// Copyright 2021 yuzu Emulator Project
// Licensed under GPLv2 or any later version
// Refer to the license.txt file included.
// This file implements the SSA rewriting algorithm proposed in
//
// Simple and Efficient Construction of Static Single Assignment Form.
// Braun M., Buchwald S., Hack S., Leiba R., Mallon C., Zwinkau A. (2013)
// In: Jhala R., De Bosschere K. (eds)
// Compiler Construction. CC 2013.
// Lecture Notes in Computer Science, vol 7791.
// Springer, Berlin, Heidelberg
//
// https://link.springer.com/chapter/10.1007/978-3-642-37051-9_6
//
#include <ranges>
#include <span>
#include <variant>
#include <vector>
#include <boost/container/flat_map.hpp>
#include <boost/container/flat_set.hpp>
#include "shader_recompiler/frontend/ir/basic_block.h"
#include "shader_recompiler/frontend/ir/microinstruction.h"
#include "shader_recompiler/frontend/ir/opcodes.h"
#include "shader_recompiler/frontend/ir/pred.h"
#include "shader_recompiler/frontend/ir/reg.h"
#include "shader_recompiler/ir_opt/passes.h"
namespace Shader::Optimization {
namespace {
struct FlagTag {
auto operator<=>(const FlagTag&) const noexcept = default;
};
struct ZeroFlagTag : FlagTag {};
struct SignFlagTag : FlagTag {};
struct CarryFlagTag : FlagTag {};
struct OverflowFlagTag : FlagTag {};
struct GotoVariable : FlagTag {
GotoVariable() = default;
explicit GotoVariable(u32 index_) : index{index_} {}
auto operator<=>(const GotoVariable&) const noexcept = default;
u32 index;
};
struct IndirectBranchVariable {
auto operator<=>(const IndirectBranchVariable&) const noexcept = default;
};
using Variant = std::variant<IR::Reg, IR::Pred, ZeroFlagTag, SignFlagTag, CarryFlagTag,
OverflowFlagTag, GotoVariable, IndirectBranchVariable>;
using ValueMap = boost::container::flat_map<IR::Block*, IR::Value, std::less<IR::Block*>>;
struct DefTable {
[[nodiscard]] ValueMap& operator[](IR::Reg variable) noexcept {
return regs[IR::RegIndex(variable)];
}
[[nodiscard]] ValueMap& operator[](IR::Pred variable) noexcept {
return preds[IR::PredIndex(variable)];
}
[[nodiscard]] ValueMap& operator[](GotoVariable goto_variable) {
return goto_vars[goto_variable.index];
}
[[nodiscard]] ValueMap& operator[](IndirectBranchVariable) {
return indirect_branch_var;
}
[[nodiscard]] ValueMap& operator[](ZeroFlagTag) noexcept {
return zero_flag;
}
[[nodiscard]] ValueMap& operator[](SignFlagTag) noexcept {
return sign_flag;
}
[[nodiscard]] ValueMap& operator[](CarryFlagTag) noexcept {
return carry_flag;
}
[[nodiscard]] ValueMap& operator[](OverflowFlagTag) noexcept {
return overflow_flag;
}
std::array<ValueMap, IR::NUM_USER_REGS> regs;
std::array<ValueMap, IR::NUM_USER_PREDS> preds;
boost::container::flat_map<u32, ValueMap> goto_vars;
ValueMap indirect_branch_var;
ValueMap zero_flag;
ValueMap sign_flag;
ValueMap carry_flag;
ValueMap overflow_flag;
};
IR::Opcode UndefOpcode(IR::Reg) noexcept {
return IR::Opcode::UndefU32;
}
IR::Opcode UndefOpcode(IR::Pred) noexcept {
return IR::Opcode::UndefU1;
}
IR::Opcode UndefOpcode(const FlagTag&) noexcept {
return IR::Opcode::UndefU1;
}
IR::Opcode UndefOpcode(IndirectBranchVariable) noexcept {
return IR::Opcode::UndefU32;
}
[[nodiscard]] bool IsPhi(const IR::Inst& inst) noexcept {
return inst.Opcode() == IR::Opcode::Phi;
}
class Pass {
public:
template <typename Type>
void WriteVariable(Type variable, IR::Block* block, const IR::Value& value) {
current_def[variable].insert_or_assign(block, value);
}
template <typename Type>
IR::Value ReadVariable(Type variable, IR::Block* block) {
const ValueMap& def{current_def[variable]};
if (const auto it{def.find(block)}; it != def.end()) {
return it->second;
}
return ReadVariableRecursive(variable, block);
}
void SealBlock(IR::Block* block) {
const auto it{incomplete_phis.find(block)};
if (it != incomplete_phis.end()) {
for (auto& [variant, phi] : it->second) {
std::visit([&](auto& variable) { AddPhiOperands(variable, *phi, block); }, variant);
}
}
sealed_blocks.insert(block);
}
private:
template <typename Type>
IR::Value ReadVariableRecursive(Type variable, IR::Block* block) {
IR::Value val;
if (!sealed_blocks.contains(block)) {
// Incomplete CFG
IR::Inst* phi{&*block->PrependNewInst(block->begin(), IR::Opcode::Phi)};
incomplete_phis[block].insert_or_assign(variable, phi);
val = IR::Value{&*phi};
} else if (const std::span imm_preds{block->ImmediatePredecessors()};
imm_preds.size() == 1) {
// Optimize the common case of one predecessor: no phi needed
