SPIRV-Tools/source/opt/dead_branch_elim_pass.cpp
GregF 7c3de19ce7 DeadBranchElim: Fix dead block detection to ignore backedges
- DeadBranchElim: Make sure to mark orphan'd merge blocks and continue
targets as live.
- Add test with loop in dead branch
- Add test that orphan'd merge block is handled.

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/776
2017-08-30 13:37:46 -04:00

392 lines
13 KiB
C++

// Copyright (c) 2017 The Khronos Group Inc.
// Copyright (c) 2017 Valve Corporation
// Copyright (c) 2017 LunarG Inc.
//
// 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 "iterator.h"
namespace spvtools {
namespace opt {
namespace {
const uint32_t kBranchCondConditionalIdInIdx = 0;
const uint32_t kBranchCondTrueLabIdInIdx = 1;
const uint32_t kBranchCondFalseLabIdInIdx = 2;
const uint32_t kSelectionMergeMergeBlockIdInIdx = 0;
const uint32_t kPhiVal0IdInIdx = 0;
const uint32_t kPhiLab0IdInIdx = 1;
const uint32_t kPhiVal1IdInIdx = 2;
const uint32_t kLoopMergeMergeBlockIdInIdx = 0;
const uint32_t kLoopMergeContinueBlockIdInIdx = 1;
} // anonymous namespace
uint32_t DeadBranchElimPass::MergeBlockIdIfAny(
const ir::BasicBlock& blk, uint32_t* cbid) const {
auto merge_ii = blk.cend();
--merge_ii;
uint32_t mbid = 0;
*cbid = 0;
if (merge_ii != blk.cbegin()) {
--merge_ii;
if (merge_ii->opcode() == SpvOpLoopMerge) {
mbid = merge_ii->GetSingleWordInOperand(kLoopMergeMergeBlockIdInIdx);
*cbid = merge_ii->GetSingleWordInOperand(kLoopMergeContinueBlockIdInIdx);
}
else if (merge_ii->opcode() == SpvOpSelectionMerge) {
mbid = merge_ii->GetSingleWordInOperand(
kSelectionMergeMergeBlockIdInIdx);
}
}
return mbid;
}
void DeadBranchElimPass::ComputeStructuredSuccessors(ir::Function* func) {
// If header, make merge block first successor. If a loop header, make
// the second successor the continue target.
for (auto& blk : *func) {
uint32_t cbid;
uint32_t mbid = MergeBlockIdIfAny(blk, &cbid);
if (mbid != 0) {
block2structured_succs_[&blk].push_back(id2block_[mbid]);
if (cbid != 0)
block2structured_succs_[&blk].push_back(id2block_[cbid]);
}
// add true successors
blk.ForEachSuccessorLabel([&blk, this](uint32_t sbid) {
block2structured_succs_[&blk].push_back(id2block_[sbid]);
});
}
}
void DeadBranchElimPass::ComputeStructuredOrder(
ir::Function* func, std::list<ir::BasicBlock*>* order) {
// Compute structured successors and do DFS
ComputeStructuredSuccessors(func);
auto ignore_block = [](cbb_ptr) {};
auto ignore_edge = [](cbb_ptr, cbb_ptr) {};
auto get_structured_successors = [this](const ir::BasicBlock* block) {
return &(block2structured_succs_[block]); };
// TODO(greg-lunarg): Get rid of const_cast by making moving const
// out of the cfa.h prototypes and into the invoking code.
auto post_order = [&](cbb_ptr b) {
order->push_front(const_cast<ir::BasicBlock*>(b)); };
spvtools::CFA<ir::BasicBlock>::DepthFirstTraversal(
&*func->begin(), get_structured_successors, ignore_block, post_order,
ignore_edge);
}
void DeadBranchElimPass::GetConstCondition(
uint32_t condId, bool* condVal, bool* condIsConst) {
ir::Instruction* cInst = def_use_mgr_->GetDef(condId);
switch (cInst->opcode()) {
case SpvOpConstantFalse: {
*condVal = false;
*condIsConst = true;
} break;
case SpvOpConstantTrue: {
*condVal = true;
*condIsConst = true;
} break;
case SpvOpLogicalNot: {
bool negVal;
(void)GetConstCondition(cInst->GetSingleWordInOperand(0),
&negVal, condIsConst);
if (*condIsConst)
*condVal = !negVal;
} break;
default: {
*condIsConst = false;
} break;
}
}
void DeadBranchElimPass::AddBranch(uint32_t labelId, ir::BasicBlock* bp) {
std::unique_ptr<ir::Instruction> newBranch(
new ir::Instruction(SpvOpBranch, 0, 0,
{{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {labelId}}}));
def_use_mgr_->AnalyzeInstDefUse(&*newBranch);
bp->AddInstruction(std::move(newBranch));
}
void DeadBranchElimPass::AddSelectionMerge(uint32_t labelId,
ir::BasicBlock* bp) {
