c - 是否有 GCC 关键字允许结构重新排序？

Question

我知道为什么默认情况下 GCC 不对结构的成员重新排序，但我很少编写依赖于结构顺序的代码，所以有什么方法可以将我的结构标记为自动重新排序？

Answer 1

以前的 GCC 版本有 -fipa-struct-reorg option 允许在 -fwhole-program + -combine 模式下重新排序

• -fipa-struct-reorg

  Perform structure reorganization optimization, that change C-like structures layout in order to better utilize spatial locality. This transformation is affective for programs containing arrays of structures. Available in two compilation modes: profile-based (enabled with -fprofile-generate) or static (which uses built-in heuristics). Require -fipa-type-escape to provide the safety of this transformation. It works only in whole program mode, so it requires -fwhole-program and -combine to be enabled. Structures considered 'cold' by this transformation are not affected (see --param struct-reorg-cold-struct-ratio=value).

由于 release note 中的以下原因，它从 GCC 4.8.x 开始被删除

The struct reorg and matrix reorg optimizations (command-line options -fipa-struct-reorg and -fipa-matrix-reorg) have been removed. They did not always work correctly, nor did they work with link-time optimization (LTO), hence were only applicable to programs consisting of a single translation unit.

但是您仍然可以在 GCC SVN 或 github mirror 上尝试 struct-reorg-branch，风险自负，因为它仍在积极开发中。

您还可以使用 clang-reorder-fields 中的 clang-tools-extra 工具对字段重新排序

另见

• Automated field re-ordering in C structs to avoid padding