[Bug middle-end/93806] Wrong optimization: instability of floating-point results with -funsafe-math-optimizations leads to nonsense

[ch3root at openwall dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93806

--- Comment #8 from Alexander Cherepanov <ch3root at openwall dot com> ---
A similar problem happens with -fno-signed-zeros -fno-trapping-math. Not sure
if a separate PR should be filed...

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#include <stdio.h>

__attribute__((noipa)) // imagine it in a separate TU
static double opaque(double d) { return d; }

int main()
{
    int zero = opaque(0);

    double x = opaque(-0.);
    int a = 1 / x == 1 / 0.;

    printf("zero = %d\n", zero);

    opaque(a);
    if (zero == a) {
        opaque(0);
        if (x == 0) {
            opaque(0);
            if (a) {
                opaque(0);
                if (zero == 1)
                    printf("zero = %d\n", zero);
            }
        }
    }
}
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$ gcc -std=c11 -pedantic -Wall -Wextra -fno-signed-zeros -fno-trapping-math -O3
test.c && ./a.out
zero = 0
zero = 1
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gcc x86-64 version: gcc (GCC) 10.0.1 20200220 (experimental)
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