std::unique_ptr<ir::Instruction> newMerge(
new ir::Instruction(SpvOpSelectionMerge, 0, 0,
{{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {labelId}},
{spv_operand_type_t::SPV_OPERAND_TYPE_LITERAL_INTEGER, {0}}}));
def_use_mgr_->AnalyzeInstDefUse(&*newMerge);
bp->AddInstruction(std::move(newMerge));
}
void DeadBranchElimPass::AddBranchConditional(uint32_t condId,
uint32_t trueLabId, uint32_t falseLabId, ir::BasicBlock* bp) {
std::unique_ptr<ir::Instruction> newBranchCond(
new ir::Instruction(SpvOpBranchConditional, 0, 0,
{{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {condId}},
{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {trueLabId}},
{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {falseLabId}}}));
def_use_mgr_->AnalyzeInstDefUse(&*newBranchCond);
bp->AddInstruction(std::move(newBranchCond));
}
void DeadBranchElimPass::KillAllInsts(ir::BasicBlock* bp) {
bp->ForEachInst([this](ir::Instruction* ip) {
KillNamesAndDecorates(ip);
def_use_mgr_->KillInst(ip);
});
}
bool DeadBranchElimPass::GetConstConditionalSelectionBranch(ir::BasicBlock* bp,
ir::Instruction** branchInst, ir::Instruction** mergeInst,
uint32_t *condId, bool *condVal) {
auto ii = bp->end();
--ii;
*branchInst = &*ii;
if ((*branchInst)->opcode() != SpvOpBranchConditional)
return false;
if (ii == bp->begin())
return false;
--ii;
*mergeInst = &*ii;
if ((*mergeInst)->opcode() != SpvOpSelectionMerge)
return false;
bool condIsConst;
*condId = (*branchInst)->GetSingleWordInOperand(
kBranchCondConditionalIdInIdx);
(void) GetConstCondition(*condId, condVal, &condIsConst);
return condIsConst;
}
bool DeadBranchElimPass::HasNonPhiRef(uint32_t labelId) {
analysis::UseList* uses = def_use_mgr_->GetUses(labelId);
if (uses == nullptr)
return false;
for (auto u : *uses)
if (u.inst->opcode() != SpvOpPhi)
return true;
return false;
}
bool DeadBranchElimPass::EliminateDeadBranches(ir::Function* func) {
// Traverse blocks in structured order
std::list<ir::BasicBlock*> structuredOrder;
ComputeStructuredOrder(func, &structuredOrder);
std::unordered_set<ir::BasicBlock*> elimBlocks;
bool modified = false;
for (auto bi = structuredOrder.begin(); bi != structuredOrder.end(); ++bi) {
// Skip blocks that are already in the elimination set
if (elimBlocks.find(*bi) != elimBlocks.end())
continue;
// Skip blocks that don't have constant conditional branch preceded
// by OpSelectionMerge
ir::Instruction* br;
ir::Instruction* mergeInst;
uint32_t condId;
bool condVal;
if (!GetConstConditionalSelectionBranch(*bi, &br, &mergeInst, &condId,
&condVal))
continue;
// Replace conditional branch with unconditional branch
const uint32_t trueLabId =
br->GetSingleWordInOperand(kBranchCondTrueLabIdInIdx);
const uint32_t falseLabId =
br->GetSingleWordInOperand(kBranchCondFalseLabIdInIdx);
const uint32_t mergeLabId =
mergeInst->GetSingleWordInOperand(kSelectionMergeMergeBlockIdInIdx);
const uint32_t liveLabId = condVal == true ? trueLabId : falseLabId;
const uint32_t deadLabId = condVal == true ? falseLabId : trueLabId;
AddBranch(liveLabId, *bi);
def_use_mgr_->KillInst(br);
def_use_mgr_->KillInst(mergeInst);
// Initialize live block set to the live label
std::unordered_set<uint32_t> liveLabIds;
liveLabIds.insert(liveLabId);
// Iterate to merge block adding dead blocks to elimination set
auto dbi = bi;
++dbi;
uint32_t dLabId = (*dbi)->id();
while (dLabId != mergeLabId) {
if (liveLabIds.find(dLabId) == liveLabIds.end()) {
// Kill use/def for all instructions and mark block for elimination
KillAllInsts(*dbi);
elimBlocks.insert(*dbi);
}
else {
// Mark all successors as live
(*dbi)->ForEachSuccessorLabel([&liveLabIds](const uint32_t succId){
liveLabIds.insert(succId);
});
// Mark merge and continue blocks as live
(*dbi)->ForMergeAndContinueLabel([&liveLabIds](const uint32_t succId){
liveLabIds.insert(succId);
});
}
++dbi;
dLabId = (*dbi)->id();
}
// Process phi instructions in merge block.
