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 //===----------------------------------------------------------------------===//
10 /// This file provides an interface for laying out a sequence of fields
11 /// as a struct in a way that attempts to minimizes the total space
12 /// requirements of the struct while still satisfying the layout
13 /// requirements of the individual fields. The resulting layout may be
14 /// substantially more compact than simply laying out the fields in their
17 /// Fields may be pre-assigned fixed offsets. They may also be given sizes
18 /// that are not multiples of their alignments. There is no currently no
19 /// way to describe that a field has interior padding that other fields may
20 /// be allocated into.
22 /// This algorithm does not claim to be "optimal" for several reasons:
24 /// - First, it does not guarantee that the result is minimal in size.
25 /// There is no known efficient algoorithm to achieve minimality for
26 /// unrestricted inputs. Nonetheless, this algorithm
28 /// - Second, there are other ways that a struct layout could be optimized
29 /// besides space usage, such as locality. This layout may have a mixed
30 /// impact on locality: less overall memory may be used, but adjacent
31 /// fields in the original array may be moved further from one another.
33 //===----------------------------------------------------------------------===//
35 #ifndef LLVM_SUPPORT_OPTIMIZEDSTRUCTLAYOUT_H
36 #define LLVM_SUPPORT_OPTIMIZEDSTRUCTLAYOUT_H
38 #include "llvm/Support/Alignment.h"
39 #include "llvm/ADT/ArrayRef.h"
44 /// A field in a structure.
45 struct OptimizedStructLayoutField {
46 /// A special value for Offset indicating that the field can be moved
48 static constexpr uint64_t FlexibleOffset = ~(uint64_t)0;
50 OptimizedStructLayoutField(const void *Id, uint64_t Size, Align Alignment,
51 uint64_t FixedOffset = FlexibleOffset)
52 : Offset(FixedOffset), Size(Size), Id(Id), Alignment(Alignment) {
53 assert(Size > 0 && "adding an empty field to the layout");
56 /// The offset of this field in the final layout. If this is
57 /// initialized to FlexibleOffset, layout will overwrite it with
58 /// the assigned offset of the field.
61 /// The required size of this field in bytes. Does not have to be
62 /// a multiple of Alignment. Must be non-zero.
65 /// A opaque value which uniquely identifies this field.
68 /// Private scratch space for the algorithm. The implementation
69 /// must treat this as uninitialized memory on entry.
72 /// The required alignment of this field.
75 /// Return true if this field has been assigned a fixed offset.
76 /// After layout, this will be true of all the fields.
77 bool hasFixedOffset() const {
78 return (Offset != FlexibleOffset);
81 /// Given that this field has a fixed offset, return the offset
82 /// of the first byte following it.
83 uint64_t getEndOffset() const {
84 assert(hasFixedOffset());
89 /// Compute a layout for a struct containing the given fields, making a
90 /// best-effort attempt to minimize the amount of space required.
92 /// Two features are supported which require a more careful solution
93 /// than the well-known "sort by decreasing alignment" solution:
95 /// - Fields may be assigned a fixed offset in the layout. If there are
96 /// gaps among the fixed-offset fields, the algorithm may attempt
97 /// to allocate flexible-offset fields into those gaps. If that's
98 /// undesirable, the caller should "block out" those gaps by e.g.
99 /// just creating a single fixed-offset field that represents the
102 /// - The size of a field is not required to be a multiple of, or even
103 /// greater than, the field's required alignment. The only constraint
104 /// on fields is that they must not be zero-sized.
106 /// To simplify the implementation, any fixed-offset fields in the
107 /// layout must appear at the start of the field array, and they must
108 /// be ordered by increasing offset.
110 /// The algorithm will produce a guaranteed-minimal layout with no
111 /// interior padding in the following "C-style" case:
113 /// - every field's size is a multiple of its required alignment and
114 /// - either no fields have initially fixed offsets, or the fixed-offset
115 /// fields have no interior padding and end at an offset that is at
116 /// least as aligned as all the flexible-offset fields.
118 /// Otherwise, while the algorithm will make a best-effort attempt to
119 /// avoid padding, it cannot guarantee a minimal layout, as there is
120 /// no known efficient algorithm for doing so.
122 /// The layout produced by this algorithm may not be stable across LLVM
123 /// releases. Do not use this anywhere where ABI stability is required.
125 /// Flexible-offset fields with the same size and alignment will be ordered
126 /// the same way they were in the initial array. Otherwise the current
127 /// algorithm makes no effort to preserve the initial order of
128 /// flexible-offset fields.
130 /// On return, all fields will have been assigned a fixed offset, and the
131 /// array will be sorted in order of ascending offsets. Note that this
132 /// means that the fixed-offset fields may no longer form a strict prefix
133 /// if there's any padding before they end.
135 /// The return value is the total size of the struct and its required
136 /// alignment. Note that the total size is not rounded up to a multiple
137 /// of the required alignment; clients which require this can do so easily.
138 std::pair<uint64_t, Align> performOptimizedStructLayout(
139 MutableArrayRef<OptimizedStructLayoutField> Fields);