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Re: simple session question [message #175749 is a reply to message #175742] Sun, 23 October 2011 07:44 Go to previous messageGo to previous message
The Natural Philosoph is currently offline  The Natural Philosoph
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Richard Damon wrote:
> On 10/22/11 8:12 PM, Thomas Mlynarczyk wrote:
>> Richard Damon schrieb:
>>
>>> I suspect that there is a difference between the execution model of
>>> C/C++ and PHP here, do in part to the fact that C/C++ is (normally) a
>>> compiled language with the goal of allowing the compiler to generate
>>> as efficient of code as possible, while PHP is designed as a
>>> interpreted language.
>>
>> Hm. But PHP is written in C, so I would assume it to just follow C here.
>>
>>> x = x; /* perform the = */
>>> x = x+1; /* perform the ++ */
>>
>> That could be further optimized by dropping the x = x. But here the
>> assignment seems to have a higher precedence than the increment.
>>
>>> temp = x;
>>> x = x+1;
>>> ... do what ever with temp
>>>
>>> Which being an interpreted language makes some sense, why put off
>>> doing something, and recording somewhere that you need to do it, when
>>> you can do it now, the possible savings that C might have been able to
>>> make, get swamped by the other overhead in PHP.
>>
>> Now I wonder why x = x++ is undefined in C. If it was defined to be a no
>> op as it behaves in PHP, then it could be optimized away completely.
>>
>> Greetings,
>> Thomas
>>
>
> The issue is C is that there is a general rule (to allow optimizations)
> that defines the behavior to be undefined if an expression causes a
> variable to be written to twice, or have a read from and write to (where
> the read from is not needed to determine the value to write to) without
> an intervening sequence point, like the end of an expression.
>
> The statement x = x++; has two different writes to x, so we meet the
> requirement for undefined behavior. Not also that C does not limit when
> the ++ part happens, only that this side effect will finish by the next
> sequence point.
>
> Part of the problem with this example is that it is a bit to simple, and
> that simplicity hides some of the issues. Let us make the expression
> just slightly more complicated to make it cleared. Let us use x = 5*x++;
>
> This expression has two different value paths:
>
> x = 5 * x; (the main expression) and
> x = x + 1; (the side effect of x++)
>
> The only ordering that C puts on these two expressions is that the read
> for x in the first, must happen before the write of x in the second (due
> to the definition of x++).
>
> Thus in C the net expression is likely to be one of:
>
> x = x+1; (if the first expression finishes first, and then the second,
> using the original value of x)
> x = 5*x; (if the second expression finishes first, and then the first,
> using the original value of x)
> x = 5*x+1; (if the first expression finishes first, and then the second,
> using the new value of x)
>
> It is even possible on some machines that the program will trap, as C
> doesn't limit the possible effect on undefined behavior. This might
> happen on a machine which is pipelined, and there is a restriction on
> either writing a value and reading it back to quickly, or doing two
> writes to closely.
>
> This example may seem simple enough that the compiler should catch it,
> but what if the statement was:
>
> *y = x++;
>
> and y just happened to point to x.
>
> PHP, not trying to allow the compiler to generate as efficient code,
> doesn't need to allow for the ambiguity in when the increment occurs,
> and it makes sense to do it right away. Knowing C, I don't see them
> making that explicit promise, but the way it is worded, if you didn't
> know C, it seems to be what would be expected.


Forget the foo=foo++ and consider the cases

bar=foo++;
and
bar=(foo++).


In C they give different results. in PHP they do not.

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