Ternary operator did not work in visual studio ARM64 MSVC release version.

Question

Env:

PC: Qualcomm X Elite(ARM64)

Visual Studio 2022 PRO

Platform Toolset: MSVC v143(14.43.34808)

Windows SDK version: 10.0.26100.0(can be others)

below is the sample code:

#include <iostream>


int main()
{


    typedef unsigned long long int ulonglong; /* ulong or unsigned long long */
    typedef long long int	longlong;
    
    long long int result;
    std::cin >> result;
    bool unsigned_flag;
    std::cin >> unsigned_flag;
    double d;

	d = unsigned_flag ? (double)((ulonglong)result) : (double)result;
    std::cout << d << "\n"; 
    return 0;
}

I found that ternary operator can not be operated properly in ARM64 release compilation. In the sample, if result variable is -11, then whether unsigned_flag variable is true or false, the d variable is 1.84467e+19 (0xFFFFFFFFFFFFFFF5), which means d is assigned by (double)((ulonglong)result).

Also, I checked the generated assembly code as follow:

sp+8 is the address of result variable, after ldr executed in address 0x00007FF6BB161108, result variable is directly converted to d0 using ucvtf, which assumes X8 is a unsigned variable and do not care the value of unsigned_flag variable.

But if I run debug version, the generated assembly code would check unsigned_flag variable, and use scvtf and ucvtf instructions correctly.

ps: If I use clang-cl platform toolset, the result is correct.

Answer 1

@Cheng Yang, Welcome to Microsoft Q&A, based on my test, I find that I always can get fasle for unsigned_flag either I input true or false. The problem is that the code std::cin >> unsigned_flag; cannot accept the correct value from console.

I recommend that you could use the following code to get accept value for unsigned_flag.

std::cin >> std::boolalpha >> unsigned_flag;

Please feel free to let me know if you still meet the problem.

Best Regards,

Jack

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