3 files changed, 164 insertions, 1 deletions

diff --git a/src/shader_recompiler/ir_opt/identity_removal_pass.cpp b/src/shader_recompiler/ir_opt/identity_removal_pass.cpp
index f9bb063fb..7f8500087 100644
--- a/src/shader_recompiler/ir_opt/identity_removal_pass.cpp
+++ b/src/shader_recompiler/ir_opt/identity_removal_pass.cpp
@@ -28,7 +28,6 @@ void IdentityRemovalPass(IR::Block& block) {
             ++inst;
         }
     }
-
     for (IR::Inst* const inst : to_invalidate) {
         inst->Invalidate();
     }
diff --git a/src/shader_recompiler/ir_opt/passes.h b/src/shader_recompiler/ir_opt/passes.h
index fe5454e9a..83f094d73 100644
--- a/src/shader_recompiler/ir_opt/passes.h
+++ b/src/shader_recompiler/ir_opt/passes.h
@@ -5,12 +5,21 @@
 #pragma once
 
 #include "shader_recompiler/frontend/ir/basic_block.h"
+#include "shader_recompiler/frontend/ir/function.h"
 
 namespace Shader::Optimization {
 
+template <typename Func>
+void Invoke(Func&& func, IR::Function& function) {
+    for (const auto& block : function.blocks) {
+        func(*block);
+    }
+}
+
 void DeadCodeEliminationPass(IR::Block& block);
 void GetSetElimination(IR::Block& block);
 void IdentityRemovalPass(IR::Block& block);
+void SsaRewritePass(IR::Function& function);
 void VerificationPass(const IR::Block& block);
 
 } // namespace Shader::Optimization
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 <map>
+
+#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/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<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)];
+    }
+
+    std::array<ValueMap, IR::NUM_USER_REGS> regs;
+    std::array<ValueMap, IR::NUM_USER_PREDS> 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