Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing handling of underflows,
On 12/27/2006 12:44 PM, Bruce Momjian wrote:
The only unsolved issue is the one with underflow checks. I have added
comments explaining the problem in case someone ever figures out how to
address it.
This will behave better for float4:
Datum float4pl(PG_FUNCTION_ARGS)
{
---float4 arg1 = PG_GETARG_FLOAT4(0);
---float4 arg2 = PG_GETARG_FLOAT4(1);
+++double arg1 = PG_GETARG_FLOAT4(0);
+++double arg2 = PG_GETARG_FLOAT4(1);
double result;
result = arg1 + arg2;
CheckFloat4Val(result,isinf(arg1) || isinf(arg2));
PG_RETURN_FLOAT4((float4) result);
}
Roman
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Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing
Tom Lane wrote:
No, because you are still comparing against FLOAT4_MAX. I'm suggesting
that only an actual infinity should be rejected. Even that is contrary
to IEEE spec, though.
The other problem with this coding technique is that it must invoke
isinf three times when the typical case really only requires one (if the
output isn't inf there is no need to perform isinf on the inputs).
If we're going to check for overflow at all, I think we should lose the
subroutine and just do
if (isinf(result)
!(isinf(arg1) || isinf(arg2)))
ereport(...OVERFLOW...);
I wasn't excited about doing one isinf() call to avoid three, so I just
made a fast isinf() macro:
/*We call isinf() a lot, so we use a fast version in this file */
#define fast_isinf(val) (((val) DBL_MIN || (val) DBL_MAX)
isinf(val))
and used that instead of the direct isinf() call. (We do call fabs() in
the Check* routines. Should we be using our own Abs()?) The new patch
also uses float8 for float4 computations, and adds a comment about why
(avoid underflow in some cases).
In looking at the idea of checking for zero as an underflow, I found
most transcendental functions already had such a check, so I moved the check
into the Check*() routines, and added checks for multiplication/division
underflow to zero. The only outstanding uncaught underflow is from
addition/subtraction.
--
Bruce Momjian [EMAIL PROTECTED]
EnterpriseDBhttp://www.enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
Index: src/backend/utils/adt/float.c
===
RCS file: /cvsroot/pgsql/src/backend/utils/adt/float.c,v
retrieving revision 1.131
diff -c -c -r1.131 float.c
*** src/backend/utils/adt/float.c 23 Dec 2006 02:13:24 - 1.131
--- src/backend/utils/adt/float.c 28 Dec 2006 20:32:32 -
***
*** 87,92
--- 87,95
#define NAN (*(const double *) nan)
#endif
+ /* We call isinf() a lot, so we use a fast version in this file */
+ #define fast_isinf(val) (((val) DBL_MIN || (val) DBL_MAX) isinf(val))
+
/* not sure what the following should be, but better to make it over-sufficient */
#define MAXFLOATWIDTH 64
#define MAXDOUBLEWIDTH 128
***
*** 104,111
int extra_float_digits = 0; /* Added to DBL_DIG or FLT_DIG */
! static void CheckFloat4Val(double val);
! static void CheckFloat8Val(double val);
static int float4_cmp_internal(float4 a, float4 b);
static int float8_cmp_internal(float8 a, float8 b);
--- 107,114
int extra_float_digits = 0; /* Added to DBL_DIG or FLT_DIG */
! static void CheckFloat4Val(double val, bool has_inf_args, bool zero_is_valid);
! static void CheckFloat8Val(double val, bool has_inf_args, bool zero_is_valid);
static int float4_cmp_internal(float4 a, float4 b);
static int float8_cmp_internal(float8 a, float8 b);
***
*** 211,219
* raise an ereport() error if it is
*/
static void
! CheckFloat4Val(double val)
{
! if (fabs(val) FLOAT4_MAX)
ereport(ERROR,
(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
errmsg(type \real\ value out of range: overflow)));
--- 214,223
* raise an ereport() error if it is
