Re: BUG #19593: area(circle) silently returns Infinity instead of raising "value out of range: overflow"

Hi Michael, all,

I've opened PR in Bugzilla GCC for the issue you reported:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126631

I'll follow up there with any progress.

сб, 1 авг. 2026 г. в 20:16, Andrey Rachitskiy <pl0h0yp1(at)gmail(dot)com>:

> Hi, Michael!
>
> Agreed — PostgreSQL's check is fine, this is a gcc wrong-code bug.
>
> circle_ar() is just two inlined float8_mul() calls:
> ```
> return float8_mul(float8_mul(circle->radius, circle->radius), M_PI);
> ```
> For radius 1e154, r*r is still finite (~1e308), but (r*r)*pi overflows.
> float8_mul() is supposed to catch that with:
> ```
> result = val1 * val2;
> if (unlikely(isinf(result)) && !isinf(val1) && !isinf(val2))
> float_overflow_error();
> ```
> That check is present in the source and survives at -O0, with clang,
> and for the equivalent SQL expression
> 1e154::float8 * 1e154::float8 * pi() (as in the report). With gcc -O1
> and above it is deleted for the outer multiply.
>
> What gcc does is jump threading (-fthread-jumps; enabled by default at
> -O1 and above). After the inner check it knows r*r is finite, M_PI is
> a finite constant, and it incorrectly treats "finite * finite yields
> Inf" as impossible. The outer float_overflow_error() call disappears;
> the function just returns Infinity. -O2 -fno-thread-jumps restores the
> correct behaviour. This is with a normal -O2 build; we do not use
> -ffinite-math-only / -ffast-math, so gcc is not entitled to assume that
> finite*finite cannot produce Infinity.
>
> Reduced C reproducer (same shape as circle_ar / float8_mul). On the
> correct path it prints the same message as PostgreSQL:
> ```
> ERROR: value out of range: overflow
> ```
> On the buggy path it prints Infinity (and exits 1):
> ```
> Host: Linux x86_64
> gcc: gcc (Ubuntu 15.2.0-16ubuntu1) 15.2.0
> clang: Ubuntu clang version 21.1.8
>
> compiler / flags result
> ------------------------------- ------------------------------------
> gcc -O0 OK (ERROR: value out of range...)
> gcc -O1 BUG (Infinity)
> gcc -O2 BUG (Infinity)
> gcc -O2 -fno-thread-jumps OK (ERROR: value out of range...)
> clang -O0 OK (ERROR: value out of range...)
> clang -O1 OK (ERROR: value out of range...)
> clang -O2 OK (ERROR: value out of range...)
> ```
> So this is not "PostgreSQL does not check circle areas". The check is
> there, gcc optimizes it away, clang does not. The same pattern can
> affect other inlined float4/8 helpers when both operands are proven
> finite.
>
> One possible workaround on our side would be to store the operation
> result in a volatile temporary in those helpers, so the isinf() check
> must inspect the computed value. But it seems to me that we should
> address the root cause first: file this with gcc bugzilla and decide
> what, if anything, to do in PostgreSQL based on their response.
>
> сб, 1 авг. 2026 г. в 18:02, PG Bug reporting form <noreply(at)postgresql(dot)org
> >:
>
>> The following bug has been logged on the website:
>>
>> Bug reference: 19593
>> Logged by: Michael Malis
>> Email address: malis(at)pgrust(dot)com
>> PostgreSQL version: 18.3
>> Operating system: Debian (official Docker image), aarch64
>> Description:
>>
>> I'm not sure what you'll want to do with this one, but I figured I would
>> at
>> least report it. The cause seems to be a bug in gcc.
>>
>> area(circle) returns Infinity where it must raise ERROR 22003 "value out
>> of
>> range: overflow". The overflow check in float8_mul() is present in the
>> source
