Is there a GCC keyword to allow structure-reordering?

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 or the github mirror out on your own risk as it's still in active development.

You can also reorder the fields with the clang-reorder-fields tool in clang-tools-extra

See also

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

As a side note, the Linux kernel implements a gcc plugin to introduce an attribute named randomize_layout. The goal is to use it in the definition of the structures to make the compiler randomize the order of the fields. Linux kernel uses it for the sake of security to counter attacks that need to know the layout of structures. For example the cred structure is defined as follow in include/linux/cred.h:

struct cred {
    atomic_t    usage;
#ifdef CONFIG_DEBUG_CREDENTIALS
    atomic_t    subscribers;    /* number of processes subscribed */
    void        *put_addr;
[...]
    struct user_struct *user;   /* real user ID subscription */
    struct user_namespace *user_ns; /* user_ns the caps and keyrings are relative to. */
    struct group_info *group_info;  /* supplementary groups for euid/fsgid */
    /* RCU deletion */
    union {
        int non_rcu;            /* Can we skip RCU deletion? */
        struct rcu_head rcu;        /* RCU deletion hook */
    };
} __randomize_layout;

The __randomize_layout tag is defined in include/linux/compiler-gcc.h of the Linux source tree as:

#define __randomize_layout __attribute__((randomize_layout))

There is no such option in GCC. And, I sure, it can not be introduced in any sensible fashion. About padding optimizations please look at this discussion.

The only exception I know is hot/cold structure fields splitting, that can be done in some cases (still I am not sure, that GCC can do it even in profile-guided mode, I know ICC can). This feature is not under user control and is performed on call-graphs where conservativeness of such transformation over data-flow is provable.