*/
static void
! CheckFloat4Val(double val, bool has_inf_args, bool zero_is_valid)
{
! /* If one of the input arguments was infinity, allow an infinite result */
! if (fabs(val) FLOAT4_MAX (!fast_isinf(val) || !has_inf_args))
ereport(ERROR,
(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
errmsg(type \real\ value out of range: overflow)));
***
*** 230,242
* raise an ereport() error if it is
*/
static void
! CheckFloat8Val(double val)
{
! if (fabs(val) FLOAT8_MAX)
ereport(ERROR,
(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
errmsg(type \double precision\ value out of range: overflow)));
! if (val != 0.0 fabs(val) FLOAT8_MIN)
ereport(ERROR,
(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
errmsg(type \double precision\ value out of range: underflow)));
--- 234,263
* raise an ereport() error if it is
*/
static void
! CheckFloat8Val(double val, bool has_inf_args, bool zero_is_valid)
{
! /*
! * Computations that slightly exceed FLOAT8_MAX are non-Infinity,
! * but those that greatly exceed FLOAT8_MAX become Infinity. Therefore
! * it is difficult to tell if a value is really infinity or the result
! * of an overflow. The solution is to use a boolean indicating if
! * the input arguments were infiity, meaning an infinite result is
! * probably not the result of an overflow. This allows various
! * computations like SELECT 'Inf'::float8 + 5.
! */
! if (fabs(val) FLOAT8_MAX (!fast_isinf(val) || !has_inf_args))
ereport(ERROR,
(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
errmsg(type \double precision\ value out of range: overflow)));
! /*
Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing
Bruce Momjian [EMAIL PROTECTED] writes: I wasn't excited about doing one isinf() call to avoid three, so I just made a fast isinf() macro: /*We call isinf() a lot, so we use a fast version in this file */ #define fast_isinf(val) (((val) DBL_MIN || (val) DBL_MAX) isinf(val)) This is *not* going in the right direction :-( regards, tom lane ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing handling of
I have made some more progress on this patch. I have fixed the issue
with aggregates:
test= select avg(ff) from tt;
ERROR: type double precision value out of range: overflow
and tested the performance overhead of the new CheckFloat8Val() calls
the fix requires using:
EXPLAIN ANALYZE SELECT AVG(x.COL)
FROM (SELECT 323.2 AS COL FROM generate_series(1,100)) AS x;
and could not measure any overhead.
I also found a few more bugs, this one with float4 negation:
test= SELECT -('0'::float4);
?column?
--
-0
(1 row)
test= SELECT -('0'::float8);
?column?
--
0
(1 row)
and this one with casting 'Nan' to an integer:
test= SELECT 'Nan'::float8::int4;
int4
-
-2147483648
(1 row)
I have fixed these as well:
test= SELECT -('0'::float4);
?column?
--
0
(1 row)
test= SELECT 'Nan'::float8::int4;
ERROR: integer out of range
The only unsolved issue is the one with underflow checks. I have added
comments explaining the problem in case someone ever figures out how to
address it.
If I don't receive further comments, I will apply the new attached patch
shortly.
---
bruce wrote:
Roman Kononov wrote:
The following bug has been logged online:
Bug reference: 2846
Logged by: Roman Kononov
Email address: [EMAIL PROTECTED]
PostgreSQL version: 8.2.0 and older
Operating system: linux 2.6.15-27-amd64 ubuntu
Description:inconsistent and confusing handling of underflows, NaNs
and INFs
Details:
This is a very interesting bug report. It seems you have done some good
analysis of PostgreSQL and how it handles certain corner cases,
infinity, and NaN.
I have researched your findings and will show some fixes below:
Please compare the results of the simple queries.