>> but is absent from the generated code, because gcc 13 and later delete it
>> at
>> -O1 and above.
>>
>> The same binary raises the error correctly for the equivalent SQL-level
>> expression, and for a circle whose radius overflows one step earlier, so
>> this
>> is not "PostgreSQL does not check circle areas".
>>
>> -- WRONG: no error, returns Infinity
>> SELECT area(circle '<(0,0),1e154>');
>> area
>> ------------------------
>> Infinity
>>
>> -- CORRECT (control): the *inner* multiply overflows, so the surviving
>> -- check fires
>> SELECT area(circle '<(0,0),1e200>');
>> ERROR: value out of range: overflow
>>
>> -- CORRECT (control): the same arithmetic, expressed in SQL
>> SELECT 1e154::float8 * 1e154::float8 * pi();
>> ERROR: value out of range: overflow
>>
>> -- sane value, for reference
>> SELECT area(circle '<(0,0),1e10>');
>> area
>> -------------------------
>> 3.1415926535897933e+20
>>
>> 1e154 * 1e154 = 1e308, which is finite (below DBL_MAX); multiplying that
>> by
>> pi
>> overflows. Behaviour is identical whether the expression is
>> constant-folded
>> at
>> plan time or evaluated at runtime:
>>
>> SELECT area(c) FROM (VALUES (circle '<(0,0),1e154>')) t(c); --
>> Infinity
>>
>> NaN and Infinity radii behave correctly (NaN -> NaN, Infinity ->
>> Infinity).
>>
>>
>> WHERE IT COMES FROM
>>
>> src/backend/utils/adt/geo_ops.c:5159
>>
>> static float8
>> circle_ar(CIRCLE *circle)
>> {
>> return float8_mul(float8_mul(circle->radius, circle->radius),
>> M_PI);
>> }
>>
>> src/include/utils/float.h:207
>>
>> static inline float8
>> float8_mul(const float8 val1, const float8 val2)
>> {
>> float8 result;
>>
>> result = val1 * val2;
>> if (unlikely(isinf(result)) && !isinf(val1) && !isinf(val2))
>> float_overflow_error();
>> ...
>>
>> Two float8_mul calls are inlined into one function. gcc keeps the first
>> copy's
>> overflow check and deletes the second's.
>>
>> Disassembly of the shipped binary (circle_area; symbols are present in
>> .dynsym).
>> gcc lowers isinf(x) to |x| > DBL_MAX, with d30 = 0x7fefffffffffffff:
>>
>> ; inner multiply -- check intact
>> 5540dc fmul d31, d29, d29 ; r*r
>> 5540e0 fcmp d31, d30
>> 5540e4 b.le 5540fc
>> 5540e8 fabs d29, d29 ; |r| <- the !isinf(val1) test
>> 5540ec fcmp d29, d30
>> 5540f0 b.le 554154
>> 554154 bl float_overflow_error ; raises
>>
>> ; outer multiply -- operand test gone
>> 554104 adrp x0, 76c000
>> 554108 ldr d29, [x0, #568] ; M_PI
>> 55410c fmul d31, d31, d29 ; (r*r) * M_PI
>> 554110 fcmp d31, d30
>> 554114 b.le 554120
>> 554118 mov x0, #0x7ff0000000000000 ; returns +Infinity
>> 55411c b 55412c
>>
>> Control reaches the outer multiply only via the b.le at 5540e4, i.e. only
>> when
>> r*r <= DBL_MAX, and M_PI is a finite constant. So
>> "!isinf(val1) && !isinf(val2)" is true on that path and
>> float_overflow_error()
>> must be called.
>>
>> Building from an unmodified 18.3 tree (git tag stamp 62d6c7d) with gcc
>> 14.2
>> and
>> the same CFLAGS reproduces it, so this is not specific to Debian's
>> packaging:
>>
>> ./configure --without-readline --without-zlib --without-icu \
>> CFLAGS="-g -O2 -fno-strict-aliasing -fwrapv
>> -fexcess-precision=standard"
>> make -C src/backend submake-generated-headers
>> make -C src/backend/utils/adt geo_ops.o
>> objdump -d geo_ops.o
>>
>> circle_area then contains 3 fmul but only 1 call to float_overflow_error.
>> From
>> pristine source the outer check is removed entirely: there is no DBL_MAX
>> comparison after the second fmul at all, only the underflow test.
>>
>>
>>
>>
>>
>
> --
> Regards,
> Rachitskiy Andrey
>

--
Regards,
Rachitskiy Andrey