From: | Peter Eisentraut <peter_e(at)gmx(dot)net> |
---|---|
To: | Gregory Stark <stark(at)enterprisedb(dot)com> |
Cc: | "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org, "Bruce Momjian" <bruce(at)momjian(dot)us>, "David Fetter" <david(at)fetter(dot)org> |
Subject: | Re: Macros for typtype (was Re: Arrays of Complex Types) |
Date: | 2007-04-01 10:04:06 |
Message-ID: | 200704011204.07451.peter_e@gmx.net |
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Lists: | pgsql-hackers pgsql-patches |
Gregory Stark wrote:
> > The width is 4 both for the macro and the enum case. Both
> >
> > #define TYPTYPE_BASE 'b'
> >
> > and
> >
> > enum ... {
> > TYPTYPE_BASE = 'b',
> >
> > effectively generate int constants named TYPTYPE_BASE with decimal
> > value 98. So there are no storage advantages either way.
>
> That's not accurate at all.
How so?
> The macro case gives you a constant you
> can only use to initialize integer variables and members that are
> explicitly declared with some integral type. If we consistently
> declare them "char" then they'll be predictably 1 byte long.
But character constants are actually ints, so when you do what you
describe then the compiler has to generate code to copy a four-byte
integer into a single byte. (Of course that can be optimized away,
probably.)
> The enum case does two things. It defines a syntactic meaning for the
> label, *and* it defines a thing "enum typtype" which can be used to
> define variables and members. If the latter is used then Tom is
> saying the standard doesn't specify what width the variable or member
> will be.
The standard says that enums are the same as ints. So when you assign
an enum label to a char variable, then compiler has to generate code to
copy a four-byte integer into a single byte. (Of course that can be
optimized away, probably.)
The fact that you can also declare variables of the enum type is not
under consideration here.
QED
--
Peter Eisentraut
http://developer.postgresql.org/~petere/
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