KytyPS5/src/graphics/shader/recompiler/ir/passes/ReadLaneElimination.cpp

#include "graphics/shader/recompiler/ir/passes/ReadLaneElimination.h"

#include <algorithm>

#include <queue>

#include <unordered_map>
#include <unordered_set>
#include <vector>

namespace Libs::Graphics::ShaderRecompiler::IR {
namespace {

struct ChainResult {
	Value value;
	Inst* write = nullptr;
};

ChainResult SearchChain(Value value, uint32_t lane, uint32_t wave_size) {
	for (;;) {
		value      = value.Resolve();
		auto* inst = value.TryInstruction();
		if (inst == nullptr || inst->GetOpcode() != ValueOpcode::WriteLane) {
			return {value};
		}
		const auto selector = inst->Arg(2).Resolve();
		if (!selector.IsImmediate() || selector.GetType() != Type::U32) {
			return {value};
		}
		if (selector.U32() % wave_size == lane) {
			return {value, inst};
		}
		value = inst->Arg(0);
	}
}

bool IsPossibleToEliminate(Value source, uint32_t lane, uint32_t wave_size) {
	std::queue<Value>         queue;
	std::unordered_set<Inst*> visited;
	queue.push(source);

	while (!queue.empty()) {
		const auto chain = SearchChain(queue.front(), lane, wave_size);
		queue.pop();
		if (chain.write != nullptr) {
			continue;
		}
		auto* inst = chain.value.TryInstruction();
		if (inst == nullptr || inst->GetOpcode() != ValueOpcode::Phi || inst->NumArgs() == 0) {
			return false;
		}
		if (!visited.insert(inst).second) {
			continue;
		}
		for (size_t index = inst->NumArgs(); index-- > 0;) {
			queue.push(inst->Arg(index));
		}
	}
	return true;
}

using PhiMap = std::unordered_map<Inst*, Inst*>;

Value GetRealValue(PhiMap& phi_map, Value source, uint32_t lane, uint32_t wave_size) {
	const auto chain = SearchChain(source, lane, wave_size);
	if (chain.write != nullptr) {
		return chain.write->Arg(1);
	}

	auto* inst = chain.value.ResolveInstruction();
	EXIT_IF(inst->GetOpcode() != ValueOpcode::Phi);
	const auto [entry, is_new] = phi_map.try_emplace(inst);
	if (!is_new) {
		return Value(entry->second);
	}

	auto* block           = inst->Parent();
	auto  insertion_point = std::find_if(block->begin(), block->end(),
	                                     [&](const Inst& candidate) { return &candidate == inst; });
	EXIT_IF(insertion_point == block->end());
	auto& phi = *block->PrependNewInst(insertion_point, ValueOpcode::Phi);
	phi.SetFlags(Type::U32);
	entry->second = &phi;

	std::vector<Value> arguments;
	arguments.reserve(inst->NumArgs());
	for (size_t index = 0; index < inst->NumArgs(); index++) {
		arguments.push_back(GetRealValue(phi_map, inst->Arg(index), lane, wave_size));
	}
	const auto first = arguments.front().Resolve();
	if (std::ranges::all_of(arguments,
	                        [&](Value argument) { return argument.Resolve() == first; })) {
		phi.ReplaceUsesWith(first);
	} else {
		for (size_t index = 0; index < arguments.size(); index++) {
			phi.AddPhiOperand(inst->PhiBlock(index), arguments[index]);
		}
	}
	return Value(&phi);
}

} // namespace

ReadLaneStats EliminateReadLane(Program& program, uint32_t wave_size) {
	ReadLaneStats stats;
	if (wave_size != 32u && wave_size != 64u) {
		return stats;
	}

	for (auto* block: program.blocks) {
		for (auto& inst: *block) {
			if (inst.GetOpcode() != ValueOpcode::ReadLane) {
				continue;
			}
			const auto selector = inst.Arg(1).Resolve();
			if (!selector.IsImmediate() || selector.GetType() != Type::U32) {
				continue;
			}

			const auto lane  = selector.U32() % wave_size;
			const auto chain = SearchChain(inst.Arg(0), lane, wave_size);
			if (chain.write != nullptr) {
				inst.ReplaceUsesWith(chain.write->Arg(1));
				stats.rewritten_reads++;
				continue;
			}
			auto* producer = chain.value.TryInstruction();
			if (producer == nullptr || producer->GetOpcode() != ValueOpcode::Phi ||
			    !IsPossibleToEliminate(chain.value, lane, wave_size)) {
				continue;
			}

			PhiMap phi_map;
			inst.ReplaceUsesWith(GetRealValue(phi_map, chain.value, lane, wave_size));
			stats.rewritten_reads++;
		}
	}
	return stats;
}

} // namespace Libs::Graphics::ShaderRecompiler::IR