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[spirv-reader] Refactor bookkeeping for localy defined values 

Bug: tint:3
Change-Id: Ibe26b802ae61c6271f8a75e641ac31db36df6000
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/24124
Reviewed-by: dan sinclair <dsinclair@google.com>
This commit is contained in:
David Neto 2020-06-30 19:03:17 +00:00
parent e88c0f6f8a
commit 2c11bd6b68
2 changed files with 172 additions and 96 deletions
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114
src/reader/spirv/function.cc
View File

@ -409,7 +409,13 @@ class StructuredTraverser {
BlockInfo::BlockInfo(const spvtools::opt::BasicBlock& bb)
: basic_block(&bb), id(bb.id()) {}
BlockInfo::~BlockInfo() {}
BlockInfo::~BlockInfo() = default;
DefInfo::DefInfo(const spvtools::opt::Instruction& def_inst,
uint32_t the_block_pos)
: inst(def_inst), block_pos(the_block_pos) {}
DefInfo::~DefInfo() = default;
FunctionEmitter::FunctionEmitter(ParserImpl* pi,
const spvtools::opt::Function& function)
@ -623,7 +629,6 @@ bool FunctionEmitter::EmitBody() {
}
// TODO(dneto): register phis
// TODO(dneto): register SSA values which need to be hoisted
RegisterValuesNeedingNamedOrHoistedDefinition();
if (!EmitFunctionVariables()) {
@ -2416,7 +2421,8 @@ bool FunctionEmitter::EmitConstDefOrWriteToHoistedVar(
const spvtools::opt::Instruction& inst,
TypedExpression ast_expr) {
const auto result_id = inst.result_id();
if (needs_hoisted_def_.count(result_id) != 0) {
const auto* def_info = GetDefInfo(result_id);
if (def_info && def_info->requires_hoisted_def) {
// Emit an assignment of the expression to the hoisted variable.
AddStatement(std::make_unique<ast::AssignmentStatement>(
std::make_unique<ast::IdentifierExpression>(namer_.Name(result_id)),
@ -2430,21 +2436,25 @@ bool FunctionEmitter::EmitStatement(const spvtools::opt::Instruction& inst) {
const auto result_id = inst.result_id();
// Handle combinatorial instructions.
auto combinatorial_expr = MaybeEmitCombinatorialValue(inst);
const auto* def_info = GetDefInfo(result_id);
if (combinatorial_expr.expr != nullptr) {
if ((needs_hoisted_def_.count(result_id) == 0) &&
(needs_named_const_def_.count(result_id) == 0) &&
(def_use_mgr_->NumUses(&inst) == 1)) {
// If it's used once, and doesn't need a named constant definition,
// then defer emitting the expression until it's used. Any supporting
// statements have already been emitted.
if (def_info == nullptr) {
return Fail() << "internal error: result ID %" << result_id
<< " is missing a def_info";
}
if (def_info->requires_hoisted_def || def_info->requires_named_const_def ||
def_info->num_uses != 1) {
// Generate a const definition or an assignment to a hoisted definition
// now and later use the const or variable name at the uses of this value.
return EmitConstDefOrWriteToHoistedVar(inst,
std::move(combinatorial_expr));
}
// It is harmless to defer emitting the expression until it's used.
// Any supporting statements have already been emitted.
singly_used_values_.insert(
std::make_pair(result_id, std::move(combinatorial_expr)));
return success();
}
// Otherwise, generate a const definition for it now and later use
// the const's name at the uses of the value.
return EmitConstDefOrWriteToHoistedVar(inst, std::move(combinatorial_expr));
}
if (failed()) {
return false;
}
@ -2906,61 +2916,68 @@ TypedExpression FunctionEmitter::MakeVectorShuffle(
}
void FunctionEmitter::RegisterValuesNeedingNamedOrHoistedDefinition() {
// Maps a result ID to the block position where it is last used.
std::unordered_map<uint32_t, uint32_t> id_to_last_use_pos;
// List of pairs of (result id, block position of the definition).
std::vector<std::pair<uint32_t, uint32_t>> id_def_pos;
// Create a DefInfo for each value definition in this function.
for (auto block_id : block_order_) {
const auto* block_info = GetBlockInfo(block_id);
const auto block_pos = block_info->pos;
for (const auto& inst : *(block_info->basic_block)) {
const auto result_id = inst.result_id();
if (result_id != 0) {
id_def_pos.emplace_back(
std::pair<uint32_t, uint32_t>{result_id, block_pos});
if ((result_id == 0) || inst.opcode() == SpvOpLabel) {
continue;
}
def_info_[result_id] = std::make_unique<DefInfo>(inst, block_pos);
}
}
inst.ForEachInId(
[&id_to_last_use_pos, block_pos](const uint32_t* id_ptr) {
// If the id is not in the map already, this will create
// an entry with value 0.
auto& pos = id_to_last_use_pos[*id_ptr];
// Update the entry.
