// 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 <boost/container/flat_map.hpp>
#include "shader_recompiler/frontend/ir/basic_block.h"
#include "shader_recompiler/frontend/ir/function.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 {
using ValueMap = boost::container::flat_map<IR::Block*, IR::Value, std::less<IR::Block*>>;
struct FlagTag {};
struct ZeroFlagTag : FlagTag {};
struct SignFlagTag : FlagTag {};
struct CarryFlagTag : FlagTag {};
struct OverflowFlagTag : FlagTag {};
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[](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;
ValueMap zero_flag;
ValueMap sign_flag;
ValueMap carry_flag;
ValueMap overflow_flag;
};
IR::Opcode UndefOpcode(IR::Reg) noexcept {
return IR::Opcode::Undef32;
}
IR::Opcode UndefOpcode(IR::Pred) noexcept {
return IR::Opcode::Undef1;
}
IR::Opcode UndefOpcode(const FlagTag&) noexcept {
return IR::Opcode::Undef1;
}
[[nodiscard]] bool IsPhi(const IR::Inst& inst) noexcept {
return inst.Opcode() == IR::Opcode::Phi;
}
class Pass {
public:
void WriteVariable(auto variable, IR::Block* block, const IR::Value& value) {
current_def[variable].insert_or_assign(block, value);
}
IR::Value ReadVariable(auto variable, IR::Block* block) {
auto& def{current_def[variable]};
if (const auto it{def.find(block)}; it != def.end()) {
return it->second;
}
return ReadVariableRecursive(variable, block);
}
private:
IR::Value ReadVariableRecursive(auto variable, IR::Block* block) {
IR::Value val;
if (const std::span preds{block->ImmediatePredecessors()}; preds.size() == 1) {
val = ReadVariable(variable, 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;
}
IR::Value AddPhiOperands(auto variable, IR::Inst& phi, IR::Block* block) {
for (IR::Block* const pred : block->ImmediatePredecessors()) {
phi.AddPhiOperand(pred, ReadVariable(variable, 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 == same || 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
const auto first_not_phi{std::ranges::find_if_not(block->Instructions(), IsPhi)};
same = IR::Value{&*block->PrependNewInst(first_not_phi, undef_opcode)};
}
// 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;
}
DefTable current_def;
};
} // Anonymous namespace
void SsaRewritePass(IR::Function& function) {
Pass pass;
for (IR::Block* const block : function.blocks) {
for (IR::Inst& inst : block->Instructions()) {
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::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::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;
}
}
}
}
} // namespace Shader::Optimization
