CODEWITHSUNDEEP
c++macosclangcross-platform
Vladimir Bershov
Vladimir Bershov

Reputation: 2831

Porting to Mac OS X error

I have the cross-platform audio processing app. It is written using Qt and PortAudio libraries. I also use Chaotic-Daw sources for some audio processing functions (Vibarto effect and Soft-Knee Dynamic range compression). The problem is that I cannot port my app from Windows to Mac OSX because of I get the compiler errors for __asm parts (I use Mac OSX Yosemite and Qt Creator 3.4.1 IDE):

/Users/admin/My projects/MySound/daw/basics/rosic_NumberManipulations.h:69:
error: expected '(' after 'asm' { ^

for such lines:

INLINE int floorInt(double x)
{
    const float round_towards_m_i = -0.5f;
    int i;

    #ifndef LINUX
    __asm
    { // <========= error indicates that row
      fld x;
      fadd st, st (0);
      fadd round_towards_m_i;
      fistp i;
      sar i, 1;
    }
    #else
    i = (int) floor(x);
    #endif

    return (i);
}

How can I resolve this problem?

Upvotes: 0

Views: 130

Answers (1)

Cody Gray
Cody Gray

Reputation: 244992

The code was clearly written for Microsoft's Visual C++ compiler, as that is the syntax it uses for inline assembly. It uses the Intel syntax and is rather simplistic, which makes it easy to write but hinders its optimization potential.

Clang and GCC both use a different format for inline assembly. In particular, they use the GNU AT&T syntax. It is more complicated to write, but much more expressive. The compiler error is basically Clang's way of telling you, "I can tell you're trying to write inline assembly, but you've formatted it all wrong!"

Therefore, to make this code compile, you will need to convert the MSVC-style inline assembly into GAS-format inline assembly. It might look like this:

int floorInt(double x)
{
    const float round_towards_m_i = -0.5f;
    int i;

    __asm__("fadd   %[x], %[x]  \n\t"
            "fadds  %[adj]      \n\t"
            "fistpl %[i]        \n\t"
            "sarl   $1, %[i]"
           : [i]   "=m" (i)    // store result in memory (as required by FISTP)
           : [x]   "t"  (x),   // load input onto top of x87 stack (equivalent to FLD)
             [adj] "m"  (round_towards_m_i)
           : "st");

    return (i);
}

But, because of the additional expressivity of the GAS style, we can offload more of the work to the built-in optimizer, which may yield even more optimal object code:

int floorInt(double x)
{
    const float round_towards_m_i = -0.5f;
    int i;

    x += x;                  // equivalent to the first FADD
    x += round_towards_m_i;  // equivalent to the second FADD
    __asm__("fistpl %[i]"
           : [i]   "=m" (i)
           : [x]   "t"  (x)
           : "st");
    return (i >> 1);         // equivalent to the final SAR
}

Live demonstration
(Note that, technically, a signed right-shift like that done by the last line is implementation-defined in C and would normally be inadvisable. However, if you're using inline assembly, you have already made the decision to target a specific platform and can therefore rely on implementation-specific behavior. In this case, I know and it can easily be demonstrated that all C compilers will generate SAR instructions to do an arithmetic right-shift on signed integer values.)

That said, it appears that the authors of the code intended for the inline assembly to be used only when you are compiling for a platform other than LINUX (presumably, that would be Windows, on which they expected you to be using Microsoft's compiler). So you could get the code to compile simply by ensuring that you are defining LINUX, either on the command line or in your makefile.

I'm not sure why that decision was made; Clang and GCC are both going to generate the same inefficient code that MSVC does (assuming that you are targeting the older generation of x86 processors and unable to use SSE2 instructions). It is up to you: the code will run either way, but it will be slower without the use of inline assembly to force the use of this clever optimization.

Upvotes: 1

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