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// Copyright (c) 2017 Google Inc.
// 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 <functional>
#include <queue>
#include <set>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#include "source/opt/ir_context.h"
#include "source/opt/module.h"
namespace spvtools {
namespace opt {
// Represents a CFG control edge.
struct Edge {
Edge(BasicBlock* b1, BasicBlock* b2) : source(b1), dest(b2) {
assert(source && "CFG edges cannot have a null source block.");
assert(dest && "CFG edges cannot have a null destination block.");
BasicBlock* source;
BasicBlock* dest;
bool operator<(const Edge& o) const {
return std::make_pair(source->id(), dest->id()) <
std::make_pair(o.source->id(), o.dest->id());
// This class implements a generic value propagation algorithm based on the
// conditional constant propagation algorithm proposed in
// Constant propagation with conditional branches,
// Wegman and Zadeck, ACM TOPLAS 13(2):181-210.
// A Propagation Engine for GCC
// Diego Novillo, GCC Summit 2005
// The purpose of this implementation is to act as a common framework for any
// transformation that needs to propagate values from statements producing new
// values to statements using those values. Simulation proceeds as follows:
// 1- Initially, all edges of the CFG are marked not executable and the CFG
// worklist is seeded with all the statements in the entry basic block.
// 2- Every instruction I is simulated by calling a pass-provided function
// |visit_fn|. This function is responsible for three things:
// (a) Keep a value table of interesting values. This table maps SSA IDs to
// their values. For instance, when implementing constant propagation,
// given a store operation 'OpStore %f %int_3', |visit_fn| should assign
// the value 3 to the table slot for %f.
// In general, |visit_fn| will need to use the value table to replace its
// operands, fold the result and decide whether a new value needs to be
// stored in the table. |visit_fn| should only create a new mapping in
// the value table if all the operands in the instruction are known and
// present in the value table.
// (b) Return a status indicator to direct the propagator logic. Once the
// instruction is simulated, the propagator needs to know whether this
// instruction produced something interesting. This is indicated via
// |visit_fn|'s return value:
// SSAPropagator::kNotInteresting: Instruction I produces nothing of
// interest and does not affect any of the work lists. The
// propagator will visit the statement again if any of its operands
// produce an interesting value in the future.
// |visit_fn| should always return this value when it is not sure
// whether the instruction will produce an interesting value in the
// future or not. For instance, for constant propagation, an OpIAdd
// instruction may produce a constant if its two operands are
// constant, but the first time we visit the instruction, we still
// may not have its operands in the value table.
// SSAPropagator::kVarying: The value produced by I cannot be determined
// at compile time. Further simulation of I is not required. The
// propagator will not visit this instruction again. Additionally,
// the propagator will add all the instructions at the end of SSA
// def-use edges to be simulated again.
// If I is a basic block terminator, it will mark all outgoing edges
// as executable so they are traversed one more time. Eventually
// the kVarying attribute will be spread out to all the data and
// control dependents for I.
// It is important for propagation to use kVarying as a bottom value
// for the propagation lattice. It should never be possible for an
// instruction to return kVarying once and kInteresting on a second
// visit. Otherwise, propagation would not stabilize.
// SSAPropagator::kInteresting: Instruction I produces a value that can
// be computed at compile time. In this case, |visit_fn| should
// create a new mapping between I's result ID and the produced
// value. Much like the kNotInteresting case, the propagator will
// visit this instruction again if any of its operands changes.
// This is useful when the statement changes from one interesting
// state to another.
// (c) For conditional branches, |visit_fn| may decide which edge to take out
// of I's basic block. For example, if the operand for an OpSwitch is
// known to take a specific constant value, |visit_fn| should figure out
// the destination basic block and pass it back by setting the second
// argument to |visit_fn|.
// At the end of propagation, values in the value table are guaranteed to be
// stable and can be replaced in the IR.
// 3- The propagator keeps two work queues. Instructions are only added to
// these queues if they produce an interesting or varying value. None of this
// should be handled by |visit_fn|. The propagator keeps track of this
// automatically (see SSAPropagator::Simulate for implementation).
// CFG blocks: contains the queue of blocks to be simulated.
// Blocks are added to this queue if their incoming edges are
// executable.
// SSA Edges: An SSA edge is a def-use edge between a value-producing
// instruction and its use instruction. The SSA edges list
// contains the statements at the end of a def-use edge that need
// to be re-visited when an instruction produces a kVarying or
// kInteresting result.
// 4- Simulation terminates when all work queues are drained.
// EXAMPLE: Basic constant store propagator.
// Suppose we want to propagate all constant assignments of the form "OpStore
// %id %cst" where "%id" is some variable and "%cst" an OpConstant. The
// following code builds a table |values| where every id that was assigned a
// constant value is mapped to the constant value it was assigned.
