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.

That's gcc syntax, it won't work in VS like that. You will need to `mov` the inputs and outputs yourself, but I would be surprised if here was no winapi function you could just call to get this information. — Jester, Mar 08 '16 at 18:05
Buf... I can´t write assembler... Where should the `mov` goes to ? — Mendes, Mar 08 '16 at 18:06
Besides it being _GCC_ inline assembly templates that won't work with MSVC++, the other issue is that the _ASM_ inline assembly isn't available when developing 64-bit applications.You can create separate object files built using _MASM_, but inline assembler in 64-bit code isn't allowed.See Jester's answer for a solution using compiler intrinsics. In your case you could be creatinging 32-bit program but __ asm__ isn't an MSVC keyword, although *__asm* is — Michael Petch, Mar 08 '16 at 19:37

score 6 · Answer 1 · answered Mar 08 '16 at 18:58

6

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];
    // ...
}

answered Mar 08 '16 at 18:58

Jester

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And there is an ``__cpuidex`` as well if you need to get to the CPUID extensions that require the sub function in ``ECX`` – Chuck Walbourn Mar 08 '16 at 21:30

score 2 · Accepted Answer · answered Mar 08 '16 at 18:22

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

answered Mar 08 '16 at 18:22

Ben

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Which one gives me Number of `Processors Cores` and `Number of Logic Processors` as shown in Windows 10 Task Management->Performance ??? – Mendes Mar 08 '16 at 18:39
2

You can also use [GetLogicalProcessorInformation](https://msdn.microsoft.com/en-us/library/windows/desktop/ms683194.aspx) for more details. – Chuck Walbourn Mar 08 '16 at 21:29

VS2012 support for __asm__ __volatile__ assembler code

2 Answers2

VS2012 support for asm volatile assembler code