val = ReadVariable(variable, imm_preds.front());
} else {
// Break potential cycles with operandless phi
IR::Inst& phi_inst{*block->PrependNewInst(block->begin(), IR::Opcode::Phi)};
val = IR::Value{&phi_inst};
WriteVariable(variable, block, val);
val = AddPhiOperands(variable, phi_inst, block);
}
WriteVariable(variable, block, val);
return val;
}
template <typename Type>
IR::Value AddPhiOperands(Type variable, IR::Inst& phi, IR::Block* block) {
for (IR::Block* const imm_pred : block->ImmediatePredecessors()) {
phi.AddPhiOperand(imm_pred, ReadVariable(variable, imm_pred));
}
return TryRemoveTrivialPhi(phi, block, UndefOpcode(variable));
}
IR::Value TryRemoveTrivialPhi(IR::Inst& phi, IR::Block* block, IR::Opcode undef_opcode) {
IR::Value same;
const size_t num_args{phi.NumArgs()};
for (size_t arg_index = 0; arg_index < num_args; ++arg_index) {
const IR::Value& op{phi.Arg(arg_index)};
if (op.Resolve() == same.Resolve() || op == IR::Value{&phi}) {
// Unique value or self-reference
continue;
}
if (!same.IsEmpty()) {
// The phi merges at least two values: not trivial
return IR::Value{&phi};
}
same = op;
}
if (same.IsEmpty()) {
// The phi is unreachable or in the start block
// First remove the phi node from the block, it will be reinserted
IR::Block::InstructionList& list{block->Instructions()};
list.erase(IR::Block::InstructionList::s_iterator_to(phi));
// Insert an undef instruction after all phi nodes (to keep phi instructions on top)
const auto first_not_phi{std::ranges::find_if_not(list, IsPhi)};
same = IR::Value{&*block->PrependNewInst(first_not_phi, undef_opcode)};
// Insert the phi node after the undef opcode, this will be replaced with an identity
list.insert(first_not_phi, phi);
}
// Reroute all uses of phi to same and remove phi
phi.ReplaceUsesWith(same);
// TODO: Try to recursively remove all phi users, which might have become trivial
return same;
}
boost::container::flat_set<IR::Block*> sealed_blocks;
boost::container::flat_map<IR::Block*, boost::container::flat_map<Variant, IR::Inst*>>
incomplete_phis;
DefTable current_def;
};
void VisitInst(Pass& pass, IR::Block* block, IR::Inst& inst) {
switch (inst.Opcode()) {
case IR::Opcode::SetRegister:
if (const IR::Reg reg{inst.Arg(0).Reg()}; reg != IR::Reg::RZ) {
pass.WriteVariable(reg, block, inst.Arg(1));
}
break;
case IR::Opcode::SetPred:
if (const IR::Pred pred{inst.Arg(0).Pred()}; pred != IR::Pred::PT) {
pass.WriteVariable(pred, block, inst.Arg(1));
}
break;
case IR::Opcode::SetGotoVariable:
pass.WriteVariable(GotoVariable{inst.Arg(0).U32()}, block, inst.Arg(1));
break;
case IR::Opcode::SetIndirectBranchVariable:
pass.WriteVariable(IndirectBranchVariable{}, block, inst.Arg(0));
break;
case IR::Opcode::SetZFlag:
pass.WriteVariable(ZeroFlagTag{}, block, inst.Arg(0));
break;
case IR::Opcode::SetSFlag:
pass.WriteVariable(SignFlagTag{}, block, inst.Arg(0));
break;
case IR::Opcode::SetCFlag:
pass.WriteVariable(CarryFlagTag{}, block, inst.Arg(0));
break;
case IR::Opcode::SetOFlag:
pass.WriteVariable(OverflowFlagTag{}, block, inst.Arg(0));
break;
case IR::Opcode::GetRegister:
if (const IR::Reg reg{inst.Arg(0).Reg()}; reg != IR::Reg::RZ) {
inst.ReplaceUsesWith(pass.ReadVariable(reg, block));
}
break;
case IR::Opcode::GetPred:
if (const IR::Pred pred{inst.Arg(0).Pred()}; pred != IR::Pred::PT) {
inst.ReplaceUsesWith(pass.ReadVariable(pred, block));
}
break;
case IR::Opcode::GetGotoVariable:
inst.ReplaceUsesWith(pass.ReadVariable(GotoVariable{inst.Arg(0).U32()}, block));
break;
case IR::Opcode::GetIndirectBranchVariable:
inst.ReplaceUsesWith(pass.ReadVariable(IndirectBranchVariable{}, block));
break;
case IR::Opcode::GetZFlag:
inst.ReplaceUsesWith(pass.ReadVariable(ZeroFlagTag{}, block));
break;
case IR::Opcode::GetSFlag:
inst.ReplaceUsesWith(pass.ReadVariable(SignFlagTag{}, block));
break;
case IR::Opcode::GetCFlag:
inst.ReplaceUsesWith(pass.ReadVariable(CarryFlagTag{}, block));
break;
case IR::Opcode::GetOFlag:
inst.ReplaceUsesWith(pass.ReadVariable(OverflowFlagTag{}, block));
break;
default:
break;
}
}
void VisitBlock(Pass& pass, IR::Block* block) {
for (IR::Inst& inst : block->Instructions()) {
VisitInst(pass, block, inst);
}
pass.SealBlock(block);
}
} // Anonymous namespace
void SsaRewritePass(IR::Program& program) {
Pass pass;
for (IR::Block* const block : program.post_order_blocks | std::views::reverse) {
VisitBlock(pass, block);
}
}
} // namespace Shader::Optimization
