是否有一个GCC关键字，允许结构重新排序？ [英] Is there a GCC keyword to allow structure-reordering?

问题描述

我知道为什么GCC不重新排序默认情况下，结构的成员，但我很少写code依靠结构的顺序，那么有没有什么办法可以标记我的结构是全自动重新排序？

解决方案

previous版本的GCC有<一个href=\"https://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Optimize-Options.html\"><$c$c>-fipa-struct-reorg选项​​允许结构重排 -fwhole程序 + -combine 模式。

  

  
• -fipa-结构-REORG 结果
    进行结构优化重组，这种变化类似于C的结构布局，以更好地利用空间局部性。这种转变是情感包含结构的阵列方案。有两种编译模式：个人资料为基础的（启用了 -fprofile生成）或静态（使用内置的启发式）。要求 -fipa型逃生来提供这种转变的安全性。它的工作原理只有在整个程序模式，所以它需要 -fwhole程序和 -combine 启用。结构被认为冷通过这种转变不会受到影响（见 - 参数结构-REORG-冷结构比=值）。

  

这是自GCC 4.8.x删除，由于在发行说明

  

该结构REORG和REORG矩阵优化（命令行选项 -fipa-结构-REORG 和 -fipa矩阵-REORG ）已被删除。他们并不总是正确，也没带链接时优化（LTO）工作，因此只适用于由单个翻译单元的节目。

但是你仍然可以尝试结构-REORG分支在 GCC SVN 的在你自己的风险，因为它仍处于积极发展。

I know why GCC doesn't re-order members of a structure by default, but I seldom write code that relies on the order of the structure, so is there some way I can flag my structures to be automaticly reordered?

解决方案

Previous GCC versions have the -fipa-struct-reorg option to allow structure reordering in -fwhole-program + -combine mode.

• -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).

It was removed since GCC 4.8.x due to the below reasons in the release note

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.

However you can still try the struct-reorg-branch on GCC SVN out on your own risk as it's still in active development.

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