SPIRV-Tools/source/opt/dead_branch_elim_pass.cpp

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// Copyright (c) 2017 The Khronos Group Inc.
// Copyright (c) 2017 Valve Corporation
// Copyright (c) 2017 LunarG Inc.
// Copyright (c) 2018 Google Inc.
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//
// 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 "dead_branch_elim_pass.h"
#include "cfa.h"
#include "ir_context.h"
#include "iterator.h"
#include "make_unique.h"
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namespace spvtools {
namespace opt {
namespace {
const uint32_t kBranchCondTrueLabIdInIdx = 1;
const uint32_t kBranchCondFalseLabIdInIdx = 2;
} // anonymous namespace
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bool DeadBranchElimPass::GetConstCondition(uint32_t condId, bool* condVal) {
bool condIsConst;
Instruction* cInst = get_def_use_mgr()->GetDef(condId);
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switch (cInst->opcode()) {
case SpvOpConstantFalse: {
*condVal = false;
condIsConst = true;
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} break;
case SpvOpConstantTrue: {
*condVal = true;
condIsConst = true;
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} break;
case SpvOpLogicalNot: {
bool negVal;
condIsConst =
GetConstCondition(cInst->GetSingleWordInOperand(0), &negVal);
if (condIsConst) *condVal = !negVal;
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} break;
default: { condIsConst = false; } break;
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}
return condIsConst;
}
bool DeadBranchElimPass::GetConstInteger(uint32_t selId, uint32_t* selVal) {
Instruction* sInst = get_def_use_mgr()->GetDef(selId);
uint32_t typeId = sInst->type_id();
Instruction* typeInst = get_def_use_mgr()->GetDef(typeId);
if (!typeInst || (typeInst->opcode() != SpvOpTypeInt)) return false;
// TODO(greg-lunarg): Support non-32 bit ints
if (typeInst->GetSingleWordInOperand(0) != 32) return false;
if (sInst->opcode() == SpvOpConstant) {
*selVal = sInst->GetSingleWordInOperand(0);
return true;
} else if (sInst->opcode() == SpvOpConstantNull) {
*selVal = 0;
return true;
}
return false;
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}
void DeadBranchElimPass::AddBranch(uint32_t labelId, BasicBlock* bp) {
assert(get_def_use_mgr()->GetDef(labelId) != nullptr);
std::unique_ptr<Instruction> newBranch(
new Instruction(context(), SpvOpBranch, 0, 0,
{{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {labelId}}}));
context()->AnalyzeDefUse(&*newBranch);
context()->set_instr_block(&*newBranch, bp);
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bp->AddInstruction(std::move(newBranch));
}
BasicBlock* DeadBranchElimPass::GetParentBlock(uint32_t id) {
return context()->get_instr_block(get_def_use_mgr()->GetDef(id));
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}
bool DeadBranchElimPass::MarkLiveBlocks(
Function* func, std::unordered_set<BasicBlock*>* live_blocks) {
std::unordered_set<BasicBlock*> continues;
std::vector<BasicBlock*> stack;
stack.push_back(&*func->begin());
bool modified = false;
while (!stack.empty()) {
BasicBlock* block = stack.back();
stack.pop_back();
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// Live blocks doubles as visited set.
if (!live_blocks->insert(block).second) continue;
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uint32_t cont_id = block->ContinueBlockIdIfAny();
if (cont_id != 0) continues.insert(GetParentBlock(cont_id));
Instruction* terminator = block->terminator();
uint32_t live_lab_id = 0;
// Check if the terminator has a single valid successor.
if (terminator->opcode() == SpvOpBranchConditional) {
bool condVal;
if (GetConstCondition(terminator->GetSingleWordInOperand(0u), &condVal)) {
live_lab_id = terminator->GetSingleWordInOperand(
condVal ? kBranchCondTrueLabIdInIdx : kBranchCondFalseLabIdInIdx);
}
} else if (terminator->opcode() == SpvOpSwitch) {
uint32_t sel_val;
if (GetConstInteger(terminator->GetSingleWordInOperand(0u), &sel_val)) {
// Search switch operands for selector value, set live_lab_id to
// corresponding label, use default if not found.
uint32_t icnt = 0;
uint32_t case_val;
terminator->WhileEachInOperand(
[&icnt, &case_val, &sel_val, &live_lab_id](const uint32_t* idp) {
if (icnt == 1) {
// Start with default label.
live_lab_id = *idp;
} else if (icnt > 1) {
if (icnt % 2 == 0) {
case_val = *idp;
} else {
if (case_val == sel_val) {
live_lab_id = *idp;
return false;
}
}
}
++icnt;
return true;
});
}
}
// Don't simplify branches of continue blocks. A path from the continue to
// the header is required.
