SPIRV-Tools/source/opt/local_redundancy_elimination.h
Steven Perron 5d602abd66 Add global redundancy elimination
Adds a pass that looks for redundant instruction in a function, and
removes them.  The algorithm is a hash table based value numbering
algorithm that traverses the dominator tree.

This pass removes completely redundant instructions, not partially
redundant ones.
2017-12-07 18:35:38 -05:00

65 lines
2.5 KiB
C++

// 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
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#ifndef LIBSPIRV_OPT_LOCAL_REDUNDANCY_ELIMINATION_H_
#define LIBSPIRV_OPT_LOCAL_REDUNDANCY_ELIMINATION_H_
#include "ir_context.h"
#include "pass.h"
#include "value_number_table.h"
namespace spvtools {
namespace opt {
// This pass implements local redundancy elimination. Its goal is to reduce the
// number of times the same value is computed. It works on each basic block
// independently, ie local. For each instruction in a basic block, it gets the
// value number for the result id, |id|, of the instruction. If that value
// number has already been computed in the basic block, it tries to replace the
// uses of |id| by the id that already contains the same value. Then the
// current instruction is deleted.
class LocalRedundancyEliminationPass : public Pass {
public:
const char* name() const override { return "local-redundancy-elimination"; }
Status Process(ir::IRContext*) override;
virtual ir::IRContext::Analysis GetPreservedAnalyses() override {
return ir::IRContext::kAnalysisDefUse |
ir::IRContext::kAnalysisInstrToBlockMapping |
ir::IRContext::kAnalysisDecorations |
ir::IRContext::kAnalysisCombinators | ir::IRContext::kAnalysisCFG |
ir::IRContext::kAnalysisDominatorAnalysis;
}
protected:
// Deletes instructions in |block| whose value is in |value_to_ids| or is
// computed earlier in |block|.
//
// |vnTable| must have computed a value number for every result id defined
// in |bb|.
//
// |value_to_ids| is a map from value number to ids. If {vn, id} is in
// |value_to_ids| then vn is the value number of id, and the definition of id
// dominates |bb|.
//
// Returns true if the module is changed.
bool EliminateRedundanciesInBB(ir::BasicBlock* block,
const ValueNumberTable& vnTable,
std::map<uint32_t, uint32_t>* value_to_ids);
};
} // namespace opt
} // namespace spvtools
#endif // LIBSPIRV_OPT_LOCAL_REDUNDANCY_ELIMINATION_H_