pos = std::max(pos, block_pos);
});
// Mark vector operands of OpVectorShuffle as needing a named definition,
// but only if they are defined in this function as well.
for (auto& id_def_info_pair : def_info_) {
const auto& inst = id_def_info_pair.second->inst;
if (inst.opcode() == SpvOpVectorShuffle) {
// We might access the vector operands multiple times. Make sure they
// are evaluated only once.
for (auto index : std::array<uint32_t, 2>{0, 1}) {
auto id = inst.GetSingleWordInOperand(index);
if (constant_mgr_->FindDeclaredConstant(id)) {
// If it's constant, then avoid making a const definition
// in the wrong place; it would be wrong if it didn't
// dominate its uses.
continue;
}
// Othewrise, register it.
needs_named_const_def_.insert(id);
auto* operand_def = GetDefInfo(id);
if (operand_def) {
operand_def->requires_named_const_def = true;
}
}
}
}
// Scan uses of locally defined IDs, to determine the block position
// of its last use.
for (auto block_id : block_order_) {
const auto* block_info = GetBlockInfo(block_id);
const auto block_pos = block_info->pos;
for (const auto& inst : *(block_info->basic_block)) {
// Update the usage span for IDs used by this instruction.
inst.ForEachInId([this, block_pos](const uint32_t* id_ptr) {
auto* def_info = GetDefInfo(*id_ptr);
if (def_info) {
def_info->num_uses++;
def_info->last_use_pos = std::max(def_info->last_use_pos, block_pos);
}
});
}
}
// For an ID defined in this function, if it is used in a different construct
// than its definition, then it needs a named constant definition. Otherwise
// we might sink an expensive computation into control flow, and hence change
// performance.
for (const auto& id_and_pos : id_def_pos) {
const auto id = id_and_pos.first;
const auto def_pos = id_and_pos.second;
for (auto& id_def_info_pair : def_info_) {
const auto def_id = id_def_info_pair.first;
auto* def_info = id_def_info_pair.second.get();
if (def_info->num_uses == 0) {
// There is no need to adjust the location of the declaration.
continue;
}
const auto last_use_pos = def_info->last_use_pos;
auto last_use_where = id_to_last_use_pos.find(id);
if (last_use_where != id_to_last_use_pos.end()) {
const auto last_use_pos = last_use_where->second;
const auto* const def_in_construct =
GetBlockInfo(block_order_[def_pos])->construct;
GetBlockInfo(block_order_[def_info->block_pos])->construct;
const auto* const construct_with_last_use =
GetBlockInfo(block_order_[last_use_pos])->construct;
@ -2977,14 +2994,13 @@ void FunctionEmitter::RegisterValuesNeedingNamedOrHoistedDefinition() {
if (def_in_construct != construct_with_last_use) {
if (enclosing_construct == def_in_construct) {
// We can use a plain 'const' definition.
needs_named_const_def_.insert(id);
def_info->requires_named_const_def = true;
} else {
// We need to make a hoisted variable definition.
// TODO(dneto): Handle non-storable types, particularly pointers.
needs_hoisted_def_.insert(id);
def_info->requires_hoisted_def = true;
auto* hoist_to_block = GetBlockInfo(enclosing_construct->begin_id);
hoist_to_block->hoisted_ids.push_back(id);
}
hoist_to_block->hoisted_ids.push_back(def_id);
}
}
}

102
src/reader/spirv/function.h
View File

@ -159,8 +159,8 @@ struct BlockInfo {
std::string flow_guard_name = "";
/// The result IDs that this block is responsible for declaring as a
/// hoisted variable. See the |needs_hoisted_def_| member of
/// FunctionEmitter for an explanation.
/// hoisted variable. See the |requires_hoisted_def| member of
/// DefInfo for an explanation.
std::vector<uint32_t> hoisted_ids;
};
@ -174,6 +174,62 @@ inline std::ostream& operator<<(std::ostream& o, const BlockInfo& bi) {
return o;
}
/// Bookkeeping info for a SPIR-V ID defined in the function.
/// This will be valid for result IDs for:
/// - instructions that are not OpLabel, OpVariable, and OpFunctionParameter
/// - are defined in a basic block visited in the block-order for the function.
struct DefInfo {
/// Constructor.
/// @param def_inst the SPIR-V instruction defining the ID
/// @param block_pos the position of the basic block where the ID is defined.
DefInfo(const spvtools::opt::Instruction& def_inst, uint32_t block_pos);
/// Destructor.
