext::kAnalysisInstrToBlockMapping);
builder.AddBranch(if_merge_block->id());
builder.SetInsertPoint(&*loop_merge_block->begin());
cfg.RegisterBlock(loop_merge_block);
def_use_mgr->AnalyzeInstDef(loop_merge_block->GetLabelInst());
// Update CFG.
if_merge_block->ForEachPhiInst(
[loop_merge_block, &builder, this](ir::Instruction* phi) {
ir::Instruction* cloned = phi->Clone(context_);
builder.AddInstruction(std::unique_ptr<ir::Instruction>(cloned));
phi->SetInOperand(0, {cloned->result_id()});
phi->SetInOperand(1, {loop_merge_block->id()});
for (uint32_t j = phi->NumInOperands() - 1; j > 1; j--)
phi->RemoveInOperand(j);
});
// Copy the predecessor list (will get invalidated otherwise).
std::vector<uint32_t> preds = cfg.preds(if_merge_block->id());
for (uint32_t pid : preds) {
if (pid == loop_merge_block->id()) continue;
ir::BasicBlock* p_bb = cfg.block(pid);
p_bb->ForEachSuccessorLabel(
[if_merge_block, loop_merge_block](uint32_t* id) {
if (*id == if_merge_block->id()) *id = loop_merge_block->id();
});
cfg.AddEdge(pid, loop_merge_block->id());
}
cfg.RemoveNonExistingEdges(if_merge_block->id());
// Update loop descriptor.
if (ir::Loop* ploop = loop_->GetParent()) {
ploop->AddBasicBlock(loop_merge_block);
loop_desc_.SetBasicBlockToLoop(loop_merge_block->id(), ploop);
}
// Update the dominator tree.
DominatorTreeNode* loop_merge_dtn =
dom_tree->GetOrInsertNode(loop_merge_block);
DominatorTreeNode* if_merge_block_dtn =
dom_tree->GetOrInsertNode(if_merge_block);
loop_merge_dtn->parent_ = if_merge_block_dtn->parent_;
loop_merge_dtn->children_.push_back(if_merge_block_dtn);
loop_merge_dtn->parent_->children_.push_back(loop_merge_dtn);
if_merge_block_dtn->parent_->children_.erase(std::find(
if_merge_block_dtn->parent_->children_.begin(),
if_merge_block_dtn->parent_->children_.end(), if_merge_block_dtn));
loop_->SetMergeBlock(loop_merge_block);
}
////////////////////////////////////////////////////////////////////////////
// Step 2: Build a new preheader for |loop_|, use the old one
// for the constant branch.
////////////////////////////////////////////////////////////////////////////
ir::BasicBlock* if_block = loop_->GetPreHeaderBlock();
// If this preheader is the parent loop header,
// we need to create a dedicated block for the if.
ir::BasicBlock* loop_pre_header =
CreateBasicBlock(++FindBasicBlockPosition(if_block));
opt::InstructionBuilder(context_, loop_pre_header,
ir::IRContext::kAnalysisDefUse |
ir::IRContext::kAnalysisInstrToBlockMapping)
.AddBranch(loop_->GetHeaderBlock()->id());
if_block->tail()->SetInOperand(0, {loop_pre_header->id()});
// Update loop descriptor.
if (ir::Loop* ploop = loop_desc_[if_block]) {
ploop->AddBasicBlock(loop_pre_header);
loop_desc_.SetBasicBlockToLoop(loop_pre_header->id(), ploop);
}
// Update the CFG.
cfg.RegisterBlock(loop_pre_header);
def_use_mgr->AnalyzeInstDef(loop_pre_header->GetLabelInst());
cfg.AddEdge(if_block->id(), loop_pre_header->id());
cfg.RemoveNonExistingEdges(loop_->GetHeaderBlock()->id());
loop_->GetHeaderBlock()->ForEachPhiInst(
[loop_pre_header, if_block](ir::Instruction* phi) {
phi->ForEachInId([loop_pre_header, if_block](uint32_t* id) {
if (*id == if_block->id()) {
*id = loop_pre_header->id();
}
});
});
loop_->SetPreHeaderBlock(loop_pre_header);
// Update the dominator tree.
