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. 44 struct OptimizedStructLayoutField { 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 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"); 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. 72 Align Alignment; 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 { 83 assert(hasFixedOffset()); 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( 138 MutableArrayRef<OptimizedStructLayoutField> Fields); 139 140 } // namespace llvm 141 142 #endif 143