==
test=# select ('NaN'::float4)::int2;
int2
--
0
(1 row)
There certainly should be an isnan() test when converting to int2
because while float can represent NaN, int2 cannot. The fix shows:
test= select ('NaN'::float4)::int2;
ERROR: smallint out of range
test=# select ('NaN'::float4)::int4;
int4
-
-2147483648
(1 row)
Same for int4:
test= select ('NaN'::float4)::int4;
ERROR: integer out of range
test=# select ('NaN'::float4)::int8;
ERROR: bigint out of range
This one was correct because it uses rint() internally.
test=# select ('nan'::numeric)::int4;
ERROR: cannot convert NaN to integer
==
test=# select abs('INF'::float4);
abs
--
Infinity
(1 row)
Correct.
test=# select abs('INF'::float8);
ERROR: type double precision value out of range: overflow
This one was more complicated. float4/8 operations test for
results FLOAT[84]_MAX. This is because if you do this:
test= select (1e201::float8)*(1e200::float8);
the result internally is Infinity, so they check for Inf as a check for
overflow. The bottom line is that while the current code allows
infinity to be entered, it does not allow the value to operate in many
context because it is assumes Inf to be an overflow indicator. I have
fixed this by passing a boolean to indicate if any of the operands were
infinity, and if so, allow an infinite result, so this now works:
test= select abs('INF'::float8);
abs
--
Infinity
(1 row)
==
test=# select -('INF'::float4);
?column?
---
-Infinity
(1 row)
test=# select -('INF'::float8);
ERROR: type double precision value out of range: overflow
And this now works too:
test= select -('INF'::float8);
?column?
---
-Infinity
(1 row)
==
test=# select (1e-37::float4)*(1e-22::float4);
?column?
--
0
(1 row)
This one is quite complex. For overflow, there is a range of values
that is represented as FLOAT8_MAX, but for values very large, the
result becomes Inf. The old code assumed an Inf result was an overflow,
and threw an error, as I outlined above. The new code does a better
job.
Now, for underflow. For underflow, we again have a range slightly
smaller than DBL_MIN where we can detect an underflow, and throw an
error, but just like overflow, if the underflow is too small, the result
becomes zero. The bad news is that unlike Inf, zero isn't a special
value. With Inf, we could say if we got an infinite result from
non-infinite arguments, we had an overflow,
Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing handling of
Bruce Momjian [EMAIL PROTECTED] writes: I have made some more progress on this patch. I'm not convinced that you're fixing things so much as doing your best to destroy IEEE-compliant float arithmetic behavior. I think what we should probably consider is removing CheckFloat4Val and CheckFloat8Val altogether, and just letting the float arithmetic have its head. Most modern hardware gets float arithmetic right per spec, and we shouldn't be second-guessing it. A slightly less radical proposal is to reject only the case where isinf(result) and neither input isinf(); and perhaps likewise with respect to NaNs. regards, tom lane ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing handling of
Roman Kononov wrote:
On 12/27/2006 12:44 PM, Bruce Momjian wrote:
The only unsolved issue is the one with underflow checks. I have added
comments explaining the problem in case someone ever figures out how to
address it.
This will behave better for float4:
Datum float4pl(PG_FUNCTION_ARGS)
{
---float4 arg1 = PG_GETARG_FLOAT4(0);
---float4 arg2 = PG_GETARG_FLOAT4(1);
+++double arg1 = PG_GETARG_FLOAT4(0);
+++double arg2 = PG_GETARG_FLOAT4(1);
double result;
result = arg1 + arg2;
CheckFloat4Val(result,isinf(arg1) || isinf(arg2));
PG_RETURN_FLOAT4((float4) result);
}
Are you sure? As I remember, computation automatically upgrades to
'double'. See this program and output:
$ cat tst1.c
#include stdio.h
#include stdlib.h
int
main(int argc, char *argv[])
{
float a = 1e30, b = 1e30;
double c;
c = a * b;
printf(%e\n, c);
return 0;
}
$ tst1
1.00e+60
--
Bruce Momjian [EMAIL PROTECTED]
EnterpriseDBhttp://www.enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
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Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing
Tom Lane wrote:
Bruce Momjian [EMAIL PROTECTED] writes:
I have made some more progress on this patch.
