「耶穌說，你來罷。彼得就從船上下去，在水面上走
讀者，你看見嗎？彼得的小信是一個極大的障礙；他既然已經開步走了
讀者，當主在水面上叫你：「來罷」的時候，歡歡喜喜地向前走去
有頌謙
之後又跑回市區吃冰去, 挖挖挖... 觀光客進來啦...
「你們要....儆醒禱告。」（聖經彼得前書四章 7 節）
朋友，不要冒險和世界接觸，如果你還沒有禱告。當你晚上跪下禱告的
你一點都不儆醒禱告！你忽略了儆醒；這樣的忽略能不能補救的呢
你不儆醒禱告，是一件何等嚴重的事！試探在你面前，你無法對付
信徒們，不要作無謂的冒險，把儆醒禱告這件事忽略了
Most of the time, when the kernel compares a variable against some external value to see whether a given condition is met, the result is extremely likely to be predictable. This is pretty common, for example, with code that enforces sanity checks. The kernel uses the likely and unlikely macros, respectively, to wrap comparisons that are likely to return a true (1) or false (0) result. Those macros take advantage of a feature of the gcc compiler that can optimize the compilation of the code based on that information.
Here is an example. Let's suppose you need to call the do_something function, and that in case of failure, you must handle it with the handle_error function:
err = do_something(x,y,z);
if (err)
handle_error(err);
Under the assumption that do_something rarely fails, you can rewrite the code as follows:
err = do_something(x,y,z);
if (unlikely(err))
handle_error(err);
An example of the optimization made possible by the likely and unlikely macros is in handling options in the IP header. The use of IP options is limited to very specific cases, and the kernel can safely assume that most IP packets do not carry IP options. When the kernel forwards an IP packet, it needs to take care of options according to the rules described in Chapter 18. The last stage of forwarding an IP packet is taken care of by ip_forward_finish. This function uses the unlikely macro to wrap the condition that checks whether there is any IP option to take care of. See the section "ip_forward_finish Function" in Chapter 20.
基本上跟先前的認知是一樣的, 但發覺 ARM, MIPS 跟 PowerPC 用的 GNU crosscompiler 似乎都不會去做這些個 optimization... x86 的就會...
