add details about optimizer passes

[realFlowControl]

Did some digging into optimizer passes in php/php-src#19658 and figured I could just document things a bit more user friendly

[realFlowControl]

There are really just a few external links in the docs, would you think it would be beneficial to link to Nikitas blog post?
As there are only a few external links, I am not sure how this is handled usually

[arnaud-lb]

This looks good to me, but I will defer to doc maintainers for the final review.

I think that linking to the blog post would make sense, as it can be considered a reference, but I'm not sure about the policy either. I also defer to doc maintainers.

[Girgias]

Please try to use <simpara> for new text as PhD generates an HTML <p> tag rather than a <div> as <para> does.

[realFlowControl]

Thanks @Girgias for your review, I've applied your suggestions

[Girgias]

@realFlowControl can you rebase the PR as I performed some bulk XML changes on reference/opcache.

[realFlowControl]

@Girgias rebase ✅

[Girgias]

Does it ignore only overloding from do_operation or also comparison overloading?

[realFlowControl]

AFAIK it is only about the binary and unary operators (so the do_operation you mentioned), more specifically about tracking a MAY_BE_OBJECT (and MAY_BE_FALSE) for the result of that operation to unlock optimisation further down, see https://github.com/php/php-src/blob/aeb8524584967bdebffec0e7896ca358a8071f3e/Zend/Optimizer/zend_inference.c#L2267-L2274
For comparison the result is always a boolean (or int for the spaceship operator), it can't be an object, so this does not apply.

[Girgias]

Right, I guess it would be good to indicate that this only affects classes overloading the do_operator object handle, as many more internal classes overload the compare object handler (e.g. ext/data) and if someone doesn't use ext/gmp, ext/bcmatch enabling this optimization might be sensible.

[realFlowControl]

Oh yes, that is good idea, documenting the "unsafe" bit and when/why it is safe. I'll have a look and add this

[realFlowControl]

I've added docs and an example on the unsafe optimisations

[Girgias]

Sorry this took a while. Should be live within a few hours! :)