Re: BUG #15519: Casting float4 into int4 gets the wrong sign instead of "integer out of range" error

>>>>> "PG" == PG Bug reporting form <noreply(at)postgresql(dot)org> writes:

PG> The following bug has been logged on the website:
PG> Bug reference: 15519
PG> Logged by: Victor Petrovykh
PG> Email address: victor(at)magic(dot)io
PG> PostgreSQL version: 10.5
PG> Operating system: Gentoo Linux 4.14.20
PG> Description:

PG> Offending examples:
PG> SELECT ((2147483647::float4) - 1.0::float4)::int4;
PG> SELECT ((2147483590::float4) - 1.0::float4)::int4;
PG> SELECT ((2147483647::float4) + 1.0::float4)::int4;

PG> They all produce the same result: -2147483648

The bug here is that the ftoi4 conversion function thinks it can do
this:

if (num < INT_MIN || num > INT_MAX || isnan(num))

but this causes INT_MIN and INT_MAX to be implicitly converted to floats
(not doubles), which can't represent their values exactly and ends up
with the conditions being false in the edge cases. (Specifically,
(float)INT_MAX is a bit larger than INT_MAX.)

Probably this should be changed to use ((float8) INT_MAX) etc. instead?

PG> I understand that a float4 cannot represent large integers with the
PG> same precision as int4, that's OK. What surprised me is that
PG> instead of getting an "overflow error" or "integer out of range" I
PG> simply got a negative result for a value that is actually close to
PG> maximum int4. To contrast this, the query:

PG> SELECT ((2147483647::float4) + 200.0::float4)::int4;
PG> The above produces the expected "ERROR: integer out of range"

Likewise you also get that error if you cast the float4 to a float8
before casting to integer.

--
Andrew (irc:RhodiumToad)