Disable AVX2 functions on non-Haswell processors

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终归单人心
终归单人心 2021-02-11 02:29

I have written some AVX2 code to run on a Haswell i7 processor. The same codebase is also used on non-Haswell processors, where the same code should be replaced with their SSE e

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  • 2021-02-11 02:53

    If you just want to do this at compile-time then you can do this:

    #ifdef __AVX2__
        // AVX2 code
    #elif __SSE__
        // SSE code
    #else
        // scalar code
    #endif
    

    Note that when you compile with gcc -mavx2 ... then __AVX2__ gets defined automatically. Similarly for __SSE__. (Note also that you can check what's pre-defined by your compiler for any given command line switching using the incantation gcc -dM -E -mavx2 - < /dev/null.)

    If you want to do run-time dispatching though then that's a little more complicated.

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  • 2021-02-11 02:56

    I don't think it's a good idea to make separate executable unless you have to. In your case you can make a CPU dispatcher. I did this recently for GCC and Visual studio.

    Let's assume you have a function called product for SSE and AVX. You put the SSE version in a file product_SSE.cpp and the AVX2 version in a file product_AVX2.cpp. You compile each one separately (e.g. with -msse2 and -mavx2). Then make a module like this:

    extern "C" void product_SSE(float *a, float *b, float *c, int n);
    extern "C" void product_AVX2(float *a, float *b, float *c, int n); 
               void product_dispatch(float *a, float *b, float *c, int n); 
    void (*fp)(float* a, float *b, float *c, int n) = product_dispatch;
    
    inline void product_dispatch(float *a, float *b, float *c, int n) {
        int iset = instrset_detect();
        if(iset==8) {
            fp = product_AVX2
        }
        else {
            fp = product_SSE
        }
        fp(a,b,c,n);
    }
    
    inline void product(float *a, float *b, float*c, int bs) {
        fp(a,b,c,n);
    }
    

    You compile that module with the lower common instruction set (e.g. with SSE2). Now when you call product it first calls product_dispatch sets the function pointer fpto either product_AVX2 or product_SSE and then calls the function from the function pointer. The second time you call productit jumps right to product_AVX2or product_SSE. This way you don't have to have separate executable.

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