GCC -fobjc-direct-dispatch选项到底做了什么？ [英] What exactly does GCC -fobjc-direct-dispatch option do?

问题描述

GCC手册说：

The GCC manual says:

-fobjc-direct-dispatch

允许快速跳转到消息分派器。在
达尔文这是通过通信页面完成。

Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.

我可以假设这个标志消除了动态分派吗？它是如何工作的？
我相信它应该和C函数调用一样快，如果它直接链接的话。

Can I assume this flag eliminates dynamic dispatch? How does it work? I believe it should be as fast as a C function call if it is linked directly.

推荐答案

不，动态调度仍然存在（调用仍然通过 objc_msgSend 路由）。而且这个选项并没有在x86（-64）中引入任何差异。

No, the dynamic dispatch is still there (calls still route through objc_msgSend). And this option doesn't introduce any difference currently with x86(-64).

From http://developer.apple.com/legacy/mac/library/documentation/DeveloperTools/gcc-3.3/gcc/Objective_002dC-Dialect-Options.html ：
$ b

From http://developer.apple.com/legacy/mac/library/documentation/DeveloperTools/gcc-3.3/gcc/Objective_002dC-Dialect-Options.html:

对于Objective-C非常频繁地调用的某些函数（例如 objc_msgSend ）程序中，高内存中可能会直接跳转到特定的入口点（例如，通过PowerPC上的 bla 指令）以提高性能。 fobjc-direct-dispatch 选项将导致产生这种跳转。该选项只能与NeXT运行时一起使用;此外，使用 -fobjc-direct-dispatch 选项构建的程序只能在Mac OS X 10.4（Tiger）或更高版本的系统上运行。

For some functions (such as objc_msgSend) called very frequently by Objective-C programs, special entry points exist in high memory that may be jumped to directly (e.g., via the "bla" instruction on the PowerPC) for improved performance. The fobjc-direct-dispatch option will cause such jumps to be generated. This option is only available in conjunction with the NeXT runtime; furthermore, programs built with the -fobjc-direct-dispatch option will only run on Mac OS X 10.4 (Tiger) or later systems.

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