// auto ctx = BuildModule(...);
// std::map<uint32_t, uint32_t> values;
// const auto visit_fn = [&ctx, &values](Instruction* instr,
// BasicBlock** dest_bb) {
// if (instr->opcode() == SpvOpStore) {
// uint32_t rhs_id = instr->GetSingleWordOperand(1);
// Instruction* rhs_def = ctx->get_def_use_mgr()->GetDef(rhs_id);
// if (rhs_def->opcode() == SpvOpConstant) {
// uint32_t val = rhs_def->GetSingleWordOperand(2);
// values[rhs_id] = val;
// return SSAPropagator::kInteresting;
// }
// }
// return SSAPropagator::kVarying;
// };
// SSAPropagator propagator(ctx.get(), &cfg, visit_fn);
// propagator.Run(&fn);
// Given the code:
// %int_4 = OpConstant %int 4
// %int_3 = OpConstant %int 3
// %int_1 = OpConstant %int 1
// OpStore %x %int_4
// OpStore %y %int_3
// OpStore %z %int_1
// After SSAPropagator::Run returns, the |values| map will contain the entries:
// values[%x] = 4, values[%y] = 3, and, values[%z] = 1.
class SSAPropagator {
// Lattice values used for propagation. See class documentation for
// a description.
enum PropStatus { kNotInteresting, kInteresting, kVarying };
using VisitFunction = std::function<PropStatus(Instruction*, BasicBlock**)>;
SSAPropagator(IRContext* context, const VisitFunction& visit_fn)
: ctx_(context), visit_fn_(visit_fn) {}
// Runs the propagator on function |fn|. Returns true if changes were made to
// the function. Otherwise, it returns false.
bool Run(Function* fn);
// Returns true if the |i|th argument for |phi| comes through a CFG edge that
// has been marked executable. |i| should be an index value accepted by
// Instruction::GetSingleWordOperand.
bool IsPhiArgExecutable(Instruction* phi, uint32_t i) const;
// Returns true if |inst| has a recorded status. This will be true once |inst|
// has been simulated once.
bool HasStatus(Instruction* inst) const { return statuses_.count(inst); }
// Returns the current propagation status of |inst|. Assumes
// |HasStatus(inst)| returns true.
PropStatus Status(Instruction* inst) const {
return statuses_.find(inst)->second;
// Records the propagation status |status| for |inst|. Returns true if the
// status for |inst| has changed or set was set for the first time.
bool SetStatus(Instruction* inst, PropStatus status);
// Initialize processing.
void Initialize(Function* fn);
// Simulate the execution |block| by calling |visit_fn_| on every instruction
// in it.
bool Simulate(BasicBlock* block);
// Simulate the execution of |instr| by replacing all the known values in
// every operand and determining whether the result is interesting for
// propagation. This invokes the callback function |visit_fn_| to determine
// the value computed by |instr|.
bool Simulate(Instruction* instr);
// Returns true if |instr| should be simulated again.
bool ShouldSimulateAgain(Instruction* instr) const {
return do_not_simulate_.find(instr) == do_not_simulate_.end();
// Add |instr| to the set of instructions not to simulate again.
void DontSimulateAgain(Instruction* instr) { do_not_simulate_.insert(instr); }
// Returns true if |block| has been simulated already.
bool BlockHasBeenSimulated(BasicBlock* block) const {
return simulated_blocks_.find(block) != simulated_blocks_.end();
// Marks block |block| as simulated.
void MarkBlockSimulated(BasicBlock* block) {
// Marks |edge| as executable. Returns false if the edge was already marked
// as executable.
bool MarkEdgeExecutable(const Edge& edge) {
return executable_edges_.insert(edge).second;
// Returns true if |edge| has been marked as executable.
bool IsEdgeExecutable(const Edge& edge) const {
return executable_edges_.find(edge) != executable_edges_.end();
// Returns a pointer to the def-use manager for |ctx_|.
analysis::DefUseManager* get_def_use_mgr() const {
return ctx_->get_def_use_mgr();
// If the CFG edge |e| has not been executed, this function adds |e|'s
// destination block to the work list.
void AddControlEdge(const Edge& e);
// Adds all the instructions that use the result of |instr| to the SSA edges
// work list. If |instr| produces no result id, this does nothing.
void AddSSAEdges(Instruction* instr);
// IR context to use.
IRContext* ctx_;
// Function that visits instructions during simulation. The output of this
// function is used to determine if the simulated instruction produced a value
// interesting for propagation. The function is responsible for keeping
// track of interesting values by storing them in some user-provided map.
VisitFunction visit_fn_;
// SSA def-use edges to traverse. Each entry is a destination statement for an
// SSA def-use edge as returned by |def_use_manager_|.
std::queue<Instruction*> ssa_edge_uses_;
// Blocks to simulate.
std::queue<BasicBlock*> blocks_;
// Blocks simulated during propagation.
std::unordered_set<BasicBlock*> simulated_blocks_;
// Set of instructions that should not be simulated again because they have
// been found to be in the kVarying state.
std::unordered_set<Instruction*> do_not_simulate_;
// Map between a basic block and its predecessor edges.
// TODO(dnovillo): Move this to CFG and always build them. Alternately,
// move it to IRContext and build CFG preds/succs on-demand.
std::unordered_map<BasicBlock*, std::vector<Edge>> bb_preds_;
// Map between a basic block and its successor edges.
// TODO(dnovillo): Move this to CFG and always build them. Alternately,
// move it to IRContext and build CFG preds/succs on-demand.
std::unordered_map<BasicBlock*, std::vector<Edge>> bb_succs_;
// Set of executable CFG edges.
std::set<Edge> executable_edges_;
// Tracks instruction propagation status.
std::unordered_map<Instruction*, SSAPropagator::PropStatus> statuses_;
std::ostream& operator<<(std::ostream& str,
const SSAPropagator::PropStatus& status);
} // namespace opt
} // namespace spvtools