When building for 32-bit x86 but with SSE2 floating-point enabled:
-m32 -msse2 -mfpmath=sse

the conversion from unsigned int 0 to double produces the result of -0.0 when floating-point rounding mode is set to FE_DOWNWARD.

I used -frounding-math and #pragma STDC FENV_ACCESS ON.

This bug is not present on x87 nor x86_64 builds.

The bug seems to be present at least since GCC 5.


#include <fenv.h>

#pragma STDC FENV_ACCESS ON

__attribute__((noinline)) double u32_to_f64(unsigned x) {
  return static_cast<double>(x);
}

int main() {
  fesetround(FE_DOWNWARD);

  double d = u32_to_f64(0);

  return __builtin_signbit(d) != 0;  // signbit should be 0
}


The assembly:

u32_to_f64(unsigned int):
        sub     esp, 12
        pxor    xmm0, xmm0
        mov     eax, DWORD PTR [esp+16]
        add     eax, -2147483648
        cvtsi2sd        xmm0, eax
        addsd   xmm0, QWORD PTR .LC0
        movsd   QWORD PTR [esp], xmm0
        fld     QWORD PTR [esp]
        add     esp, 12
        ret
main:
        lea     ecx, [esp+4]
        and     esp, -16
        push    DWORD PTR [ecx-4]
        push    ebp
        mov     ebp, esp
        push    ecx
        sub     esp, 32
        push    1024
        call    fesetround
        mov     DWORD PTR [esp], 0
        call    u32_to_f64(unsigned int)
        mov     ecx, DWORD PTR [ebp-4]
        add     esp, 16
        fstp    QWORD PTR [ebp-16]
        movsd   xmm0, QWORD PTR [ebp-16]
        leave
        lea     esp, [ecx-4]
        movmskpd        eax, xmm0
        and     eax, 1
        ret
.LC0:
        .long   0
        .long   1105199104


https://godbolt.org/z/rrMWY9jsG