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https://github.com/KhronosGroup/SPIRV-Tools
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7d01643132
When doing if-conversion, we do not currently move code out of the side nodes. The reason for this is that it can increase the number of instructions that get executed because both side nods will have to be executed now. In this commit, we add code to move an instruction, and all of the instructions it depends on, out of a side node and into the header of the selection construct. However to keep the cost down, we only do it when the two values in the OpPhi node compute the same value. This way we have to move only one of the instructions and the other becomes unused most of the time. So no real extra cost. Makes the value number table an alalysis in the ir context. Added more opcodes to list of code motion safe opcodes. Fixes #1526.
96 lines
3.2 KiB
C++
96 lines
3.2 KiB
C++
// Copyright (c) 2017 Google Inc.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef LIBSPIRV_OPT_VALUE_NUMBER_TABLE_H_
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#define LIBSPIRV_OPT_VALUE_NUMBER_TABLE_H_
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#include <cstdint>
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#include <unordered_map>
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#include "instruction.h"
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namespace spvtools {
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namespace ir {
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class IRContext;
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}
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} // namespace spvtools
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namespace spvtools {
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namespace opt {
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// Returns true if the two instructions compute the same value. Used by the
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// value number table to compare two instructions.
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class ComputeSameValue {
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public:
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bool operator()(const ir::Instruction& lhs, const ir::Instruction& rhs) const;
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};
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// The hash function used in the value number table.
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class ValueTableHash {
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public:
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std::size_t operator()(const spvtools::ir::Instruction& inst) const;
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};
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// This class implements the value number analysis. It is using a hash-based
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// approach to value numbering. It is essentially doing dominator-tree value
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// numbering described in
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//
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// Preston Briggs, Keith D. Cooper, and L. Taylor Simpson. 1997. Value
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// numbering. Softw. Pract. Exper. 27, 6 (June 1997), 701-724.
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// https://www.cs.rice.edu/~keith/Promo/CRPC-TR94517.pdf.gz
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//
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// The main difference is that because we do not perform redundancy elimination
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// as we build the value number table, we do not have to deal with cleaning up
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// the scope.
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class ValueNumberTable {
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public:
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ValueNumberTable(ir::IRContext* ctx) : context_(ctx), next_value_number_(1) {
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BuildDominatorTreeValueNumberTable();
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}
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// Returns the value number of the value computed by |inst|. |inst| must have
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// a result id that will hold the computed value. If no value number has been
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// assigned to the result id, then the return value is 0.
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uint32_t GetValueNumber(spvtools::ir::Instruction* inst) const;
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// Returns the value number of the value contain in |id|. Returns 0 if it
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// has not been assigned a value number.
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uint32_t GetValueNumber(uint32_t id) const;
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ir::IRContext* context() const { return context_; }
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private:
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// Assigns a value number to every result id in the module.
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void BuildDominatorTreeValueNumberTable();
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// Returns the new value number.
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uint32_t TakeNextValueNumber() { return next_value_number_++; }
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// Assigns a new value number to the result of |inst| if it does not already
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// have one. Return the value number for |inst|. |inst| must have a result
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// id.
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uint32_t AssignValueNumber(ir::Instruction* inst);
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std::unordered_map<spvtools::ir::Instruction, uint32_t, ValueTableHash,
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ComputeSameValue>
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instruction_to_value_;
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std::unordered_map<uint32_t, uint32_t> id_to_value_;
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ir::IRContext* context_;
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uint32_t next_value_number_;
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};
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} // namespace opt
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} // namespace spvtools
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#endif // LIBSPIRV_OPT_VALUE_NUMBER_TABLE_H_
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