DominatorTreeNode* loop_pre_header_dtn =
dom_tree->GetOrInsertNode(loop_pre_header);
DominatorTreeNode* if_block_dtn = dom_tree->GetTreeNode(if_block);
loop_pre_header_dtn->parent_ = if_block_dtn;
assert(
if_block_dtn->children_.size() == 1 &&
"A loop preheader should only have the header block as a child in the "
"dominator tree");
loop_pre_header_dtn->children_.push_back(if_block_dtn->children_[0]);
if_block_dtn->children_.clear();
if_block_dtn->children_.push_back(loop_pre_header_dtn);
// Make domination queries valid.
dom_tree->ResetDFNumbering();
// Compute an ordered list of basic block to clone: loop blocks + pre-header
// + merge block.
loop_->ComputeLoopStructuredOrder(&ordered_loop_blocks_, true, true);
/////////////////////////////
// Do the actual unswitch: //
// - Clone the loop //
// - Connect exits //
// - Specialize the loop //
/////////////////////////////
ir::Instruction* iv_condition = &*switch_block_->tail();
SpvOp iv_opcode = iv_condition->opcode();
ir::Instruction* condition =
def_use_mgr->GetDef(iv_condition->GetOperand(0).words[0]);
analysis::ConstantManager* cst_mgr = context_->get_constant_mgr();
const analysis::Type* cond_type =
context_->get_type_mgr()->GetType(condition->type_id());
// Build the list of value for which we need to clone and specialize the
// loop.
std::vector<std::pair<ir::Instruction*, ir::BasicBlock*>> constant_branch;
// Special case for the original loop
ir::Instruction* original_loop_constant_value;
ir::BasicBlock* original_loop_target;
if (iv_opcode == SpvOpBranchConditional) {
constant_branch.emplace_back(
cst_mgr->GetDefiningInstruction(cst_mgr->GetConstant(cond_type, {0})),
nullptr);
original_loop_constant_value =
cst_mgr->GetDefiningInstruction(cst_mgr->GetConstant(cond_type, {1}));
} else {
// We are looking to take the default branch, so we can't provide a
// specific value.
original_loop_constant_value = nullptr;
for (uint32_t i = 2; i < iv_condition->NumInOperands(); i += 2) {
constant_branch.emplace_back(
cst_mgr->GetDefiningInstruction(cst_mgr->GetConstant(
cond_type, iv_condition->GetInOperand(i).words)),
nullptr);
}
}
// Get the loop landing pads.
std::unordered_set<uint32_t> if_merging_blocks;
std::function<bool(uint32_t)> is_from_original_loop;
if (loop_->GetHeaderBlock()->GetLoopMergeInst()) {
if_merging_blocks.insert(if_merge_block->id());
is_from_original_loop = [this](uint32_t id) {
return loop_->IsInsideLoop(id) || loop_->GetMergeBlock()->id() == id;
};
} else {
loop_->GetExitBlocks(&if_merging_blocks);
is_from_original_loop = [this](uint32_t id) {
return loop_->IsInsideLoop(id);
};
}
for (auto& specialisation_pair : constant_branch) {
ir::Instruction* specialisation_value = specialisation_pair.first;
//////////////////////////////////////////////////////////
// Step 3: Duplicate |loop_|.
//////////////////////////////////////////////////////////
LoopUtils::LoopCloningResult clone_result;
ir::Loop* cloned_loop =
loop_utils.CloneLoop(&clone_result, ordered_loop_blocks_);
specialisation_pair.second = cloned_loop->GetPreHeaderBlock();
////////////////////////////////////
// Step 4: Specialize the loop. //
////////////////////////////////////
{
std::unordered_set<uint32_t> dead_blocks;
std::unordered_set<uint32_t> unreachable_merges;
SimplifyLoop(
ir::make_range(
ir::UptrVectorIterator<ir::BasicBlock>(
&clone_result.cloned_bb_, clone_result.cloned_bb_.begin()),
ir::UptrVectorIterator<ir::BasicBlock>(
&clone_result.cloned_bb_, clone_result.cloned_bb_.end())),
cloned_loop, condition, specialisation_value, &dead_blocks);
// We tagged dead blocks, create the loop before we invalidate any basic
// block.
