VS2012 support for __asm__ __volatile__ assembler code

Question

I´m using VS2012 C++ Windows 7 and I need to get information about CPU multithreading to calculate the number of available logic processors.

I´m using this code (from This SO Post )

typedef __int32 int32_t;
typedef unsigned __int32 uint32_t;

uint32_t registers[4];
__asm__ __volatile__ ("cpuid " :
                      "=a" (registers[0]),
                      "=b" (registers[1]),
                      "=c" (registers[2]),
                      "=d" (registers[3])
                      : "a" (1), "c" (0));

unsigned CPUFeatureSet = registers[3];
bool hyperthreading = CPUFeatureSet & (1 << 28);

This assembly does not compile, given the following error:

error C2065: '__asm__' : undeclared identifier

I´ve tried changing to __asm __volatile and putting everything in a single line as:

__asm __volatile ("cpuid " :   "=a" (registers[0]), "=b" (registers[1]), "=c" (registers[2]), "=d" (registers[3])  : "a" (1), "c" (0));

This did not work also, leading to:

error C2400: inline assembler syntax error in 'opcode'; found '('

Help appreciated to solve that.

Answer 1

If you insist on using cpuid, you should use the __cpuid() intrinsic function. The msdn page even comes with sample code. Something like this:

#include <intrin.h>

void foo()
{
    uint32_t registers[4];
    __cpuid(registers, 1);
    unsigned CPUFeatureSet = registers[3];
    // ...
}

Answer 2

You can query the architecture of the current machine, including processor and core count, and NUMA architecture, using the provided APIs.

• GetProcessorCount: https://msdn.microsoft.com/en-us/library/dd470844.aspx
• GetSystemInfo: https://msdn.microsoft.com/en-us/library/windows/desktop/ms724958(v=vs.85).aspx
• IsProcessorFeaturePresent: https://msdn.microsoft.com/en-us/library/windows/desktop/ms724482(v=vs.85).aspx