// elimBlocks are now blocks which cannot precede merge block. Also,
// if eliminated branch is to merge label, remember the conditional block
// also cannot precede merge block.
uint32_t deadCondLabId = 0;
if (deadLabId == mergeLabId)
deadCondLabId = (*bi)->id();
(*dbi)->ForEachPhiInst([&elimBlocks, &deadCondLabId, this](
ir::Instruction* phiInst) {
const uint32_t phiLabId0 =
phiInst->GetSingleWordInOperand(kPhiLab0IdInIdx);
const bool useFirst =
elimBlocks.find(id2block_[phiLabId0]) == elimBlocks.end() &&
phiLabId0 != deadCondLabId;
const uint32_t phiValIdx =
useFirst ? kPhiVal0IdInIdx : kPhiVal1IdInIdx;
const uint32_t replId = phiInst->GetSingleWordInOperand(phiValIdx);
const uint32_t phiId = phiInst->result_id();
KillNamesAndDecorates(phiId);
(void)def_use_mgr_->ReplaceAllUsesWith(phiId, replId);
def_use_mgr_->KillInst(phiInst);
});
// If merge block has no predecessors, replace the new branch with
// a MergeSelection/BranchCondition using the original constant condition
// and the mergeblock as the false branch. This is done so the merge block
// is not orphaned, which could cause invalid control flow in certain case.
// TODO(greg-lunarg): Do this only in cases where invalid code is caused.
if (!HasNonPhiRef(mergeLabId)) {
auto eii = (*bi)->end();
--eii;
ir::Instruction* nbr = &*eii;
AddSelectionMerge(mergeLabId, *bi);
if (condVal == true)
AddBranchConditional(condId, liveLabId, mergeLabId, *bi);
else
AddBranchConditional(condId, mergeLabId, liveLabId, *bi);
def_use_mgr_->KillInst(nbr);
}
modified = true;
}
// Erase dead blocks
for (auto ebi = func->begin(); ebi != func->end(); )
if (elimBlocks.find(&*ebi) != elimBlocks.end())
ebi = ebi.Erase();
else
++ebi;
return modified;
}
void DeadBranchElimPass::Initialize(ir::Module* module) {
module_ = module;
// Initialize function and block maps
id2block_.clear();
block2structured_succs_.clear();
// Initialize block map
for (auto& fn : *module_)
for (auto& blk : fn)
id2block_[blk.id()] = &blk;
// TODO(greg-lunarg): Reuse def/use from previous passes
def_use_mgr_.reset(new analysis::DefUseManager(consumer(), module_));
// Initialize extension whitelist
InitExtensions();
};
bool DeadBranchElimPass::AllExtensionsSupported() const {
// If any extension not in whitelist, return false
for (auto& ei : module_->extensions()) {
const char* extName = reinterpret_cast<const char*>(
&ei.GetInOperand(0).words[0]);
if (extensions_whitelist_.find(extName) == extensions_whitelist_.end())
return false;
}
return true;
}
Pass::Status DeadBranchElimPass::ProcessImpl() {
// Current functionality assumes structured control flow.
// TODO(greg-lunarg): Handle non-structured control-flow.
if (!module_->HasCapability(SpvCapabilityShader))
return Status::SuccessWithoutChange;
// Do not process if module contains OpGroupDecorate. Additional
// support required in KillNamesAndDecorates().
// TODO(greg-lunarg): Add support for OpGroupDecorate
for (auto& ai : module_->annotations())
if (ai.opcode() == SpvOpGroupDecorate)
return Status::SuccessWithoutChange;
// Do not process if any disallowed extensions are enabled
if (!AllExtensionsSupported())
return Status::SuccessWithoutChange;
// Collect all named and decorated ids
FindNamedOrDecoratedIds();
// Process all entry point functions
ProcessFunction pfn = [this](ir::Function* fp) {
return EliminateDeadBranches(fp);
};
bool modified = ProcessEntryPointCallTree(pfn, module_);
return modified ? Status::SuccessWithChange : Status::SuccessWithoutChange;
}
DeadBranchElimPass::DeadBranchElimPass() {}
Pass::Status DeadBranchElimPass::Process(ir::Module* module) {
Initialize(module);
return ProcessImpl();
}
void DeadBranchElimPass::InitExtensions() {
extensions_whitelist_.clear();
extensions_whitelist_.insert({
"SPV_AMD_shader_explicit_vertex_parameter",
"SPV_AMD_shader_trinary_minmax",
"SPV_AMD_gcn_shader",
"SPV_KHR_shader_ballot",
"SPV_AMD_shader_ballot",
"SPV_AMD_gpu_shader_half_float",
"SPV_KHR_shader_draw_parameters",
"SPV_KHR_subgroup_vote",
"SPV_KHR_16bit_storage",
"SPV_KHR_device_group",
"SPV_KHR_multiview",
"SPV_NVX_multiview_per_view_attributes",
"SPV_NV_viewport_array2",
"SPV_NV_stereo_view_rendering",
"SPV_NV_sample_mask_override_coverage",
"SPV_NV_geometry_shader_passthrough",
"SPV_AMD_texture_gather_bias_lod",
"SPV_KHR_storage_buffer_storage_class",
"SPV_KHR_variable_pointers",
"SPV_AMD_gpu_shader_int16",
"SPV_KHR_post_depth_coverage",
"SPV_KHR_shader_atomic_counter_ops",
});
}
} // namespace opt
} // namespace spvtools