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[Bug c/48874] Sign of zeros sometimes lost in literals
- From: "kargl at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Wed, 4 May 2011 17:22:18 +0000
- Subject: [Bug c/48874] Sign of zeros sometimes lost in literals
- Auto-submitted: auto-generated
- References: <bug-48874-4@http.gcc.gnu.org/bugzilla/>
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48874
kargl at gcc dot gnu.org changed:
What |Removed |Added
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CC| |kargl at gcc dot gnu.org
--- Comment #2 from kargl at gcc dot gnu.org 2011-05-04 17:00:31 UTC ---
(In reply to comment #0)
> Consider
>
> #include <stdio.h>
> #include <complex.h>
>
> int main()
> {
> double _Complex a = 0.0 + I*0.0;
> double _Complex b = 0.0 - I*0.0;
> double _Complex c = -0.0 + I*0.0;
> double _Complex d = -0.0 - I*0.0;
> printf("a= (%g,%g)\n", creal(a), cimag(a));
> printf("b= (%g,%g)\n", creal(b), cimag(b));
> printf("c= (%g,%g)\n", creal(c), cimag(c));
> printf("d= (%g,%g)\n", creal(d), cimag(d));
> }
>
> This program, compiled with "gcc zero1.c -O2 -pedantic -Wall -std=c99" (or
> -std=gnu99) prints
>
> a= (0,0)
> b= (0,-0)
> c= (0,0)
> d= (-0,-0)
>
> That is, the sign of the real part of "c" is lost. Add -fdump-tree-original to
> the compile flags shows the dump as
To some of us this, this is a well-known problem/issue/feature of gcc.
In the expression, -0.0 + I * 0.0, the I is treated as 0 + i 1 where
i = sqrt(-1). So, you get -0.0 + (0 + i 1) * 0.0 = -0.0 + 0.0 + i 0.0,
which yields 0.0 + i 0.0.
There were/are fun issues with NaN and +-Inf. Joseph fixed some/all
of those problems. For details, see
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24581