cloned_loop =
CleanLoopNest(cloned_loop, dead_blocks, &unreachable_merges);
CleanUpCFG(
ir::UptrVectorIterator<ir::BasicBlock>(
&clone_result.cloned_bb_, clone_result.cloned_bb_.begin()),
dead_blocks, unreachable_merges);
///////////////////////////////////////////////////////////
// Step 5: Connect convergent edges to the landing pads. //
///////////////////////////////////////////////////////////
for (uint32_t merge_bb_id : if_merging_blocks) {
ir::BasicBlock* merge = context_->cfg()->block(merge_bb_id);
// We are in LCSSA so we only care about phi instructions.
merge->ForEachPhiInst([is_from_original_loop, &dead_blocks,
&clone_result](ir::Instruction* phi) {
uint32_t num_in_operands = phi->NumInOperands();
for (uint32_t i = 0; i < num_in_operands; i += 2) {
uint32_t pred = phi->GetSingleWordInOperand(i + 1);
if (is_from_original_loop(pred)) {
pred = clone_result.value_map_.at(pred);
if (!dead_blocks.count(pred)) {
uint32_t incoming_value_id = phi->GetSingleWordInOperand(i);
// Not all the incoming value are coming from the loop.
ValueMapTy::iterator new_value =
clone_result.value_map_.find(incoming_value_id);
if (new_value != clone_result.value_map_.end()) {
incoming_value_id = new_value->second;
}
phi->AddOperand({SPV_OPERAND_TYPE_ID, {incoming_value_id}});
phi->AddOperand({SPV_OPERAND_TYPE_ID, {pred}});
}
}
}
});
}
}
function_->AddBasicBlocks(clone_result.cloned_bb_.begin(),
clone_result.cloned_bb_.end(),
++FindBasicBlockPosition(if_block));
}
// Same as above but specialize the existing loop
{
std::unordered_set<uint32_t> dead_blocks;
std::unordered_set<uint32_t> unreachable_merges;
SimplifyLoop(ir::make_range(function_->begin(), function_->end()), loop_,
condition, original_loop_constant_value, &dead_blocks);
for (uint32_t merge_bb_id : if_merging_blocks) {
ir::BasicBlock* merge = context_->cfg()->block(merge_bb_id);
// LCSSA, so we only care about phi instructions.
// If we the phi is reduced to a single incoming branch, do not
// propagate it to preserve LCSSA.
PatchPhis(merge, dead_blocks, true);
}
if (if_merge_block) {
bool has_live_pred = false;
for (uint32_t pid : cfg.preds(if_merge_block->id())) {
if (!dead_blocks.count(pid)) {
has_live_pred = true;
break;
}
}
if (!has_live_pred) unreachable_merges.insert(if_merge_block->id());
}
original_loop_target = loop_->GetPreHeaderBlock();
// We tagged dead blocks, prune the loop descriptor from any dead loops.
// After this call, |loop_| can be nullptr (i.e. the unswitch killed this
// loop).
loop_ = CleanLoopNest(loop_, dead_blocks, &unreachable_merges);
CleanUpCFG(function_->begin(), dead_blocks, unreachable_merges);
}
/////////////////////////////////////
// Finally: connect the new loops. //
/////////////////////////////////////
// Delete the old jump
context_->KillInst(&*if_block->tail());
opt::InstructionBuilder builder(context_, if_block);
if (iv_opcode == SpvOpBranchConditional) {
assert(constant_branch.size() == 1);
builder.AddConditionalBranch(
condition->result_id(), original_loop_target->id(),
constant_branch[0].second->id(),
if_merge_block ? if_merge_block->id() : kInvalidId);
} else {
std::vector<std::pair<std::vector<uint32_t>, uint32_t>> targets;
for (auto& t : constant_branch) {
targets.emplace_back(t.first->GetInOperand(0).words, t.second->id());
}
builder.AddSwitch(condition->result_id(), original_loop_target->id(),
targets,
if_merge_block ? if_merge_block->id() : kInvalidId);
}
switch_block_ = nullptr;
ordered_loop_blocks_.clear();
context_->InvalidateAnalysesExceptFor(
ir::IRContext::Analysis::kAnalysisLoopAnalysis);
}
// Returns true if the unswitch killed the original |loop_|.