I'm not convinced that you're fixing things so much as doing your best
to destroy IEEE-compliant float arithmetic behavior.
I think what we should probably consider is removing CheckFloat4Val
and CheckFloat8Val altogether, and just letting the float arithmetic
have its head. Most modern hardware gets float arithmetic right per
spec, and we shouldn't be second-guessing it.
Well, I am on an Xeon and can confirm that our computations of large
non-infinite doubles who's result greatly exceed the max double are
indeed returning infinity, as the poster reported, so something isn't
working, if it supposed to. What do people get for this computation?
#include stdio.h
#include stdlib.h
int
main(int argc, char *argv[])
{
double a = 1e300, b = 1e300;
double c;
c = a * b;
printf(%e\n, c);
return 0;
}
I get 'inf'. I am on BSD and just tested it on Fedora Core 2 and got
'inf' too.
A slightly less radical proposal is to reject only the case where
isinf(result) and neither input isinf(); and perhaps likewise with
respect to NaNs.
Uh, that's what the patch does for 'Inf':
result = arg1 + arg2;
CheckFloat4Val(result, isinf(arg1) || isinf(arg2));
I didn't touch 'Nan' because that is passed around as a value just fine
--- it isn't created or tested as part of an overflow.
--
Bruce Momjian [EMAIL PROTECTED]
EnterpriseDBhttp://www.enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
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Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing handling of underflows,
On 12/27/2006 03:23 PM, Bruce Momjian wrote:
Are you sure? As I remember, computation automatically upgrades to
'double'. See this program and output:
This is platform- and compiler- dependent:
~uname -a
Linux rklinux 2.6.15-27-amd64-generic #1 SMP PREEMPT Fri Dec 8 17:50:54 UTC
2006 x86_64 GNU/Linux
~gcc --version
gcc (GCC) 4.3.0 20061213 (experimental)
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
~cat test.c
#include stdio.h
#include stdlib.h
int
main(int argc, char *argv[])
{
float a = 1e30, b = 1e30;
double c;
c = a * b;
printf(%e\n, c);
return 0;
}
~gcc test.c
~./a.out
inf
~gcc -march=i386 -m32 test.c
~./a.out
1.00e+60
Roman
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Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing handling of
Roman Kononov wrote:
On 12/27/2006 03:23 PM, Bruce Momjian wrote:
Are you sure? As I remember, computation automatically upgrades to
'double'. See this program and output:
This is platform- and compiler- dependent:
~uname -a
Linux rklinux 2.6.15-27-amd64-generic #1 SMP PREEMPT Fri Dec 8 17:50:54 UTC
2006 x86_64 GNU/Linux
~gcc --version
gcc (GCC) 4.3.0 20061213 (experimental)
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
~cat test.c
#include stdio.h
#include stdlib.h
int
main(int argc, char *argv[])
{
float a = 1e30, b = 1e30;
double c;
c = a * b;
printf(%e\n, c);
return 0;
}
~gcc test.c
~./a.out
inf
~gcc -march=i386 -m32 test.c
~./a.out
1.00e+60
Interesting. I didn't know that, but in the float4pl() function,
because the overflow tests and result is float4, what value is there to
doing things as double --- as soon as the float4 maximum is exceeded, we
throw an error?
--
Bruce Momjian [EMAIL PROTECTED]
EnterpriseDBhttp://www.enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
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Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing handling of underflows,
On 12/27/2006 04:04 PM, Bruce Momjian wrote:
Interesting. I didn't know that, but in the float4pl() function,
because the overflow tests and result is float4, what value is there to
doing things as double --- as soon as the float4 maximum is exceeded, we
throw an error?
This is useful for underflows.
float a=1e-30;
float b=1e-30;
double r1=a*b;
double r2=(double)a*b;
r1 is zero and underflow is lost.
r2 is not zero and underflow is detected.
In float4mul() and float4div(), the computation should be double precision.