// TODO(alan-baker): They can be simplified iff there remains a path to the
// backedge. Structured control flow should guarantee one path hits the
// backedge, but I've removed the requirement for structured control flow
// from this pass.
bool simplify = live_lab_id != 0 && !continues.count(block);
if (simplify) {
modified = true;
// Replace with unconditional branch.
// Remove the merge instruction if it is a selection merge.
AddBranch(live_lab_id, block);
context()->KillInst(terminator);
Instruction* mergeInst = block->GetMergeInst();
if (mergeInst && mergeInst->opcode() == SpvOpSelectionMerge) {
context()->KillInst(mergeInst);
}
stack.push_back(GetParentBlock(live_lab_id));
} else {
// All successors are live.
const auto* const_block = block;
const_block->ForEachSuccessorLabel([&stack, this](const uint32_t label) {
stack.push_back(GetParentBlock(label));
Adding new def -> use mapping container Replaced representation of uses * Changed uses from unordered_map<uint32_t, UseList> to set<pairInstruction*, Instruction*>> * Replaced GetUses with ForEachUser and ForEachUse functions * updated passes to use new functions * partially updated tests * lots of cleanup still todo Adding an unique id to Instruction generated by IRContext Each instruction is given an unique id that can be used for ordering purposes. The ids are generated via the IRContext. Major changes: * Instructions now contain a uint32_t for unique id and a cached context pointer * Most constructors have been modified to take a context as input * unfortunately I cannot remove the default and copy constructors, but developers should avoid these * Added accessors to parents of basic block and function * Removed the copy constructors for BasicBlock and Function and replaced them with Clone functions * Reworked BuildModule to return an IRContext owning the built module * Since all instructions require a context, the context now becomes the basic unit for IR * Added a constructor to context to create an owned module internally * Replaced uses of Instruction's copy constructor with Clone whereever I found them * Reworked the linker functionality to perform clones into a different context instead of moves * Updated many tests to be consistent with the above changes * Still need to add new tests to cover added functionality * Added comparison operators to Instruction Adding tests for Instruction, IRContext and IR loading Fixed some header comments for BuildModule Fixes to get tests passing again * Reordered two linker steps to avoid use/def problems * Fixed def/use manager uses in merge return pass * Added early return for GetAnnotations * Changed uses of Instruction::ToNop in passes to IRContext::KillInst Simplifying the uses for some contexts in passes
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});
}
}
return modified;
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}
void DeadBranchElimPass::MarkUnreachableStructuredTargets(
const std::unordered_set<BasicBlock*>& live_blocks,
std::unordered_set<BasicBlock*>* unreachable_merges,
std::unordered_map<BasicBlock*, BasicBlock*>* unreachable_continues) {
for (auto block : live_blocks) {
if (auto merge_id = block->MergeBlockIdIfAny()) {
BasicBlock* merge_block = GetParentBlock(merge_id);
if (!live_blocks.count(merge_block)) {
unreachable_merges->insert(merge_block);
}
if (auto cont_id = block->ContinueBlockIdIfAny()) {
BasicBlock* cont_block = GetParentBlock(cont_id);
if (!live_blocks.count(cont_block)) {
(*unreachable_continues)[cont_block] = block;
}
}
}
}
}
bool DeadBranchElimPass::FixPhiNodesInLiveBlocks(
Function* func, const std::unordered_set<BasicBlock*>& live_blocks,
const std::unordered_map<BasicBlock*, BasicBlock*>& unreachable_continues) {
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bool modified = false;
for (auto& block : *func) {
if (live_blocks.count(&block)) {
for (auto iter = block.begin(); iter != block.end();) {
if (iter->opcode() != SpvOpPhi) {
break;
}
bool changed = false;
bool backedge_added = false;
Instruction* inst = &*iter;
std::vector<Operand> operands;
// Build a complete set of operands (not just input operands). Start
// with type and result id operands.
operands.push_back(inst->GetOperand(0u));
operands.push_back(inst->GetOperand(1u));
// Iterate through the incoming labels and determine which to keep
// and/or modify. If there in an unreachable continue block, there will
// be an edge from that block to the header. We need to keep it to
// maintain the structured control flow. If the header has more that 2
// incoming edges, then the OpPhi must have an entry for that edge.