bool WasLoopKilled() const { return loop_ == nullptr; }
private:
using ValueMapTy = std::unordered_map<uint32_t, uint32_t>;
using BlockMapTy = std::unordered_map<uint32_t, ir::BasicBlock*>;
ir::Function* function_;
ir::Loop* loop_;
ir::LoopDescriptor& loop_desc_;
ir::IRContext* context_;
ir::BasicBlock* switch_block_;
// Map between instructions and if they are dynamically uniform.
std::unordered_map<uint32_t, bool> dynamically_uniform_;
// The loop basic blocks in structured order.
std::vector<ir::BasicBlock*> ordered_loop_blocks_;
// Returns the next usable id for the context.
uint32_t TakeNextId() { return context_->TakeNextId(); }
// Patches |bb|'s phi instruction by removing incoming value from unexisting
// or tagged as dead branches.
void PatchPhis(ir::BasicBlock* bb,
const std::unordered_set<uint32_t>& dead_blocks,
bool preserve_phi) {
ir::CFG& cfg = *context_->cfg();
std::vector<ir::Instruction*> phi_to_kill;
const std::vector<uint32_t>& bb_preds = cfg.preds(bb->id());
auto is_branch_dead = [&bb_preds, &dead_blocks](uint32_t id) {
return dead_blocks.count(id) ||
std::find(bb_preds.begin(), bb_preds.end(), id) == bb_preds.end();
};
bb->ForEachPhiInst([&phi_to_kill, &is_branch_dead, preserve_phi,
this](ir::Instruction* insn) {
uint32_t i = 0;
while (i < insn->NumInOperands()) {
uint32_t incoming_id = insn->GetSingleWordInOperand(i + 1);
if (is_branch_dead(incoming_id)) {
// Remove the incoming block id operand.
insn->RemoveInOperand(i + 1);
// Remove the definition id operand.
insn->RemoveInOperand(i);
continue;
}
i += 2;
}
// If there is only 1 remaining edge, propagate the value and
// kill the instruction.
if (insn->NumInOperands() == 2 && !preserve_phi) {
phi_to_kill.push_back(insn);
context_->ReplaceAllUsesWith(insn->result_id(),
insn->GetSingleWordInOperand(0));
}
});
for (ir::Instruction* insn : phi_to_kill) {
context_->KillInst(insn);
}
}
// Removes any block that is tagged as dead, if the block is in
// |unreachable_merges| then all block's instructions are replaced by a
// OpUnreachable.
void CleanUpCFG(ir::UptrVectorIterator<ir::BasicBlock> bb_it,
const std::unordered_set<uint32_t>& dead_blocks,
const std::unordered_set<uint32_t>& unreachable_merges) {
ir::CFG& cfg = *context_->cfg();
while (bb_it != bb_it.End()) {
ir::BasicBlock& bb = *bb_it;
if (unreachable_merges.count(bb.id())) {
if (bb.begin() != bb.tail() ||
bb.terminator()->opcode() != SpvOpUnreachable) {
// Make unreachable, but leave the label.
bb.KillAllInsts(false);
opt::InstructionBuilder(context_, &bb).AddUnreachable();
cfg.RemoveNonExistingEdges(bb.id());
}
++bb_it;
} else if (dead_blocks.count(bb.id())) {
cfg.ForgetBlock(&bb);
// Kill this block.
bb.KillAllInsts(true);
bb_it = bb_it.Erase();
} else {
cfg.RemoveNonExistingEdges(bb.id());
++bb_it;
}
}
}
// Return true if |c_inst| is a Boolean constant and set |cond_val| with the
// value that |c_inst|
bool GetConstCondition(const ir::Instruction* c_inst, bool* cond_val) {
bool cond_is_const;
switch (c_inst->opcode()) {
case SpvOpConstantFalse: {
*cond_val = false;
cond_is_const = true;
} break;
case SpvOpConstantTrue: {
*cond_val = true;
cond_is_const = true;
} break;
default: { cond_is_const = false; } break;
}
return cond_is_const;
}
// Simplifies |loop| assuming the instruction |to_version_insn| takes the
// value |cst_value|. |block_range| is an iterator range returning the loop
// basic blocks in a structured order (dominator first).