In float4pl() and float4mi(), it depends on the ability of the hardware
to generate denormalized numbers. If denormalized numbers are generated,
float vs double makes no difference. If denormalized numbers are not
generated (zero is generated), then double computation is safer.
Another way to detect underflows, overflows and other junk is to use FPU
status flags after each computation. Performance will likely suffer.
#include fenv.h
Datum
float4mul(PG_FUNCTION_ARGS)
{
float4 arg1 = PG_GETARG_FLOAT4(0);
float4 arg2 = PG_GETARG_FLOAT4(1);
float4 result;
int fe_exceptions;
feclearexcept(FE_ALL_EXCEPT);
result = arg1 * arg2;
fe_exceptions=fetestexcept(FE_DIVBYZERO,FE_INVALID,FE_OVERFLOW,FE_UNDERFLOW);
if (fe_exceptions) handle_exceptions(fe_exceptions); //??
PG_RETURN_FLOAT4(result);
}
Yet another way to detect exceptions is to remove all CheckFloat4Val(),
CheckFloat8Val(), isnan(), unmask FPU exceptions and install an exception
handler.
Might have portability difficulties. Comparisons of NaNs must be done in
non-IEEE way (FPU does not compare NaNs, it generates exceptions).
Roman
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Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing
I get 'inf'. I am on BSD and just tested it on Fedora Core 2 and got 'inf' too. Ubuntu Edgy 64bit on Athlon 64X2 returns inf. Joshua D. Drake A slightly less radical proposal is to reject only the case where isinf(result) and neither input isinf(); and perhaps likewise with respect to NaNs. Uh, that's what the patch does for 'Inf': result = arg1 + arg2; CheckFloat4Val(result, isinf(arg1) || isinf(arg2)); I didn't touch 'Nan' because that is passed around as a value just fine --- it isn't created or tested as part of an overflow. -- === The PostgreSQL Company: Command Prompt, Inc. === Sales/Support: +1.503.667.4564 || 24x7/Emergency: +1.800.492.2240 Providing the most comprehensive PostgreSQL solutions since 1997 http://www.commandprompt.com/ Donate to the PostgreSQL Project: http://www.postgresql.org/about/donate ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing handling of
Bruce Momjian [EMAIL PROTECTED] writes: Tom Lane wrote: I think what we should probably consider is removing CheckFloat4Val and CheckFloat8Val altogether, and just letting the float arithmetic have its head. Most modern hardware gets float arithmetic right per spec, and we shouldn't be second-guessing it. Well, I am on an Xeon and can confirm that our computations of large non-infinite doubles who's result greatly exceed the max double are indeed returning infinity, as the poster reported, so something isn't working, if it supposed to. What do people get for this computation? Infinity is what you are *supposed* to get, per IEEE spec. A slightly less radical proposal is to reject only the case where isinf(result) and neither input isinf(); and perhaps likewise with respect to NaNs. Uh, that's what the patch does for 'Inf': result = arg1 + arg2; CheckFloat4Val(result, isinf(arg1) || isinf(arg2)); No, because you are still comparing against FLOAT4_MAX. I'm suggesting that only an actual infinity should be rejected. Even that is contrary to IEEE spec, though. The other problem with this coding technique is that it must invoke isinf three times when the typical case really only requires one (if the output isn't inf there is no need to perform isinf on the inputs). If we're going to check for overflow at all, I think we should lose the subroutine and just do if (isinf(result) !(isinf(arg1) || isinf(arg2))) ereport(...OVERFLOW...); regards, tom lane ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing handling of underflows,
Roman Kononov [EMAIL PROTECTED] writes: On 12/27/2006 03:23 PM, Bruce Momjian wrote: Are you sure? As I remember, computation automatically upgrades to 'double'. See this program and output: This is platform- and compiler- dependent: ... and probably irrelevant, too. We should store the result into a float4 variable and then test for isinf() on that; that eliminates the question of whether the compiler did the multiply in a wider format or not. regards, tom lane ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing handling of underflows,
Roman Kononov [EMAIL PROTECTED] writes: In float4mul() and float4div(), the computation should be double precision. Why? It's going to have to fit in a float4 eventually anyway. regards, tom lane ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing handling
