Andres Freund <and...@2ndquadrant.com> writes:
> On 2013-01-12 13:16:56 -0500, Tom Lane wrote:
>> However, using a do-block with a local variable is definitely something
>> worth considering. I'm getting less enamored of the __builtin_constant_p
>> idea after finding out that the gcc boys seem to have curious ideas
>> about what its semantics ought to be:
>> https://bugzilla.redhat.com/show_bug.cgi?id=894515
> I wonder whether __builtin_choose_expr is any better?
Right offhand I don't see how that helps us. AFAICS,
__builtin_choose_expr is the same as x?y:z except that y and z are not
required to have the same datatype, which is okay because they require
the value of x to be determinable at compile time. So we couldn't just
write __builtin_choose_expr((elevel) >= ERROR, ...). That would fail
if elevel wasn't compile-time-constant. We could conceivably do
__builtin_choose_expr(__builtin_constant_p(elevel) && (elevel) >= ERROR, ...)
with the idea of making real sure that that expression reduces to a
compile time constant ... but stacking two nonstandard constructs on
each other seems to me to probably increase our exposure to gcc's
definitional randomness rather than reduce it. I mean, if
__builtin_constant_p can't already be trusted to act like a constant,
why should we trust that __builtin_choose_expr doesn't also have a
curious definition of constant-ness?
regards, tom lane
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