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// Copyright (c) 2018 Google LLC.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include <queue>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#include "source/opt/basic_block.h"
#include "source/opt/ir_context.h"
#include "source/opt/mem_pass.h"
namespace spvtools {
namespace opt {
// Utility class for passes that need to rewrite a function into SSA. This
// converts load/store operations on function-local variables into SSA IDs,
// which allows them to be the target of optimizing transformations.
// Store and load operations to these variables are converted into
// operations on SSA IDs. Phi instructions are added when needed. See the
// SSA construction paper for algorithmic details
// (
class SSARewriter {
SSARewriter(MemPass* pass) : pass_(pass) {}
// Rewrites SSA-target variables in function |fp| into SSA. This is the
// entry point for the SSA rewrite algorithm. SSA-target variables are
// locally defined variables that meet the criteria set by IsSSATargetVar.
// Returns whether the function was modified or not, and whether or not the
// rewrite was successful.
Pass::Status RewriteFunctionIntoSSA(Function* fp);
class PhiCandidate {
explicit PhiCandidate(uint32_t var, uint32_t result, BasicBlock* block)
: var_id_(var),
users_() {}
uint32_t var_id() const { return var_id_; }
uint32_t result_id() const { return result_id_; }
BasicBlock* bb() const { return bb_; }
std::vector<uint32_t>& phi_args() { return phi_args_; }
const std::vector<uint32_t>& phi_args() const { return phi_args_; }
uint32_t copy_of() const { return copy_of_; }
bool is_complete() const { return is_complete_; }
std::vector<uint32_t>& users() { return users_; }
const std::vector<uint32_t>& users() const { return users_; }
// Marks this phi candidate as a trivial copy of |orig_id|.
void MarkCopyOf(uint32_t orig_id) { copy_of_ = orig_id; }
// Marks this phi candidate as incomplete.
void MarkIncomplete() { is_complete_ = false; }
// Marks this phi candidate as complete.
void MarkComplete() { is_complete_ = true; }
// Returns true if this Phi candidate is ready to be emitted.
bool IsReady() const { return is_complete() && copy_of() == 0; }
// Pretty prints this Phi candidate into a string and returns it. |cfg| is
// needed to lookup basic block predecessors.
std::string PrettyPrint(const CFG* cfg) const;
// Registers |operand_id| as a user of this Phi candidate.
void AddUser(uint32_t operand_id) { users_.push_back(operand_id); }
// Variable ID that this Phi is merging.
uint32_t var_id_;
// SSA ID generated by this Phi (i.e., this is the result ID of the eventual
// Phi instruction).
uint32_t result_id_;
// Basic block to hold this Phi.
BasicBlock* bb_;
// Vector of operands for every predecessor block of |bb|. This vector is
// organized so that the Ith slot contains the argument coming from the Ith
// predecessor of |bb|.
std::vector<uint32_t> phi_args_;
// If this Phi is a trivial copy of another Phi, this is the ID of the
// original. If this is 0, it means that this is not a trivial Phi.
uint32_t copy_of_;
// False, if this Phi candidate has no arguments or at least one argument is
// %0.
bool is_complete_;
// List of all users for this Phi instruction. Each element is the result ID
// of the load instruction replaced by this Phi, or the result ID of a Phi
// candidate that has this Phi in its list of operands.
std::vector<uint32_t> users_;
// Type used to keep track of store operations in each basic block.
typedef std::unordered_map<BasicBlock*,
std::unordered_map<uint32_t, uint32_t>>
// Generates all the SSA rewriting decisions for basic block |bb|. This
// populates the Phi candidate table (|phi_candidate_|) and the load
// replacement table (|load_replacement_). Returns true if successful.
bool GenerateSSAReplacements(BasicBlock* bb);
// Seals block |bb|. Sealing a basic block means |bb| and all its
// predecessors of |bb| have been scanned for loads/stores.
void SealBlock(BasicBlock* bb);
// Returns true if |bb| has been sealed.
bool IsBlockSealed(BasicBlock* bb) { return sealed_blocks_.count(bb) != 0; }
// Returns the Phi candidate with result ID |id| if it exists in the table
// |phi_candidates_|. If no such Phi candidate exists, it returns nullptr.
PhiCandidate* GetPhiCandidate(uint32_t id) {
auto it = phi_candidates_.find(id);
return (it != phi_candidates_.end()) ? &it->second : nullptr;
// Replaces all the users of Phi candidate |phi_cand| to be users of
// |repl_id|.
void ReplacePhiUsersWith(const PhiCandidate& phi_cand, uint32_t repl_id);
// Returns the value ID that should replace the load ID in the given
// replacement pair |repl|. The replacement is a pair (|load_id|, |val_id|).
// If |val_id| is itself replaced by another value in the table, this function
// will look the replacement for |val_id| until it finds one that is not
// itself replaced. For instance, given:
// %34 = OpLoad %float %f1
// OpStore %t %34
// %36 = OpLoad %float %t
// Assume that %f1 is reached by a Phi candidate %42, the load
// replacement table will have the following entries:
// %34 -> %42
// %36 -> %34
// So, when looking for the replacement for %36, we should not use
// %34. Rather, we should use %42. To do this, the chain of
// replacements must be followed until we reach an element that has
// no replacement.
uint32_t GetReplacement(std::pair<uint32_t, uint32_t> repl);
// Returns the argument at index |ix| from |phi_candidate|. If argument |ix|
// comes from a trivial Phi, it follows the copy-of chain from that trivial
// Phi until it finds the original Phi candidate.