~DefInfo();
/// The SPIR-V instruction that defines the ID.
const spvtools::opt::Instruction& inst;
/// The position of the block that defines this ID, in the function block
/// order. See method |FunctionEmitter::ComputeBlockOrderAndPositions|
const uint32_t block_pos = 0;
/// The number of uses of this ID.
uint32_t num_uses = 0;
/// The block position of the last use of this ID, or 0 if it is not used
/// at all. The "last" ordering is determined by the function block order.
uint32_t last_use_pos = 0;
/// True if this ID requires a WGSL 'const' definition, due to context. It
/// might get one anyway (so this is *not* an if-and-only-if condition).
bool requires_named_const_def = false;
/// True if this ID must map to a WGSL variable declaration before the
/// corresponding position of the ID definition in SPIR-V. This compensates
/// for the difference between dominance and scoping. An SSA definition can
/// dominate all its uses, but the construct where it is defined does not
/// enclose all the uses, and so if it were declared as a WGSL constant
/// definition at the point of its SPIR-V definition, then the WGSL name
/// would go out of scope too early. Fix that by creating a variable at the
/// top of the smallest construct that encloses both the definition and all
/// its uses. Then the original SPIR-V definition maps to a WGSL assignment
/// to that variable, and each SPIR-V use becomes a WGSL read from the
/// variable.
/// TODO(dneto): This works for constants of storable type, but not, for
/// example, pointers.
bool requires_hoisted_def = false;
};
inline std::ostream& operator<<(std::ostream& o, const DefInfo& di) {
o << "DefInfo{"
<< " inst.result_id: " << di.inst.result_id()
<< " block_pos: " << di.block_pos << " num_uses: " << di.num_uses
<< " last_use_pos: " << di.last_use_pos << " requires_named_const_def: "
<< (di.requires_named_const_def ? "true" : "false")
<< " requires_hoisted_def: " << (di.requires_hoisted_def ? "true" : "false")
<< "}";
return o;
}
/// A FunctionEmitter emits a SPIR-V function onto a Tint AST module.
class FunctionEmitter {
public:
@ -288,12 +344,16 @@ class FunctionEmitter {
/// - When a SPIR-V instruction might use the dynamically computed value
/// only once, but the WGSL code might reference it multiple times.
/// For example, this occurs for the vector operands of OpVectorShuffle.
/// In this case the definition is added to |needs_named_const_def_|.
/// In this case the definition's |requires_named_const_def| property is
/// set to true.
/// - When a definition and at least one of its uses are not in the
/// same structured construct.
/// In this case the definition is added to |needs_named_const_def_|.
/// In this case the definition's |requires_named_const_def| property is
/// set to true.
/// - When a definition is in a construct that does not enclose all the
/// uses. In this case the definition is added to |needs_hoisted_def_|.
/// uses. In this case the definition's |requires_hoisted_def| property
/// is set to true.
/// Populates the |def_info_| mapping.
void RegisterValuesNeedingNamedOrHoistedDefinition();
/// Emits declarations of function variables.
@ -495,8 +555,20 @@ class FunctionEmitter {
/// @returns the block info for the given ID, if it exists, or nullptr
BlockInfo* GetBlockInfo(uint32_t id) const {
auto where = block_info_.find(id);
if (where == block_info_.end())
if (where == block_info_.end()) {
return nullptr;
}
return where->second.get();
}
/// Gets the local definition info for a result ID.
/// @param id the SPIR-V ID of local definition.
/// @returns the definition info for the given ID, if it exists, or nullptr
DefInfo* GetDefInfo(uint32_t id) const {
auto where = def_info_.find(id);
if (where == def_info_.end()) {
return nullptr;
}
return where->second.get();
}
@ -622,21 +694,6 @@ class FunctionEmitter {
std::unordered_set<uint32_t> identifier_values_;
// Mapping from SPIR-V ID that is used at most once, to its AST expression.
std::unordered_map<uint32_t, TypedExpression> singly_used_values_;
// Set of SPIR-V IDs which should get a named const definition.
std::unordered_set<uint32_t> needs_named_const_def_;
// The SPIR-V IDs that must be declared in WGSL before the corresponding
// location in SPIR-V. This compensates for the difference between dominance
// and scoping. An SSA definition can dominate all its uses, but the construct
// where it is defined does not enclose all the uses, and so if it were
// declared as a WGSL constant definition at the point of its SPIR-V
// definition, then the WGSL name would go out of scope too early. Fix that by
// creating a variable at the top of the smallest construct that encloses both
// the definition and all its uses. Then the original SPIR-V definition maps
// to a WGSL assignment to that variable, and each SPIR-V use becomes a WGSL
// read from the variable.
// TODO(dneto): This works for constants of storable type, but not, for
// example, pointers.
std::unordered_set<uint32_t> needs_hoisted_def_;
// The IDs of basic blocks, in reverse structured post-order (RSPO).
// This is the output order for the basic blocks.
@ -645,6 +702,9 @@ class FunctionEmitter {
// Mapping from block ID to its bookkeeping info.
std::unordered_map<uint32_t, std::unique_ptr<BlockInfo>> block_info_;
// Mapping from a locally-defined result ID to its bookkeeping info.
std::unordered_map<uint32_t, std::unique_ptr<DefInfo>> def_info_;
// Structured constructs, where enclosing constructs precede their children.
ConstructList constructs_;
};