On 12/27/2006 05:19 PM, Tom Lane wrote: Roman Kononov [EMAIL PROTECTED] writes: On 12/27/2006 03:23 PM, Bruce Momjian wrote: Are you sure? As I remember, computation automatically upgrades to 'double'. See this program and output: This is platform- and compiler- dependent: ... and probably irrelevant, too. We should store the result into a float4 variable and then test for isinf() on that; that eliminates the question of whether the compiler did the multiply in a wider format or not. You are right provided that you want to ignore underflows and silently produce zeros instead. If you go this way, I recommend to ignore overflows as well, and silently produce infinities and NaNs. Roman ---(end of broadcast)--- TIP 7: You can help support the PostgreSQL project by donating at http://www.postgresql.org/about/donate
Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing
Tom Lane wrote: Roman Kononov [EMAIL PROTECTED] writes: In float4mul() and float4div(), the computation should be double precision. Why? It's going to have to fit in a float4 eventually anyway. One issue is in the patch comment: !* Computations that slightly exceed FLOAT8_MAX are non-Infinity, !* but those that greatly exceed FLOAT8_MAX become Infinity. Therefore !* it is difficult to tell if a value is really infinity or the result !* of an overflow. The solution is to use a boolean indicating if !* the input arguments were infiity, meaning an infinite result is !* probably not the result of an overflow. This allows various !* computations like SELECT 'Inf'::float8 + 5. +* Underflow has similar issues to overflow, i.e. if a computation is +* slighly smaller than FLOAT8_MIN, the result is non-zero, but if it is +* much smaller than FLOAT8_MIN, the value becomes zero. However, +* unlike overflow, zero is not a special value and can be the result +* of a computation, so there is no easy way to pass a boolean +* indicating whether a zero result is reasonable or not. It might +* be possible for multiplication and division, but because of rounding, +* such tests would probably not be reliable. For overflow, it doesn't matter, but by using float8, you have a much larger range until you underflow to zero. I will make adjustments to the patch to use this, and add comments explaining its purpose. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing
Tom Lane wrote: Roman Kononov [EMAIL PROTECTED] writes: On 12/27/2006 03:23 PM, Bruce Momjian wrote: Are you sure? As I remember, computation automatically upgrades to 'double'. See this program and output: This is platform- and compiler- dependent: ... and probably irrelevant, too. We should store the result into a float4 variable and then test for isinf() on that; that eliminates the question of whether the compiler did the multiply in a wider format or not. True for overflow to Infinity, but underflow checking is better for float4 using float8. See previous email for details. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 7: You can help support the PostgreSQL project by donating at http://www.postgresql.org/about/donate
Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing
Roman Kononov wrote: On 12/27/2006 05:19 PM, Tom Lane wrote: Roman Kononov [EMAIL PROTECTED] writes: On 12/27/2006 03:23 PM, Bruce Momjian wrote: Are you sure? As I remember, computation automatically upgrades to 'double'. See this program and output: This is platform- and compiler- dependent: ... and probably irrelevant, too. We should store the result into a float4 variable and then test for isinf() on that; that eliminates the question of whether the compiler did the multiply in a wider format or not. You are right provided that you want to ignore underflows and silently produce zeros instead. If you go this way, I recommend to ignore overflows as well, and silently produce infinities and NaNs. While IEEE seems fine with that, I don't think this is good for SQL. It is best to produce a meaningful error. The major issue is that our current code is buggy because it doesn't have consistent behavior, as Roman outlined. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [PATCHES] [BUGS] BUG #2846: inconsistent and confusing handling of
Roman Kononov wrote:
The following bug has been logged online:
Bug reference: 2846
Logged by: Roman Kononov
Email address: [EMAIL PROTECTED]
PostgreSQL version: 8.2.0 and older
Operating system: linux 2.6.15-27-amd64 ubuntu
Description:inconsistent and confusing handling of underflows, NaNs
and INFs
Details:
This is a very interesting bug report. It seems you have done some good
analysis of PostgreSQL and how it handles certain corner cases,
infinity, and NaN.
