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OptimizedStructLayout.h
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1 //===-- OptimizedStructLayout.h - Struct layout algorithm ---------*- C++ -*-=//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 ///
9 /// This file provides an interface for laying out a sequence of fields
10 /// as a struct in a way that attempts to minimizes the total space
11 /// requirements of the struct while still satisfying the layout
12 /// requirements of the individual fields. The resulting layout may be
13 /// substantially more compact than simply laying out the fields in their
14 /// original order.
15 ///
16 /// Fields may be pre-assigned fixed offsets. They may also be given sizes
17 /// that are not multiples of their alignments. There is no currently no
18 /// way to describe that a field has interior padding that other fields may
19 /// be allocated into.
20 ///
21 /// This algorithm does not claim to be "optimal" for several reasons:
22 ///
23 /// - First, it does not guarantee that the result is minimal in size.
24 /// There is no known efficient algoorithm to achieve minimality for
25 /// unrestricted inputs. Nonetheless, this algorithm
26 ///
27 /// - Second, there are other ways that a struct layout could be optimized
28 /// besides space usage, such as locality. This layout may have a mixed
29 /// impact on locality: less overall memory may be used, but adjacent
30 /// fields in the original array may be moved further from one another.
31 ///
32 //===----------------------------------------------------------------------===//
33 
34 #ifndef LLVM_SUPPORT_OPTIMIZEDSTRUCTLAYOUT_H
35 #define LLVM_SUPPORT_OPTIMIZEDSTRUCTLAYOUT_H
36 
37 #include "llvm/Support/Alignment.h"
38 #include "llvm/ADT/ArrayRef.h"
39 #include <utility>
40 
41 namespace llvm {
42 
43 /// A field in a structure.
45  /// A special value for Offset indicating that the field can be moved
46  /// anywhere.
47  static constexpr uint64_t FlexibleOffset = ~(uint64_t)0;
48 
50  uint64_t FixedOffset = FlexibleOffset)
51  : Offset(FixedOffset), Size(Size), Id(Id), Alignment(Alignment) {
52  assert(Size > 0 && "adding an empty field to the layout");
53  }
54 
55  /// The offset of this field in the final layout. If this is
56  /// initialized to FlexibleOffset, layout will overwrite it with
57  /// the assigned offset of the field.
58  uint64_t Offset;
59 
60  /// The required size of this field in bytes. Does not have to be
61  /// a multiple of Alignment. Must be non-zero.
62  uint64_t Size;
63 
64  /// A opaque value which uniquely identifies this field.
65  const void *Id;
66 
67  /// Private scratch space for the algorithm. The implementation
68  /// must treat this as uninitialized memory on entry.
69  void *Scratch;
70 
71  /// The required alignment of this field.
73 
74  /// Return true if this field has been assigned a fixed offset.
75  /// After layout, this will be true of all the fields.
76  bool hasFixedOffset() const {
77  return (Offset != FlexibleOffset);
78  }
79 
80  /// Given that this field has a fixed offset, return the offset
81  /// of the first byte following it.
82  uint64_t getEndOffset() const {
84  return Offset + Size;
85  }
86 };
87 
88 /// Compute a layout for a struct containing the given fields, making a
89 /// best-effort attempt to minimize the amount of space required.
90 ///
91 /// Two features are supported which require a more careful solution
92 /// than the well-known "sort by decreasing alignment" solution:
93 ///
94 /// - Fields may be assigned a fixed offset in the layout. If there are
95 /// gaps among the fixed-offset fields, the algorithm may attempt
96 /// to allocate flexible-offset fields into those gaps. If that's
97 /// undesirable, the caller should "block out" those gaps by e.g.
98 /// just creating a single fixed-offset field that represents the
99 /// entire "header".
100 ///
101 /// - The size of a field is not required to be a multiple of, or even
102 /// greater than, the field's required alignment. The only constraint
103 /// on fields is that they must not be zero-sized.
104 ///
105 /// To simplify the implementation, any fixed-offset fields in the
106 /// layout must appear at the start of the field array, and they must
107 /// be ordered by increasing offset.
108 ///
109 /// The algorithm will produce a guaranteed-minimal layout with no
110 /// interior padding in the following "C-style" case:
111 ///
112 /// - every field's size is a multiple of its required alignment and
113 /// - either no fields have initially fixed offsets, or the fixed-offset
114 /// fields have no interior padding and end at an offset that is at
115 /// least as aligned as all the flexible-offset fields.
116 ///
117 /// Otherwise, while the algorithm will make a best-effort attempt to
118 /// avoid padding, it cannot guarantee a minimal layout, as there is
119 /// no known efficient algorithm for doing so.
120 ///
121 /// The layout produced by this algorithm may not be stable across LLVM
122 /// releases. Do not use this anywhere where ABI stability is required.
123 ///
124 /// Flexible-offset fields with the same size and alignment will be ordered
125 /// the same way they were in the initial array. Otherwise the current
126 /// algorithm makes no effort to preserve the initial order of
127 /// flexible-offset fields.
128 ///
129 /// On return, all fields will have been assigned a fixed offset, and the
130 /// array will be sorted in order of ascending offsets. Note that this
131 /// means that the fixed-offset fields may no longer form a strict prefix
132 /// if there's any padding before they end.
133 ///
134 /// The return value is the total size of the struct and its required
135 /// alignment. Note that the total size is not rounded up to a multiple
136 /// of the required alignment; clients which require this can do so easily.
137 std::pair<uint64_t, Align> performOptimizedStructLayout(
139 
140 } // namespace llvm
141 
142 #endif
bool hasFixedOffset() const
Return true if this field has been assigned a fixed offset.
This class represents lattice values for constants.
Definition: AllocatorList.h:23
uint64_t Offset
The offset of this field in the final layout.
std::pair< uint64_t, Align > performOptimizedStructLayout(MutableArrayRef< OptimizedStructLayoutField > Fields)
Compute a layout for a struct containing the given fields, making a best-effort attempt to minimize t...
OptimizedStructLayoutField(const void *Id, uint64_t Size, Align Alignment, uint64_t FixedOffset=FlexibleOffset)
Align Alignment
The required alignment of this field.
MutableArrayRef - Represent a mutable reference to an array (0 or more elements consecutively in memo...
Definition: ArrayRef.h:298
This struct is a compact representation of a valid (non-zero power of two) alignment.
Definition: Alignment.h:39
uint64_t getEndOffset() const
Given that this field has a fixed offset, return the offset of the first byte following it...
uint64_t Size
The required size of this field in bytes.
static constexpr uint64_t FlexibleOffset
A special value for Offset indicating that the field can be moved anywhere.
void * Scratch
Private scratch space for the algorithm.
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
const void * Id
A opaque value which uniquely identifies this field.