我写了一小段代码,如果尾递归被优化,则应该成功,但它却导致堆栈溢出。我是否应该得出结论:PHP不会优化尾递归?
function sumrand($n,$sum) {
if ($n== 0) {
return $sum;
}
else {
return (sumrand($n-1,$sum+rand(0,1)));
}
}
echo sumrand(500000,0)."\n";
我写了一小段代码,如果尾递归被优化,则应该成功,但它却导致堆栈溢出。我是否应该得出结论:PHP不会优化尾递归?
function sumrand($n,$sum) {
if ($n== 0) {
return $sum;
}
else {
return (sumrand($n-1,$sum+rand(0,1)));
}
}
echo sumrand(500000,0)."\n";
以下是生成的操作码(对于奇怪的表示表示抱歉):
Global
-------------------------------------------------------------------------------
BCDCAC 0003: NOP ()
BCDD24 0012: SEND_VAL (CONST: "500000")
BCDD9C 0012: SEND_VAL (CONST: NULL)
BCDE14 0012: DO_FCALL (CONST: "sumrand") -> VAR 0
BCDE8C 0012: CONCAT (VAR 0, CONST: "\n") -> TMP_VAR 1
BCDF04 0012: ECHO (TMP_VAR 1)
BCDF7C 0014: RETURN (CONST: "1")
Functions
-------------------------------------------------------------------------------
sumrand (17 op)
BCFABC 0003: RECV (CONST: "1") -> CV 0 ($n)
BCFB34 0003: RECV (CONST: "2") -> CV 1 ($sum)
BCFBAC 0004: IS_EQUAL (CV 0 ($n), CONST: NULL) -> TMP_VAR 0
BCFC24 0004: JMPZ (TMP_VAR 0, &(BCFD18+6))
BCFC9C 0005: RETURN (CV 1 ($sum))
BCFD14 0006: JMP (&(BD01C8+10))
BCFD8C 0008: INIT_FCALL_BY_NAME (NULL, CONST: "sumrand")
BCFE04 0008: SUB (CV 0 ($n), CONST: "1") -> TMP_VAR 1
BCFE7C 0008: SEND_VAL (TMP_VAR 1)
BCFEF4 0008: SEND_VAL (CONST: NULL)
BCFF6C 0008: SEND_VAL (CONST: "1")
BCFFE4 0008: DO_FCALL (CONST: "rand") -> VAR 2
BD005C 0008: ADD (CV 1 ($sum), VAR 2) -> TMP_VAR 3
BD00D4 0008: SEND_VAL (TMP_VAR 3)
BD014C 0008: DO_FCALL_BY_NAME () -> VAR 4
BD01C4 0008: RETURN (VAR 4)
BD023C 0010: RETURN (CONST: NULL)
所以,不,显然并非如此。了解PHP是一种用C语言编写的脚本语言,因此这种限制必然会出现是很重要的。优化不足也反映在底层的C语言中:
http://rosettacode.org/wiki/Find_limit_of_recursion
正如您所见,PHP并不是唯一不能优雅地处理事情的语言。
我建议使用Erlang和MyPeb PHP/Erlang桥接来解决这类问题。