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Re: Further news on Clang - spurious warnings

From: Peter Geoghegan <peter(at)2ndquadrant(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: PG Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Further news on Clang - spurious warnings
Date: 2011-08-03 11:13:20
Message-ID: CAEYLb_URKytyc_ro0zDDNFB3eZKsK74mgF85+FpEHrBcp71vgw@mail.gmail.com (view raw or flat)
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Lists: pgsql-hackers
On 3 August 2011 11:05, Peter Geoghegan <peter(at)2ndquadrant(dot)com> wrote:
> I don't believe that the standard allows for an implementation of
> enums as unsigned integers - after all, individual enum literals can
> be given corresponding negative integer values.

It actually gives leeway to implement the enum as unsigned int when
the compiler knows that it won't matter, because there are no negative
integer values that correspond to some enum literal. The hint was in
my original warning.  :-)

> This warning is only seen because the first enum literal in the enum
> is explicitly given the value 0, thus precluding the possibility of
> the value being < 0, barring some abuse of the enum.

It's also seen if no explicit values are given, and the compiler opts
to make the representation unsigned. It is not seen if it the value is
-1, for example.

Despite the fact that whether or not the value is unsigned is
implementation defined, I think that the patch is still valid - the
expression is at least logically tautological, even if it isn't
necessarily bitwise tautological, because, as I said, barring some
violation of the enum's contract, it should not be < 0. That's
precisely why the compiler has opted to make it unsigned.

-- 
Peter Geoghegan       http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training and Services

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Subject: Re: Further news on Clang - spurious warnings
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