Re: Perl and common subexpressions

Unfortunately, unlike C, Perl has to contend with something a little crazier: Tied hashes and arrays.

And function calls. In

  t = x->y->z->a->b[5];
[download]

any of y, z, or a can be methods, with or without side-effects, and with no guarantee that they'll return the same thing on subsequent calls. In the face of such flexibility, static analysis optimizations are non-starters.

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Re^2: Perl and common subexpressions
by theorbtwo (Prior) on Dec 23, 2004 at 15:08 UTC

It's trivial to tell the difference between a method call and a hash lookup for the perl compile -- you're just having issues with C vs perl syntax. (Getting e, an element of the struct pointed to by p in C is p->e, which looks a lot like calling a method, m, on an object $o in Perl -- $o->m)

On the other hand, when it /is/ a method call, perl can't tell /what/ method is being called, because it doesn't know the type of the object you're calling the method on until runtime. The m method on Foo::Bar might be side-effect free, while the m method on Baz has side-effects.

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