// The function will ignore basic blocks returned by |block_range| if they
// does not belong to the loop.
// The set |dead_blocks| will contain all the dead basic blocks.
//
// Requirements:
// - |loop| must be in the LCSSA form;
// - |cst_value| must be constant or null (to represent the default target
// of an OpSwitch).
void SimplifyLoop(
ir::IteratorRange<ir::UptrVectorIterator<ir::BasicBlock>> block_range,
ir::Loop* loop, ir::Instruction* to_version_insn,
ir::Instruction* cst_value, std::unordered_set<uint32_t>* dead_blocks) {
ir::CFG& cfg = *context_->cfg();
analysis::DefUseManager* def_use_mgr = context_->get_def_use_mgr();
std::function<bool(uint32_t)> ignore_node;
ignore_node = [loop](uint32_t bb_id) { return !loop->IsInsideLoop(bb_id); };
std::vector<std::pair<ir::Instruction*, uint32_t>> use_list;
def_use_mgr->ForEachUse(
to_version_insn, [&use_list, &ignore_node, this](
ir::Instruction* inst, uint32_t operand_index) {
ir::BasicBlock* bb = context_->get_instr_block(inst);
if (!bb || ignore_node(bb->id())) {
// Out of the loop, the specialization does not apply any more.
return;
}
use_list.emplace_back(inst, operand_index);
});
// First pass: inject the specialized value into the loop (and only the
// loop).
for (auto use : use_list) {
ir::Instruction* inst = use.first;
uint32_t operand_index = use.second;
ir::BasicBlock* bb = context_->get_instr_block(inst);
// If it is not a branch, simply inject the value.
if (!inst->IsBranch()) {
// To also handle switch, cst_value can be nullptr: this case
// means that we are looking to branch to the default target of
// the switch. We don't actually know its value so we don't touch
// it if it not a switch.
if (cst_value) {
inst->SetOperand(operand_index, {cst_value->result_id()});
def_use_mgr->AnalyzeInstUse(inst);
}
}
// The user is a branch, kill dead branches.
uint32_t live_target = 0;
std::unordered_set<uint32_t> dead_branches;
switch (inst->opcode()) {
case SpvOpBranchConditional: {
assert(cst_value && "No constant value to specialize !");
bool branch_cond = false;
if (GetConstCondition(cst_value, &branch_cond)) {
uint32_t true_label =
inst->GetSingleWordInOperand(kBranchCondTrueLabIdInIdx);
uint32_t false_label =
inst->GetSingleWordInOperand(kBranchCondFalseLabIdInIdx);
live_target = branch_cond ? true_label : false_label;
uint32_t dead_target = !branch_cond ? true_label : false_label;
cfg.RemoveEdge(bb->id(), dead_target);
}
break;
}
case SpvOpSwitch: {
live_target = inst->GetSingleWordInOperand(1);
if (cst_value) {
if (!cst_value->IsConstant()) break;
const ir::Operand& cst = cst_value->GetInOperand(0);
for (uint32_t i = 2; i < inst->NumInOperands(); i += 2) {
const ir::Operand& literal = inst->GetInOperand(i);
if (literal == cst) {
live_target = inst->GetSingleWordInOperand(i + 1);
break;
}
}
}
for (uint32_t i = 1; i < inst->NumInOperands(); i += 2) {
uint32_t id = inst->GetSingleWordInOperand(i);
if (id != live_target) {
cfg.RemoveEdge(bb->id(), id);
}
}
}
default:
break;
}
if (live_target != 0) {
// Check for the presence of the merge block.
if (ir::Instruction* merge = bb->GetMergeInst())
context_->KillInst(merge);
context_->KillInst(&*bb->tail());
opt::InstructionBuilder builder(
context_, bb,
ir::IRContext::kAnalysisDefUse |
ir::IRContext::kAnalysisInstrToBlockMapping);
builder.AddBranch(live_target);
}
}
// Go through the loop basic block and tag all blocks that are obviously
// dead.
std::unordered_set<uint32_t> visited;
for (ir::BasicBlock& bb : block_range) {
if (ignore_node(bb.id())) continue;
visited.insert(bb.id());
// Check if this block is dead, if so tag it as dead otherwise patch phi
// instructions.