I have researched your findings and will show some fixes below:
Please compare the results of the simple queries.
==
test=# select ('NaN'::float4)::int2;
int2
--
0
(1 row)
There certainly should be an isnan() test when converting to int2
because while float can represent NaN, int2 cannot. The fix shows:
test= select ('NaN'::float4)::int2;
ERROR: smallint out of range
test=# select ('NaN'::float4)::int4;
int4
-
-2147483648
(1 row)
Same for int4:
test= select ('NaN'::float4)::int4;
ERROR: integer out of range
test=# select ('NaN'::float4)::int8;
ERROR: bigint out of range
This one was correct because it uses rint() internally.
test=# select ('nan'::numeric)::int4;
ERROR: cannot convert NaN to integer
==
test=# select abs('INF'::float4);
abs
--
Infinity
(1 row)
Correct.
test=# select abs('INF'::float8);
ERROR: type double precision value out of range: overflow
This one was more complicated. float4/8 operations test for
results FLOAT[84]_MAX. This is because if you do this:
test= select (1e201::float8)*(1e200::float8);
the result internally is Infinity, so they check for Inf as a check for
overflow. The bottom line is that while the current code allows
infinity to be entered, it does not allow the value to operate in many
context because it is assumes Inf to be an overflow indicator. I have
fixed this by passing a boolean to indicate if any of the operands were
infinity, and if so, allow an infinite result, so this now works:
test= select abs('INF'::float8);
abs
--
Infinity
(1 row)
==
test=# select -('INF'::float4);
?column?
---
-Infinity
(1 row)
test=# select -('INF'::float8);
ERROR: type double precision value out of range: overflow
And this now works too:
test= select -('INF'::float8);
?column?
---
-Infinity
(1 row)
==
test=# select (1e-37::float4)*(1e-22::float4);
?column?
--
0
(1 row)
This one is quite complex. For overflow, there is a range of values
that is represented as FLOAT8_MAX, but for values very large, the
result becomes Inf. The old code assumed an Inf result was an overflow,
and threw an error, as I outlined above. The new code does a better
job.
Now, for underflow. For underflow, we again have a range slightly
smaller than DBL_MIN where we can detect an underflow, and throw an
error, but just like overflow, if the underflow is too small, the result
becomes zero. The bad news is that unlike Inf, zero isn't a special
value. With Inf, we could say if we got an infinite result from
non-infinite arguments, we had an overflow, but for underflow, how do we
know if zero is an underflow or just the correct result? For
multiplication, we could say that a zero result for non-zero arguments
is almost certainly an underflow, but I don't see how we can handle the
other operations as simply.
I was not able to fix the underflow problems you reported.
test=# select (1e-37::float4)*(1e-2::float4);
ERROR: type real value out of range: underflow
==
test=# select (1e-300::float8)*(1e-30::float8);
?column?
--
0
(1 row)
test=# select (1e-300::float8)*(1e-20::float8);
ERROR: type double precision value out of range: underflow
==
test=# select ('INF'::float8-'INF'::float8);
?column?
--
NaN
(1 row)
test=# select ('INF'::float8+'INF'::float8);
ERROR: type double precision value out of range: overflow
This works fine now:
test= select ('INF'::float8+'INF'::float8);
?column?
--
Infinity
(1 row)
==
test=# select ('INF'::float4)::float8;
float8
--
Infinity
(1 row)
test=# select ('INF'::float8)::float4;
ERROR: type real value out of range: overflow
==
test=# select cbrt('INF'::float4);
cbrt
--
Infinity
(1 row)
test=# select sqrt('INF'::float4);
ERROR: type double precision value out of range: overflow
This works fine too:
test=
