do-while是php中最快的循环吗？ [英] do-while is the fastest loop in php?

问题描述

我已经用简单的方法分析了 for ， while 和 do-while 循环：

I have profiled for, while and do-while loops with something simple:

while ($var < 1000000) {
  ++$var;
}

do {
  ++$var;
} while ($var < 1000000);

for ($var = 0; $var < 1000000; ++$var) {
  //do nothing
}

通过比较循环前后的microtime（）。

by comparing microtime() before and after the loops.

do-while循环在很大程度上是最快的循环。 do-while 实际上比 while 快近一半。我知道它们有不同的用途（ while 在循环执行之前检查条件，而 do-while 至少执行一次）。

The do-while loop is by a considerable amount the fastest loop. do-while is actually faster than while by almost half. I know that they are for different purposes ( while checks the condition before the loop executes and do-while executes at least once ).

我知道普遍的共识是，虽然不赞成使用循环，而做循环则更是如此。

I know the general consensus is that while loops are frowned upon and do-while even more so.

我的问题是为什么？考虑到PHP应用程序中使用了多少个for循环，不应该更多地使用 do-while 吗？即使使用 if 语句在循环执行之前检查条件，其性能提升还是相当可观的。

My question is why? Considering how many for loops are used in PHP applications, shouldn't do-while be used more? Even with an if statement to check a condition before the loop executes, the performance boost is considerable.

我目前接受的答案是代码易读是可疑的。

My currently accepted answer is that code legibility is the suspect.

推荐答案

1. 微观优化是邪恶的 。它们会降低可读性，从而导致 否 可衡量的性能提升。即使您的应用程序确实具有数百万个迭代器的循环（我对此表示怀疑），差异仍然可以忽略不计。


2. while 之间的差异/ 做而小于您说的话： http：//codepad.viper- 7.com/M8cgt9



3. 要了解为什么做而快一点的原因，请查看生成的操作码：

1. Micro optimizations are evil. They reduce readability for no measurable performance gain. Even if your application does have loops with millions of iterators (which I doubt) the difference is still negligible.
2. The difference between while / do while is smaller than you say: http://codepad.viper-7.com/M8cgt9

3. To understand why do while is marginally faster, look at the generated opcodes:

line     # *  op                           fetch          ext  return  operands
---------------------------------------------------------------------------------
# while loop
   3     0  >   ASSIGN                                                   !0, 0
   4     1  >   IS_SMALLER                                       ~1      !0, 1000000
         2    > JMPZ                                                     ~1, ->5
         3  >   PRE_INC                                                  !0
         4    > JMP                                                      ->1
         5  > > RETURN                                                   1
# do while loop
   3     0  >   ASSIGN                                                   !0, 0
   4     1  >   PRE_INC                                                  !0
         2      IS_SMALLER                                       ~2      !0, 1000000
         3    > JMPNZ                                                    ~2, ->1
   4        > > RETURN                                                   1
# for loop
   3     0  >   ASSIGN                                                   !0, 0
         1  >   IS_SMALLER                                       ~1      !0, 1000000
         2    > JMPZNZ                                        5          ~1, ->6
         3  >   PRE_INC                                                  !0
         4    > JMP                                                      ->1
         5  > > JMP                                                      ->3
         6  > > RETURN                                                   1

do while 循环仅具有一个跳转语句（ JMPNZ ），而 while 循环则需要两个（ JMPZ ， JMP ）。 for 循环需要三个跳转语句（ JMPZNZ ， JMP ， JMP ），并且逻辑通常更复杂。

The do while loop only has one jump statement (JMPNZ), whereas the while loop needs two (JMPZ, JMP). The for loop needs three jump statements (JMPZNZ, JMP, JMP) and has generally more complex logic.

这篇关于do-while是php中最快的循环吗？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！