Double cast to unsigned int on Win32 is truncating to 2,147,483,648

Question

Compiling the following code:

double getDouble()
{
    double value = 2147483649.0;
    return value;
}

int main()
{
           


        

Answer 1

Following @AnttiHaapala's answer, I tested the code using optimization /Ox and found that this will remove the bug as __ftol2_sse is no longer used:

//; 17   :     printf("Direct cast value: %u\n", (unsigned int)getDouble());

    push    -2147483647             //; 80000001H
    push    OFFSET $SG10116
    call    _printf

//; 18   :     double d = getDouble();
//; 19   :     printf("Indirect cast value: %u\n", (unsigned int)d);

    push    -2147483647             //; 80000001H
    push    OFFSET $SG10117
    call    _printf
    add esp, 28                 //; 0000001cH

The optimizations inlined getdouble() and added constant expression evaluation thus removing the need for a conversion at runtime making the bug go away.

Just out of curiosity, I made some more tests, namely changing the code to force float-to-int conversion at runtime. In this case the result is still correct, the compiler, with optimization, uses __dtoui3 in both conversions:

//; 19   :     printf("Direct cast value: %u\n", (unsigned int)getDouble(d));

    movsd   xmm0, QWORD PTR _d$[esp+24]
    add esp, 12                 //; 0000000cH
    call    __dtoui3
    push    eax
    push    OFFSET $SG9261
    call    _printf

//; 20   :     double db = getDouble(d);
//; 21   :     printf("Indirect cast value: %u\n", (unsigned int)db);

    movsd   xmm0, QWORD PTR _d$[esp+20]
    add esp, 8
    call    __dtoui3
    push    eax
    push    OFFSET $SG9262
    call    _printf

However, preventing inlining, __declspec(noinline) double getDouble(){...} will bring the bug back:

//; 17   :     printf("Direct cast value: %u\n", (unsigned int)getDouble(d));

    movsd   xmm0, QWORD PTR _d$[esp+76]
    add esp, 4
    movsd   QWORD PTR [esp], xmm0
    call    _getDouble
    call    __ftol2_sse
    push    eax
    push    OFFSET $SG9261
    call    _printf

//; 18   :     double db = getDouble(d);

    movsd   xmm0, QWORD PTR _d$[esp+80]
    add esp, 8
    movsd   QWORD PTR [esp], xmm0
    call    _getDouble

//; 19   :     printf("Indirect cast value: %u\n", (unsigned int)db);

    call    __ftol2_sse
    push    eax
    push    OFFSET $SG9262
    call    _printf

__ftol2_sse is called in both conversions making the output 2147483648 in both situations, @zwol suspicions were correct.

---

Compilation details:

• Using command line:

cl /permissive- /GS /analyze- /W3 /Gm- /Ox /sdl /D "WIN32" program.c        

• In Visual Studio:

  • Disabling RTC in Project -> Properties -> Code Generation and setting Basic Runtime Checks to default.

  • Enabling optimization in Project -> Properties -> Optimization and setting Optimization to /Ox.

  • With debugger in x86 mode.