One way of looking at it is that a naive implementation would have have every eval, require, try etc push a new jumplevel and enter a new runops loop. That way, when an exception occurs and a longjump occurs, the inner runops loop execution is popped of the C stack, and control returns to pp_entertry or whereever, which can then handle the exception how it wants.

An optimisation to this is to have a flag (je_mustcatch) that can be set to indicate that the caller of the current runops loop can catch longjumps, handle the exception, and if necessary restart a new runops loop. In this case, there's no need to push a new jumplevel and runops loop each time.

The top-level execution of a perl program consists of setting up a jump level and entering a runops loop, so it sets je_mustcatch to false there: the top-level is already set up to handle expections.

Whenever perl code is called "from C" rather than directly from perl, such as FETCHes, overload code, or perl code called from XS, then it creates a new runops loop, but in this case the loop isn't protected by a new jumplevel and expection handler. So the caller of FETCH sets je_mustcatch to true, to notify any potential eval ops that they should set up their own handler.

So the net effect is that in practice, eval {} almost never has to push a new jump level and runops.

Dave.

In reply to Re^4: How does CATCH_SET optimize efficiency? by dave_the_m
in thread How does CATCH_SET optimize efficiency? by PerlOnTheWay

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