// However, if there is only one other incoming edge, the OpPhi can be
// eliminated.
for (uint32_t i = 1; i < inst->NumInOperands(); i += 2) {
BasicBlock* inc = GetParentBlock(inst->GetSingleWordInOperand(i));
auto cont_iter = unreachable_continues.find(inc);
if (cont_iter != unreachable_continues.end() &&
cont_iter->second == &block && inst->NumInOperands() > 4) {
if (get_def_use_mgr()
->GetDef(inst->GetSingleWordInOperand(i - 1))
->opcode() == SpvOpUndef) {
// Already undef incoming value, no change necessary.
operands.push_back(inst->GetInOperand(i - 1));
operands.push_back(inst->GetInOperand(i));
backedge_added = true;
} else {
// Replace incoming value with undef if this phi exists in the
// loop header. Otherwise, this edge is not live since the
// unreachable continue block will be replaced with an
// unconditional branch to the header only.
operands.emplace_back(
SPV_OPERAND_TYPE_ID,
std::initializer_list<uint32_t>{Type2Undef(inst->type_id())});
operands.push_back(inst->GetInOperand(i));
changed = true;
backedge_added = true;
}
} else if (live_blocks.count(inc) && inc->IsSuccessor(&block)) {
// Keep live incoming edge.
operands.push_back(inst->GetInOperand(i - 1));
operands.push_back(inst->GetInOperand(i));
} else {
// Remove incoming edge.
changed = true;
}
}
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if (changed) {
modified = true;
uint32_t continue_id = block.ContinueBlockIdIfAny();
if (!backedge_added && continue_id != 0 &&
unreachable_continues.count(GetParentBlock(continue_id)) &&
operands.size() > 4) {
// Changed the backedge to branch from the continue block instead
// of a successor of the continue block. Add an entry to the phi to
// provide an undef for the continue block. Since the successor of
// the continue must also be unreachable (dominated by the continue
// block), any entry for the original backedge has been removed
// from the phi operands.
operands.emplace_back(
SPV_OPERAND_TYPE_ID,
std::initializer_list<uint32_t>{Type2Undef(inst->type_id())});
operands.emplace_back(SPV_OPERAND_TYPE_ID,
std::initializer_list<uint32_t>{continue_id});
}
// Either replace the phi with a single value or rebuild the phi out
// of |operands|.
//
// We always have type and result id operands. So this phi has a
// single source if there are two more operands beyond those.
if (operands.size() == 4) {
// First input data operands is at index 2.
uint32_t replId = operands[2u].words[0];
context()->ReplaceAllUsesWith(inst->result_id(), replId);
iter = context()->KillInst(&*inst);
} else {
// We've rewritten the operands, so first instruct the def/use
// manager to forget uses in the phi before we replace them. After
// replacing operands update the def/use manager by re-analyzing
// the used ids in this phi.
get_def_use_mgr()->EraseUseRecordsOfOperandIds(inst);
inst->ReplaceOperands(operands);
get_def_use_mgr()->AnalyzeInstUse(inst);
++iter;
}
} else {
++iter;
}
}
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}
}
return modified;
}
bool DeadBranchElimPass::EraseDeadBlocks(
Function* func, const std::unordered_set<BasicBlock*>& live_blocks,
const std::unordered_set<BasicBlock*>& unreachable_merges,
const std::unordered_map<BasicBlock*, BasicBlock*>& unreachable_continues) {
bool modified = false;
for (auto ebi = func->begin(); ebi != func->end();) {
if (unreachable_merges.count(&*ebi)) {
if (ebi->begin() != ebi->tail() ||
ebi->terminator()->opcode() != SpvOpUnreachable) {
// Make unreachable, but leave the label.
