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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 <memory>
#include <unordered_map>
#include "source/opt/constants.h"
#include "source/opt/function.h"
#include "source/opt/ir_context.h"
#include "source/opt/mem_pass.h"
#include "source/opt/module.h"
#include "source/opt/propagator.h"
namespace spvtools {
namespace opt {
class CCPPass : public MemPass {
CCPPass() = default;
const char* name() const override { return "ccp"; }
Status Process() override;
IRContext::Analysis GetPreservedAnalyses() override {
return IRContext::kAnalysisDefUse |
IRContext::kAnalysisInstrToBlockMapping |
IRContext::kAnalysisDecorations | IRContext::kAnalysisCombinators |
IRContext::kAnalysisCFG | IRContext::kAnalysisDominatorAnalysis |
IRContext::kAnalysisNameMap | IRContext::kAnalysisConstants |
// Initializes the pass.
void Initialize();
// Runs constant propagation on the given function |fp|. Returns true if any
// constants were propagated and the IR modified.
bool PropagateConstants(Function* fp);
// Visits a single instruction |instr|. If the instruction is a conditional
// branch that always jumps to the same basic block, it sets the destination
// block in |dest_bb|.
SSAPropagator::PropStatus VisitInstruction(Instruction* instr,
BasicBlock** dest_bb);
// Visits an OpPhi instruction |phi|. This applies the meet operator for the
// CCP lattice. Essentially, if all the operands in |phi| have the same
// constant value C, the result for |phi| gets assigned the value C.
SSAPropagator::PropStatus VisitPhi(Instruction* phi);
// Visits an SSA assignment instruction |instr|. If the RHS of |instr| folds
// into a constant value C, then the LHS of |instr| is assigned the value C in
// |values_|.
SSAPropagator::PropStatus VisitAssignment(Instruction* instr);
// Visits a branch instruction |instr|. If the branch is conditional
// (OpBranchConditional or OpSwitch), and the value of its selector is known,
// |dest_bb| will be set to the corresponding destination block. Unconditional
// branches always set |dest_bb| to the single destination block.
SSAPropagator::PropStatus VisitBranch(Instruction* instr,
BasicBlock** dest_bb) const;
// Replaces all operands used in |fp| with the corresponding constant values
// in |values_|. Returns true if any operands were replaced, and false
// otherwise.
bool ReplaceValues();
// Marks |instr| as varying by registering a varying value for its result
// into the |values_| table. Returns SSAPropagator::kVarying.
SSAPropagator::PropStatus MarkInstructionVarying(Instruction* instr);
// Returns true if |id| is the special SSA id that corresponds to a varying
// value.
bool IsVaryingValue(uint32_t id) const;
// Constant manager for the parent IR context. Used to record new constants
// generated during propagation.
analysis::ConstantManager* const_mgr_;
// Returns a new value for |instr| by computing the meet operation between
// its existing value and |val2|.
// Given two values val1 and val2, the meet operation in the constant
// lattice uses the following rules:
// meet(val1, UNDEFINED) = val1
// meet(val1, VARYING) = VARYING
// meet(val1, val2) = val1 if val1 == val2
// meet(val1, val2) = VARYING if val1 != val2
// When two different values meet, the result is always varying because CCP
// does not allow lateral transitions in the lattice. This prevents
// infinite cycles during propagation.
uint32_t ComputeLatticeMeet(Instruction* instr, uint32_t val2);
// Constant value table. Each entry <id, const_decl_id> in this map
// represents the compile-time constant value for |id| as declared by
// |const_decl_id|. Each |const_decl_id| in this table is an OpConstant
// declaration for the current module.
// Additionally, this table keeps track of SSA IDs with varying values. If an
// SSA ID is found to have a varying value, it will have an entry in this
// table that maps to the special SSA id kVaryingSSAId. These values are
// never replaced in the IR, they are used by CCP during propagation.
std::unordered_map<uint32_t, uint32_t> values_;
// Propagator engine used.
std::unique_ptr<SSAPropagator> propagator_;
// Value for the module's ID bound before running CCP. Used to detect whether
// propagation created new instructions.
uint32_t original_id_bound_;
} // namespace opt
} // namespace spvtools