[Bug c/115477] New: compiling with gcc flips the sign of a float return value

[s00na at protonmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115477

            Bug ID: 115477
           Summary: compiling with gcc flips the sign of a float return
                    value
           Product: gcc
           Version: 14.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: s00na at protonmail dot com
  Target Milestone: ---

##############
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/x86_64-linux-gnu/14.1.0/lto-wrapper
Target: x86_64-linux-gnu
Configured with: /usr/src/gcc/configure --build=x86_64-linux-gnu
--disable-multilib --enable-languages=c,c++,fortran,go
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 14.1.0 (GCC)
##############
A little context. I was playing around with the fast inverse square root
algorithm (https://en.wikipedia.org/wiki/Fast_inverse_square_root) and found
that compiling my own version of it returns the correct answer, but with the
wrong sign. I ecountered this behavior first on my archlinux system with gcc
14.1.1. To verify that it wasn't anything wrong with my setup I also spun up
docker containers for the latest versions of ubuntu, arch, gcc itself and
alpine. It happened on all of these except for the alpine container (gcc
13.2.1). Using clang to compile produces the correct/expected result except for
the gcc container where clang also produces the wrong result.
In all cases I compiled with:
gcc test.c
and ran the resulting file with:
./a.out
The expected result is:
0.447141
0.447141
The actual result with gcc is:
0.447141
-0.447141
Below is the code that produces the error.
test.c:

#include <stdio.h>
float bug(float number) {
        long i;
        float y = number;
        i = * ( long * ) &y;
        i = 0x5f3759df - ( i >> 1 );
        y = * ( float * ) &i;
        return (1.5F * y) + (-0.5F * number * y * y * y);
}
int main() {
        const float test = 5;
        printf("%f\n",bug(test));
        printf("%f\n",bug(test));
        return 0;
}