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Re: simple session question [message #175769 is a reply to message #175762] Sun, 23 October 2011 19:28 Go to previous messageGo to previous message
Richard Damon is currently offline  Richard Damon
Messages: 58
Registered: August 2011
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On 10/23/11 1:00 PM, Thomas Mlynarczyk wrote:
> Richard Damon schrieb:
>
>> 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.
>
> So basically, it boils down to "programmer" vs. "compiler": The
> programmer would like to have everything defined clearly, because
> anything "undefined" simply means (s)he cannot use that construct. The
> compiler, on the other hand, can work the better, the fewer restrictions
> there are, and thus "welcomes" anything left undefined. And C focuses
> more on the compiler, while PHP focuses more on the programmer.
>
>> 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.
>
> So in C, the statement x = x++ is regarded "as a whole" and the compiler
> can do anything with it as long as certain constraints are observed,
> while PHP, being a higher level language than C, must have all the tiny
> internal steps well defined.
>
>> 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++;
>
> If I have understood you correctly, this would be
>
> In PHP:
> [1] $tmp = $x;
> [2] $x = $x + 1;
> [3] $tmp = 5 * $tmp;
> [4] $x = $tmp;
> In exactly that order (or maybe [2] and [3] swapped as it would make no
> difference).
>
> In C:
> [TODO] RHS = 5 * (value of x before increment)
> [TODO] increment x
> [TODO] LHS = RHS
> With the only restriction on the order being that the third item must
> necessarily come after the first and it's up to the compiler to choose.
>
> Greetings,
> Thomas
>

Actually, both have the same basic steps (in principle)

(1) temp1 = x
(2) temp2 = x
(3) x = temp2 + 1
(4) temp3 = 5 * temp2
(5) x = temp3

(1 & 2 may be redundant and combined)
2,3 are the x++, 1,4,5 is the x = 5*x
by necessity we need the following order (can't use a value before it is
available)

1 -> 4 -> 5
2 -> 3
and by the definition of x++, 1 -> 3

PHP appears to define the order to be

1,2 -> 3 -> 4 -> 5

which means the increment of x is done tied to the fetching of the value
of x.

C places no additional restrictions on the order of the parts.

In particular, 3 can come before or after 5, and if 3 is after 5, 2 can
be before or after 5.

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