1 //===-- OptimizedStructLayout.h - Struct layout algorithm ---------*- C++ -*-=//
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
7 //===----------------------------------------------------------------------===//
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
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.
21 /// This algorithm does not claim to be "optimal" for several reasons:
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
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.
32 //===----------------------------------------------------------------------===//
34 #ifndef LLVM_SUPPORT_OPTIMIZEDSTRUCTLAYOUT_H
35 #define LLVM_SUPPORT_OPTIMIZEDSTRUCTLAYOUT_H
37 #include "llvm/Support/Alignment.h"
38 #include "llvm/ADT/ArrayRef.h"
43 /// A field in a structure.
44 struct OptimizedStructLayoutField {
45 /// A special value for Offset indicating that the field can be moved
47 static constexpr uint64_t FlexibleOffset = ~(uint64_t)0;
49 OptimizedStructLayoutField(const void *Id, uint64_t Size, Align Alignment,
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");
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.
60 /// The required size of this field in bytes. Does not have to be
61 /// a multiple of Alignment. Must be non-zero.
64 /// A opaque value which uniquely identifies this field.
67 /// Private scratch space for the algorithm. The implementation
68 /// must treat this as uninitialized memory on entry.
71 /// The required alignment of this field.
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);
80 /// Given that this field has a fixed offset, return the offset
81 /// of the first byte following it.
82 uint64_t getEndOffset() const {
83 assert(hasFixedOffset());
88 /// Compute a layout for a struct containing the given fields, making a
89 /// best-effort attempt to minimize the amount of space required.
91 /// Two features are supported which require a more careful solution
92 /// than the well-known "sort by decreasing alignment" solution:
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
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.
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.
109 /// The algorithm will produce a guaranteed-minimal layout with no
110 /// interior padding in the following "C-style" case:
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.
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.
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.
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.
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.
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(
138 MutableArrayRef<OptimizedStructLayoutField> Fields);