Remove dead shader instruction cycles with linear liveness traversal

src/graphics/shader/recompiler/ir/Value.h

@@ -152,6 +152,8 @@ public:
 	}
 
 private:
+	friend void EliminateDeadCode(const std::vector<Block*>& blocks);
+
 	void AddUse(Inst* used, size_t operand);
 	void RemoveUse(Inst* used, size_t operand);
 	void ClearArgs();
@@ -161,6 +163,7 @@ private:
 
 	ValueOpcode         opcode;
 	uint8_t             num_args;
+	bool                live = false;
 	mutable uint32_t    evaluation_index = UINT32_MAX;
 	uint64_t            flags;
 	Block*              parent = nullptr;

src/graphics/shader/recompiler/ir/passes/DeadCodeElimination.cpp

@@ -1,5 +1,7 @@
 #include "graphics/shader/recompiler/ir/passes/DeadCodeElimination.h"
 
+#include <algorithm>
+
 namespace Libs::Graphics::ShaderRecompiler::IR {
 
 void RemoveIdentities(const BlockList& blocks) {
@@ -18,23 +20,32 @@ void RemoveIdentities(const BlockList& blocks) {
 }
 
 void EliminateDeadCode(const BlockList& blocks) {
-	bool changed;
-	do {
-		changed = false;
-		for (auto block = blocks.rbegin(); block != blocks.rend(); block++) {
-			auto& instructions = (*block)->Instructions();
-			auto  inst         = instructions.end();
-			while (inst != instructions.begin()) {
-				--inst;
-				if (inst->HasUses() || inst->MayHaveSideEffects()) {
-					continue;
-				}
-				inst->Invalidate();
-				inst    = instructions.erase(inst);
-				changed = true;
+	std::vector<Inst*> pending;
+	for (auto* block: blocks) {
+		for (auto& inst: *block) {
+			// Retained resource-planning instructions live outside the block list.
+			inst.live = inst.MayHaveSideEffects() || std::ranges::any_of(inst.Uses(),
+			    [](const Use& use) { return use.user->Parent() == nullptr; });
+			if (inst.live) pending.push_back(&inst);
+		}
+	}
+	while (!pending.empty()) {
+		const auto* inst = pending.back();
+		pending.pop_back();
+		for (size_t arg = 0; arg < inst->NumArgs(); ++arg) {
+			auto* input = inst->Arg(arg).TryInstruction();
+			if (input != nullptr && input->Parent() != nullptr && !input->live) {
+				input->live = true;
+				pending.push_back(input);
 			}
 		}
-	} while (changed);
+	}
+	// Unlink every dead operand before freeing nodes in a dead Phi cycle.
+	for (auto* block: blocks)
+		for (auto& inst: *block)
+			if (!inst.live) inst.Invalidate();
+	for (auto* block: blocks)
+		std::erase_if(block->Instructions(), [](const Inst& inst) { return !inst.live; });
 }
 
 } // namespace Libs::Graphics::ShaderRecompiler::IR

tests/ScalarProvenanceTests.cpp

@@ -650,6 +650,39 @@ void TestControlFlowValueSurvivesReadLaneFolding() {
   ValidateProgram(fixture.program, true);
 }
 
+void TestDeadPhiCyclesAndPlanningRoots() {
+  Fixture fixture(2);
+  auto &entry = fixture.BlockAt(0);
+  auto &loop = fixture.BlockAt(1);
+  entry.AddBranch(&loop);
+  loop.AddBranch(&loop);
+  const auto source = fixture.Emit(ValueOpcode::GetUserData, {Value(static_cast<ScalarReg>(2))});
+  const auto identity = fixture.Emit(ValueOpcode::Identity, {source});
+  auto &live = loop.AppendNewInst(ValueOpcode::Phi, {}, uint64_t(Type::U32));
+  live.AddPhiOperand(&entry, identity);
+  live.AddPhiOperand(&loop, Value(&live));
+  const auto reference = fixture.Emit(ValueOpcode::ReferenceU32, {Value(&live)}, 0, 1);
+  auto &dead = loop.AppendNewInst(ValueOpcode::Phi, {}, uint64_t(Type::U32));
+  const auto increment = fixture.Emit(ValueOpcode::IAdd32, {Value(&dead), Value(1u)}, 0, 1);
+  dead.AddPhiOperand(&entry, source);
+  dead.AddPhiOperand(&loop, increment);
+  const auto retained = fixture.Emit(ValueOpcode::GetUserData, {Value(static_cast<ScalarReg>(3))});
+  auto &planning = fixture.program.value_storage.emplace_back(ValueOpcode::Identity);
+  planning.SetArg(0, retained);
+
+  EliminateDeadCode(fixture.program.blocks);
+  Check(entry.Instructions().size() == 3 && loop.Instructions().size() == 2,
+        "dead Phi cycle survived or a live/planning dependency was removed");
+  Check(identity.Instruction()->Arg(0) == source && live.Arg(0) == identity &&
+            live.Arg(1) == Value(&live) && planning.Arg(0) == retained,
+        "DCE did not preserve direct Identity operands or live Phi recurrence");
+  // Dropping roots must clear prior marks and remove the newly dead SCC safely.
+  reference.Instruction()->Invalidate();
+  planning.Invalidate();
+  EliminateDeadCode(fixture.program.blocks);
+  Check(entry.empty() && loop.empty(), "DCE reused stale marks after removing its roots");
+}
+
 void TestUndefinedRuntimeValueFails() {
   Fixture fixture;
   const auto undef = fixture.Emit(ValueOpcode::UndefU32);
@@ -699,6 +732,7 @@ int main() {
     TestReadLaneElimination();
     TestOptimizationPipeline();
     TestControlFlowValueSurvivesReadLaneFolding();
+    TestDeadPhiCyclesAndPlanningRoots();
     TestUndefinedRuntimeValueFails();
     std::cout << "TypedValuePlanningTests: all cases passed\n";
     return 0;