From 6c4cc0cd062fbbba5349da1108d3c23cb330ca8a Mon Sep 17 00:00:00 2001 From: ReinUsesLisp Date: Tue, 2 Feb 2021 21:07:00 -0300 Subject: shader: SSA and dominance --- src/shader_recompiler/ir_opt/ssa_rewrite_pass.cpp | 155 ++++++++++++++++++++++ 1 file changed, 155 insertions(+) create mode 100644 src/shader_recompiler/ir_opt/ssa_rewrite_pass.cpp (limited to 'src/shader_recompiler/ir_opt/ssa_rewrite_pass.cpp') diff --git a/src/shader_recompiler/ir_opt/ssa_rewrite_pass.cpp b/src/shader_recompiler/ir_opt/ssa_rewrite_pass.cpp new file mode 100644 index 000000000..a4b256a40 --- /dev/null +++ b/src/shader_recompiler/ir_opt/ssa_rewrite_pass.cpp @@ -0,0 +1,155 @@ +// 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., Leißa 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 + +#include + +#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/opcode.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>; + +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)]; + } + + std::array regs; + std::array preds; +}; + +IR::Opcode UndefOpcode(IR::Reg) noexcept { + return IR::Opcode::Undef32; +} + +IR::Opcode UndefOpcode(IR::Pred) 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 + val = IR::Value{&*block->PrependNewInst(block->begin(), IR::Opcode::Phi)}; + WriteVariable(variable, block, val); + val = AddPhiOperands(variable, val, block); + } + WriteVariable(variable, block, val); + return val; + } + + IR::Value AddPhiOperands(auto variable, const IR::Value& phi, IR::Block* block) { + for (IR::Block* const pred : block->ImmediatePredecessors()) { + phi.Inst()->AddPhiOperand(pred, ReadVariable(variable, pred)); + } + return TryRemoveTrivialPhi(phi, block, UndefOpcode(variable)); + } + + IR::Value TryRemoveTrivialPhi(const IR::Value& phi, IR::Block* block, IR::Opcode undef_opcode) { + IR::Value same; + for (const auto& pair : phi.Inst()->PhiOperands()) { + const IR::Value& op{pair.second}; + if (op == same || op == phi) { + // Unique value or self-reference + continue; + } + if (!same.IsEmpty()) { + // The phi merges at least two values: not trivial + return 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.Inst()->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 (const auto& 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.get(), 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.get(), inst.Arg(1)); + } + break; + case IR::Opcode::GetRegister: + if (const IR::Reg reg{inst.Arg(0).Reg()}; reg != IR::Reg::RZ) { + inst.ReplaceUsesWith(pass.ReadVariable(reg, block.get())); + } + break; + case IR::Opcode::GetPred: + if (const IR::Pred pred{inst.Arg(0).Pred()}; pred != IR::Pred::PT) { + inst.ReplaceUsesWith(pass.ReadVariable(pred, block.get())); + } + break; + default: + break; + } + } + } +} + +} // namespace Shader::Optimization -- cgit v1.2.3