bool has_live_pred = false;
for (uint32_t pid : cfg.preds(bb.id())) {
if (!dead_blocks->count(pid)) {
has_live_pred = true;
break;
}
}
if (!has_live_pred) {
dead_blocks->insert(bb.id());
const ir::BasicBlock& cbb = bb;
// Patch the phis for any back-edge.
cbb.ForEachSuccessorLabel(
[dead_blocks, &visited, &cfg, this](uint32_t id) {
if (!visited.count(id) || dead_blocks->count(id)) return;
ir::BasicBlock* succ = cfg.block(id);
PatchPhis(succ, *dead_blocks, false);
});
continue;
}
// Update the phi instructions, some incoming branch have/will disappear.
PatchPhis(&bb, *dead_blocks, /* preserve_phi = */ false);
}
}
// Returns true if the header is not reachable or tagged as dead or if we
// never loop back.
bool IsLoopDead(ir::BasicBlock* header, ir::BasicBlock* latch,
const std::unordered_set<uint32_t>& dead_blocks) {
if (!header || dead_blocks.count(header->id())) return true;
if (!latch || dead_blocks.count(latch->id())) return true;
for (uint32_t pid : context_->cfg()->preds(header->id())) {
if (!dead_blocks.count(pid)) {
// Seems reachable.
return false;
}
}
return true;
}
// Cleans the loop nest under |loop| and reflect changes to the loop
// descriptor. This will kill all descriptors that represent dead loops.
// If |loop_| is killed, it will be set to nullptr.
// Any merge blocks that become unreachable will be added to
// |unreachable_merges|.
// The function returns the pointer to |loop| or nullptr if the loop was
// killed.
ir::Loop* CleanLoopNest(ir::Loop* loop,
const std::unordered_set<uint32_t>& dead_blocks,
std::unordered_set<uint32_t>* unreachable_merges) {
// This represent the pair of dead loop and nearest alive parent (nullptr if
// no parent).
std::unordered_map<ir::Loop*, ir::Loop*> dead_loops;
auto get_parent = [&dead_loops](ir::Loop* l) -> ir::Loop* {
std::unordered_map<ir::Loop*, ir::Loop*>::iterator it =
dead_loops.find(l);
if (it != dead_loops.end()) return it->second;
return nullptr;
};
bool is_main_loop_dead =
IsLoopDead(loop->GetHeaderBlock(), loop->GetLatchBlock(), dead_blocks);
if (is_main_loop_dead) {
if (ir::Instruction* merge = loop->GetHeaderBlock()->GetLoopMergeInst()) {
context_->KillInst(merge);
}
dead_loops[loop] = loop->GetParent();
} else
dead_loops[loop] = loop;
// For each loop, check if we killed it. If we did, find a suitable parent
// for its children.
for (ir::Loop& sub_loop :
ir::make_range(++opt::TreeDFIterator<ir::Loop>(loop),
opt::TreeDFIterator<ir::Loop>())) {
if (IsLoopDead(sub_loop.GetHeaderBlock(), sub_loop.GetLatchBlock(),
dead_blocks)) {
if (ir::Instruction* merge =
sub_loop.GetHeaderBlock()->GetLoopMergeInst()) {
context_->KillInst(merge);
}
dead_loops[&sub_loop] = get_parent(&sub_loop);
} else {
// The loop is alive, check if its merge block is dead, if it is, tag it
// as required.
if (sub_loop.GetMergeBlock()) {
uint32_t merge_id = sub_loop.GetMergeBlock()->id();
if (dead_blocks.count(merge_id)) {
unreachable_merges->insert(sub_loop.GetMergeBlock()->id());
}
}
}
}
if (!is_main_loop_dead) dead_loops.erase(loop);
// Remove dead blocks from live loops.
for (uint32_t bb_id : dead_blocks) {
ir::Loop* l = loop_desc_[bb_id];
if (l) {
l->RemoveBasicBlock(bb_id);
loop_desc_.ForgetBasicBlock(bb_id);
}
}
std::for_each(
dead_loops.begin(), dead_loops.end(),
[&loop, this](
std::unordered_map<ir::Loop*, ir::Loop*>::iterator::reference it) {
if (it.first == loop) loop = nullptr;
loop_desc_.RemoveLoop(it.first);
});
return loop;
}
// Returns true if |var| is dynamically uniform.
