¶std::pair<uint64_t, Align>
performOptimizedStructLayout(
    MutableArrayRef<
        llvm::OptimizedStructLayoutField> Fields)

Description

Compute a layout for a struct containing the given fields, making a best-effort attempt to minimize the amount of space required. Two features are supported which require a more careful solution than the well-known "sort by decreasing alignment" solution: - Fields may be assigned a fixed offset in the layout. If there are gaps among the fixed-offset fields, the algorithm may attempt to allocate flexible-offset fields into those gaps. If that's undesirable, the caller should "block out" those gaps by e.g. just creating a single fixed-offset field that represents the entire "header". - The size of a field is not required to be a multiple of, or even greater than, the field's required alignment. The only constraint on fields is that they must not be zero-sized. To simplify the implementation, any fixed-offset fields in the layout must appear at the start of the field array, and they must be ordered by increasing offset. The algorithm will produce a guaranteed-minimal layout with no interior padding in the following "C-style" case: - every field's size is a multiple of its required alignment and - either no fields have initially fixed offsets, or the fixed-offset fields have no interior padding and end at an offset that is at least as aligned as all the flexible-offset fields. Otherwise, while the algorithm will make a best-effort attempt to avoid padding, it cannot guarantee a minimal layout, as there is no known efficient algorithm for doing so. The layout produced by this algorithm may not be stable across LLVM releases. Do not use this anywhere where ABI stability is required. Flexible-offset fields with the same size and alignment will be ordered the same way they were in the initial array. Otherwise the current algorithm makes no effort to preserve the initial order of flexible-offset fields. On return, all fields will have been assigned a fixed offset, and the array will be sorted in order of ascending offsets. Note that this means that the fixed-offset fields may no longer form a strict prefix if there's any padding before they end. The return value is the total size of the struct and its required alignment. Note that the total size is not rounded up to a multiple of the required alignment; clients which require this can do so easily.

Declared at: llvm/include/llvm/Support/OptimizedStructLayout.h:138

Parameters

MutableArrayRef<llvm::OptimizedStructLayoutField> Fields