|
|
Posted by Dan Rossi on 07/06/05 08:55
On 06/07/2005, at 3:43 PM, Rasmus Lerdorf wrote:
> The GOTO mechanism uses a bit of a compiler trick known as
> "computed gotos" to try to make each one a very simple branch at the
> assembly level. It looks like this in C:
>
> That is, we can dynamically create a list of labels and then use a GOTO
> to jump directly to one of these dynamic labels. Not instead of
> jumping
> to ptr[1] as in the above example, imagine making that a variable, say
> the opcode number, and doing the goto directly to the code defined for
> each opcode. That's the GOTO VM. And it should theoretically be the
> fastest assuming the compiler does the right thing.
Very interesting, I am not a C man however I am getting my head down
into extension building coz I am familiar with it, even though how low
level down to the registries it is, there are helper functions in the
Zend core to do some work for you :) I would presume GOTO to be quick
as its using labeled references.
>
> The SWITCH vm was originally just a big switch(opcode) { case 1: ...;
> case 2: ...' } It's a bit different now, but you can think of it in
> those terms. Decent compilers should theoretically be able to optimize
> a clean set of cases to something close to simple branches, just like
> the computed goto case, but it can be hit and miss. With the computed
> goto way of doing it, we are trying to send a stronger hint to the
> compiler.
Well i do find a performance issue running a heap of switches in PHP so
I could presume the same here.
>
> And finally the CALL vm basically just uses a function pointer to call
> the handling code. Think of it as something alone the lines of
> ${"handler_$i"}(); in PHP terms. That is, you would have handler_1(),
> handler_2(), handler_3() functions defined and you dynamically
> determine
> which one to call based on the opcode.
>
Ahh like a dynamic callback interesting, I would prob find this the
quickest then ? I do find it more efficient to run callback functions
than switches in the php code which I have been doing for a while now.
Navigation:
[Reply to this message]
|