// Note: this is currently approximated as uniform.
bool IsDynamicallyUniform(ir::Instruction* var, const ir::BasicBlock* entry,
const DominatorTree& post_dom_tree) {
assert(post_dom_tree.IsPostDominator());
analysis::DefUseManager* def_use_mgr = context_->get_def_use_mgr();
auto it = dynamically_uniform_.find(var->result_id());
if (it != dynamically_uniform_.end()) return it->second;
analysis::DecorationManager* dec_mgr = context_->get_decoration_mgr();
bool& is_uniform = dynamically_uniform_[var->result_id()];
is_uniform = false;
dec_mgr->WhileEachDecoration(var->result_id(), SpvDecorationUniform,
[&is_uniform](const ir::Instruction&) {
is_uniform = true;
return false;
});
if (is_uniform) {
return is_uniform;
}
ir::BasicBlock* parent = context_->get_instr_block(var);
if (!parent) {
return is_uniform = true;
}
if (!post_dom_tree.Dominates(parent->id(), entry->id())) {
return is_uniform = false;
}
if (var->opcode() == SpvOpLoad) {
const uint32_t PtrTypeId =
def_use_mgr->GetDef(var->GetSingleWordInOperand(0))->type_id();
const ir::Instruction* PtrTypeInst = def_use_mgr->GetDef(PtrTypeId);
uint32_t storage_class =
PtrTypeInst->GetSingleWordInOperand(kTypePointerStorageClassInIdx);
if (storage_class != SpvStorageClassUniform &&
storage_class != SpvStorageClassUniformConstant) {
return is_uniform = false;
}
} else {
if (!context_->IsCombinatorInstruction(var)) {
return is_uniform = false;
}
}
return is_uniform = var->WhileEachInId([entry, &post_dom_tree,
this](const uint32_t* id) {
return IsDynamicallyUniform(context_->get_def_use_mgr()->GetDef(*id),
entry, post_dom_tree);
});
}
// Returns true if |insn| is constant and dynamically uniform within the loop.
bool IsConditionLoopInvariant(ir::Instruction* insn) {
assert(insn->IsBranch());
assert(insn->opcode() != SpvOpBranch);
analysis::DefUseManager* def_use_mgr = context_->get_def_use_mgr();
ir::Instruction* condition =
def_use_mgr->GetDef(insn->GetOperand(0).words[0]);
return !loop_->IsInsideLoop(condition) &&
IsDynamicallyUniform(
condition, function_->entry().get(),
context_->GetPostDominatorAnalysis(function_, *context_->cfg())
->GetDomTree());
}
};
} // namespace
Pass::Status LoopUnswitchPass::Process(ir::IRContext* c) {
InitializeProcessing(c);
bool modified = false;
ir::Module* module = c->module();
// Process each function in the module
for (ir::Function& f : *module) {
modified |= ProcessFunction(&f);
}
return modified ? Status::SuccessWithChange : Status::SuccessWithoutChange;
}
bool LoopUnswitchPass::ProcessFunction(ir::Function* f) {
bool modified = false;
std::unordered_set<ir::Loop*> processed_loop;
ir::LoopDescriptor& loop_descriptor = *context()->GetLoopDescriptor(f);
bool loop_changed = true;
while (loop_changed) {
loop_changed = false;
for (ir::Loop& loop :
ir::make_range(++opt::TreeDFIterator<ir::Loop>(
loop_descriptor.GetDummyRootLoop()),
opt::TreeDFIterator<ir::Loop>())) {
if (processed_loop.count(&loop)) continue;
processed_loop.insert(&loop);
LoopUnswitch unswitcher(context(), f, &loop, &loop_descriptor);
while (!unswitcher.WasLoopKilled() && unswitcher.CanUnswitchLoop()) {
if (!loop.IsLCSSA()) {
LoopUtils(context(), &loop).MakeLoopClosedSSA();
}
modified = true;
loop_changed = true;
unswitcher.PerformUnswitch();
}
if (loop_changed) break;
}
}
return modified;
}
} // namespace opt
} // namespace spvtools
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