1// Copyright (c) 2019 Google LLC
2//
3// Licensed under the Apache License, Version 2.0 (the "License");
4// you may not use this file except in compliance with the License.
5// You may obtain a copy of the License at
6//
7// http://www.apache.org/licenses/LICENSE-2.0
8//
9// Unless required by applicable law or agreed to in writing, software
10// distributed under the License is distributed on an "AS IS" BASIS,
11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12// See the License for the specific language governing permissions and
13// limitations under the License.
14
15#ifndef SOURCE_OPT_CODE_SINK_H_
16#define SOURCE_OPT_CODE_SINK_H_
17
18#include <unordered_map>
19
20#include "source/opt/ir_context.h"
21#include "source/opt/module.h"
22#include "source/opt/pass.h"
23
24namespace spvtools {
25namespace opt {
26
27// This pass does code sinking for OpAccessChain and OpLoad on variables in
28// uniform storage or in read only memory. Code sinking is a transformation
29// where an instruction is moved into a more deeply nested construct.
30//
31// The goal is to move these instructions as close as possible to their uses
32// without having to execute them more often or to replicate the instruction.
33// Moving the instruction in this way can lead to shorter live ranges, which can
34// lead to less register pressure. It can also cause instructions to be
35// executed less often because they could be moved into one path of a selection
36// construct.
37//
38// This optimization can cause register pressure to rise if the operands of the
39// instructions go dead after the instructions being moved. That is why we only
40// move certain OpLoad and OpAccessChain instructions. They generally have
41// constants, loop induction variables, and global pointers as operands. The
42// operands are live for a longer time in most cases.
43class CodeSinkingPass : public Pass {
44 public:
45 const char* name() const override { return "code-sink"; }
46 Status Process() override;
47
48 // Return the mask of preserved Analyses.
49 IRContext::Analysis GetPreservedAnalyses() override {
50 return IRContext::kAnalysisDefUse |
51 IRContext::kAnalysisInstrToBlockMapping |
52 IRContext::kAnalysisCombinators | IRContext::kAnalysisCFG |
53 IRContext::kAnalysisDominatorAnalysis |
54 IRContext::kAnalysisLoopAnalysis | IRContext::kAnalysisNameMap |
55 IRContext::kAnalysisConstants | IRContext::kAnalysisTypes;
56 }
57
58 private:
59 // Sinks the instructions in |bb| as much as possible. Returns true if
60 // something changes.
61 bool SinkInstructionsInBB(BasicBlock* bb);
62
63 // Tries the sink |inst| as much as possible. Returns true if the instruction
64 // is moved.
65 bool SinkInstruction(Instruction* inst);
66
67 // Returns the basic block in which to move |inst| to move is as close as
68 // possible to the uses of |inst| without increasing the number of times
69 // |inst| will be executed. Return |nullptr| if there is no need to move
70 // |inst|.
71 BasicBlock* FindNewBasicBlockFor(Instruction* inst);
72
73 // Return true if |inst| reference memory and it is possible that the data in
74 // the memory changes at some point.
75 bool ReferencesMutableMemory(Instruction* inst);
76
77 // Returns true if the module contains an instruction that has a memory
78 // semantics id as an operand, and the memory semantics enforces a
79 // synchronization of uniform memory. See section 3.25 of the SPIR-V
80 // specification.
81 bool HasUniformMemorySync();
82
83 // Returns true if there may be a store to the variable |var_inst|.
84 bool HasPossibleStore(Instruction* var_inst);
85
86 // Returns true if one of the basic blocks in |set| exists on a path from the
87 // basic block |start| to |end|.
88 bool IntersectsPath(uint32_t start, uint32_t end,
89 const std::unordered_set<uint32_t>& set);
90
91 // Returns true if |mem_semantics_id| is the id of a constant that, when
92 // interpreted as a memory semantics mask enforces synchronization of uniform
93 // memory. See section 3.25 of the SPIR-V specification.
94 bool IsSyncOnUniform(uint32_t mem_semantics_id) const;
95
96 // True if a check has for uniform storage has taken place.
97 bool checked_for_uniform_sync_;
98
99 // Cache of whether or not the module has a memory sync on uniform storage.
100 // only valid if |check_for_uniform_sync_| is true.
101 bool has_uniform_sync_;
102};
103
104} // namespace opt
105} // namespace spvtools
106
107#endif // SOURCE_OPT_CODE_SINK_H_
108