Re: [HACKERS] Is it really such a good thing for newNode() to be a macro?

Tom Lane wrote:
> I happened to be looking at nodes.h and started wondering just how
> sane this coding really is:
>
> extern PGDLLIMPORT Node *newNodeMacroHolder;
>
> #define newNode(size, tag) \
> ( \
> AssertMacro((size) >= sizeof(Node)), /* need the tag, at least */ \
> newNodeMacroHolder = (Node *) palloc0fast(size), \
> newNodeMacroHolder->type = (tag), \
> newNodeMacroHolder \
> )
>
> Given that we're calling palloc, it's not clear that saving one level of
> function call is really buying much; and what it's costing us is a store
> to a global variable that the compiler has no way to optimize away.
> On a lot of platforms, accessing global variables isn't especially
> cheap. Also, considering that palloc0fast is a nontrivial macro, and
> that there are a LOT of uses of newNode(), we're paying rather a lot of
> code space for a pretty dubious savings.

Note that the MemSetLoop macro used in palloc0fast is supposed to be
evaluated at compile time, so the code space taken by that macro isn't
that big. Turning newNode into function would force it to be evaluated
at run-time instead.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com

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