// This is only valid after all Phi candidates have been completed. It can
// only be called when generating the IR for these Phis.
uint32_t GetPhiArgument(const PhiCandidate* phi_candidate, uint32_t ix);
// Applies all the SSA replacement decisions. This replaces loads/stores to
// SSA target variables with their corresponding SSA IDs, and inserts Phi
// instructions for them.
bool ApplyReplacements();
// Registers a definition for variable |var_id| in basic block |bb| with
// value |val_id|.
void WriteVariable(uint32_t var_id, BasicBlock* bb, uint32_t val_id) {
defs_at_block_[bb][var_id] = val_id;
if (auto* pc = GetPhiCandidate(val_id)) {
// Returns the value of |var_id| at |bb| if |defs_at_block_| contains it.
// Otherwise, returns 0.
uint32_t GetValueAtBlock(uint32_t var_id, BasicBlock* bb);
// Processes the store operation |inst| in basic block |bb|. This extracts
// the variable ID being stored into, determines whether the variable is an
// SSA-target variable, and, if it is, it stores its value in the
// |defs_at_block_| map.
void ProcessStore(Instruction* inst, BasicBlock* bb);
// Processes the load operation |inst| in basic block |bb|. This extracts
// the variable ID being stored into, determines whether the variable is an
// SSA-target variable, and, if it is, it reads its reaching definition by
// calling |GetReachingDef|. Returns true if successful.
bool ProcessLoad(Instruction* inst, BasicBlock* bb);
// Reads the current definition for variable |var_id| in basic block |bb|.
// If |var_id| is not defined in block |bb| it walks up the predecessors of
// |bb|, creating new Phi candidates along the way, if needed.
// It returns the value for |var_id| from the RHS of the current reaching
// definition for |var_id|.
uint32_t GetReachingDef(uint32_t var_id, BasicBlock* bb);
// Adds arguments to |phi_candidate| by getting the reaching definition of
// |phi_candidate|'s variable on each of the predecessors of its basic
// block. After populating the argument list, it determines whether all its
// arguments are the same. If so, it returns the ID of the argument that
// this Phi copies.
uint32_t AddPhiOperands(PhiCandidate* phi_candidate);
// Creates a Phi candidate instruction for variable |var_id| in basic block
// |bb|.
// Since the rewriting algorithm may remove Phi candidates when it finds
// them to be trivial, we avoid the expense of creating actual Phi
// instructions by keeping a pool of Phi candidates (|phi_candidates_|)
// during rewriting.
// Once the candidate Phi is created, it returns its ID.
PhiCandidate& CreatePhiCandidate(uint32_t var_id, BasicBlock* bb);
// Attempts to remove a trivial Phi candidate |phi_cand|. Trivial Phis are
// those that only reference themselves and one other value |val| any number
// of times. This will try to remove any other Phis that become trivial
// after |phi_cand| is removed.
// If |phi_cand| is trivial, it returns the SSA ID for the value that should
// replace it. Otherwise, it returns the SSA ID for |phi_cand|.
uint32_t TryRemoveTrivialPhi(PhiCandidate* phi_cand);
// Finalizes |phi_candidate| by replacing every argument that is still %0
// with its reaching definition.
void FinalizePhiCandidate(PhiCandidate* phi_candidate);
// Finalizes processing of Phi candidates. Once the whole function has been
// scanned for loads and stores, the CFG will still have some incomplete and
// trivial Phis. This will add missing arguments and remove trivial Phi
// candidates.
void FinalizePhiCandidates();
// Adds DebugValues for DebugDeclares in
// |decls_invisible_to_value_assignment_|. Returns whether the function was
// modified or not, and whether or not the conversion was successful.
Pass::Status AddDebugValuesForInvisibleDebugDecls(Function* fp);
// Prints the table of Phi candidates to std::cerr.
void PrintPhiCandidates() const;
// Prints the load replacement table to std::cerr.
void PrintReplacementTable() const;
// Map holding the value of every SSA-target variable at every basic block
// where the variable is stored. defs_at_block_[block][var_id] = val_id
// means that there is a store or Phi instruction for variable |var_id| at
// basic block |block| with value |val_id|.
BlockDefsMap defs_at_block_;
// Map, indexed by Phi ID, holding all the Phi candidates created during SSA
// rewriting. |phi_candidates_[id]| returns the Phi candidate whose result
// is |id|.
std::unordered_map<uint32_t, PhiCandidate> phi_candidates_;
// Queue of incomplete Phi candidates. These are Phi candidates created at
// unsealed blocks. They need to be completed before they are instantiated
// in ApplyReplacements.
std::queue<PhiCandidate*> incomplete_phis_;
// List of completed Phi candidates. These are the only candidates that
// will become real Phi instructions.
std::vector<PhiCandidate*> phis_to_generate_;
// SSA replacement table. This maps variable IDs, resulting from a load
// operation, to the value IDs that will replace them after SSA rewriting.
// After all the rewriting decisions are made, a final scan through the IR
// is done to replace all uses of the original load ID with the value ID.
std::unordered_map<uint32_t, uint32_t> load_replacement_;
// Set of blocks that have been sealed already.
std::unordered_set<BasicBlock*> sealed_blocks_;
// Memory pass requesting the SSA rewriter.
MemPass* pass_;
// Set of DebugDeclare instructions that are not added as DebugValue because
// they are invisible to the store or phi instructions.
std::unordered_set<Instruction*> decls_invisible_to_value_assignment_;
class SSARewritePass : public MemPass {
SSARewritePass() = default;
const char* name() const override { return "ssa-rewrite"; }
Status Process() override;
} // namespace opt
} // namespace spvtools