KillAllInsts(&*ebi, false);
// Add unreachable terminator.
ebi->AddInstruction(
MakeUnique<Instruction>(context(), SpvOpUnreachable, 0, 0,
std::initializer_list<Operand>{}));
context()->set_instr_block(&*ebi->tail(), &*ebi);
modified = true;
}
++ebi;
} else if (unreachable_continues.count(&*ebi)) {
uint32_t cont_id = unreachable_continues.find(&*ebi)->second->id();
if (ebi->begin() != ebi->tail() ||
ebi->terminator()->opcode() != SpvOpBranch ||
ebi->terminator()->GetSingleWordInOperand(0u) != cont_id) {
// Make unreachable, but leave the label.
KillAllInsts(&*ebi, false);
// Add unconditional branch to header.
assert(unreachable_continues.count(&*ebi));
ebi->AddInstruction(MakeUnique<Instruction>(
context(), SpvOpBranch, 0, 0,
std::initializer_list<Operand>{{SPV_OPERAND_TYPE_ID, {cont_id}}}));
get_def_use_mgr()->AnalyzeInstUse(&*ebi->tail());
context()->set_instr_block(&*ebi->tail(), &*ebi);
modified = true;
}
++ebi;
} else if (!live_blocks.count(&*ebi)) {
// Kill this block.
KillAllInsts(&*ebi);
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ebi = ebi.Erase();
modified = true;
} else {
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++ebi;
}
}
return modified;
}
bool DeadBranchElimPass::EliminateDeadBranches(Function* func) {
bool modified = false;
std::unordered_set<BasicBlock*> live_blocks;
modified |= MarkLiveBlocks(func, &live_blocks);
std::unordered_set<BasicBlock*> unreachable_merges;
std::unordered_map<BasicBlock*, BasicBlock*> unreachable_continues;
MarkUnreachableStructuredTargets(live_blocks, &unreachable_merges,
&unreachable_continues);
modified |= FixPhiNodesInLiveBlocks(func, live_blocks, unreachable_continues);
modified |= EraseDeadBlocks(func, live_blocks, unreachable_merges,
unreachable_continues);
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return modified;
}
void DeadBranchElimPass::FixBlockOrder() {
context()->BuildInvalidAnalyses(IRContext::kAnalysisCFG |
IRContext::kAnalysisDominatorAnalysis);
// Reorders blocks according to DFS of dominator tree.
ProcessFunction reorder_dominators = [this](Function* function) {
DominatorAnalysis* dominators = context()->GetDominatorAnalysis(function);
std::vector<BasicBlock*> blocks;
for (auto iter = dominators->GetDomTree().begin();
iter != dominators->GetDomTree().end(); ++iter) {
if (iter->id() != 0) {
blocks.push_back(iter->bb_);
}
}
for (uint32_t i = 1; i < blocks.size(); ++i) {
function->MoveBasicBlockToAfter(blocks[i]->id(), blocks[i - 1]);
}
return true;
};
// Reorders blocks according to structured order.
ProcessFunction reorder_structured = [this](Function* function) {
std::list<BasicBlock*> order;
context()->cfg()->ComputeStructuredOrder(function, &*function->begin(),
&order);
std::vector<BasicBlock*> blocks;
for (auto block : order) {
blocks.push_back(block);
}
for (uint32_t i = 1; i < blocks.size(); ++i) {
function->MoveBasicBlockToAfter(blocks[i]->id(), blocks[i - 1]);
}
return true;
};
// Structured order is more intuitive so use it where possible.
if (context()->get_feature_mgr()->HasCapability(SpvCapabilityShader)) {
ProcessReachableCallTree(reorder_structured, context());
} else {
ProcessReachableCallTree(reorder_dominators, context());
}
}
Pass::Status DeadBranchElimPass::Process() {
// Do not process if module contains OpGroupDecorate. Additional
// support required in KillNamesAndDecorates().
// TODO(greg-lunarg): Add support for OpGroupDecorate
for (auto& ai : get_module()->annotations())
if (ai.opcode() == SpvOpGroupDecorate) return Status::SuccessWithoutChange;
// Process all entry point functions
ProcessFunction pfn = [this](Function* fp) {
return EliminateDeadBranches(fp);
};
bool modified = ProcessReachableCallTree(pfn, context());
if (modified) FixBlockOrder();
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return modified ? Status::SuccessWithChange : Status::SuccessWithoutChange;
}
} // namespace opt
} // namespace spvtools