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0 pointsConst-me4y ago0 comments

CPUs support these instructions for 9 years now. When ignoring these old CPUs, most languages and compilers are usually doing a good job. Example in C which does not depend on any library functions:

    double fma( double a, double b, double c )
    {
        __m128d av = _mm_set_sd( a );
        __m128d bv = _mm_set_sd( b );
        __m128d cv = _mm_set_sd( c );
        return _mm_cvtsd_f64( _mm_fmadd_sd( av, bv, cv ) );
    }

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2 comments · 1 top-level

adgjlsfhk14y ago· 1 in thread

2 problems: Julia supports cpus without FMA, and on windows, llvm will use libc to constant fold the value of fma even on computers that have fma in hardware.

Const-meOP4y ago

Hardware requirements are up to the product management. For instance, many modern videogames (they generally want compatibility because directly translates to sales) no longer support 32-bit or pre-AVX1 processors. Technically, Julia can drop the support of pre-FMA3 processors if it helps moving forward.

It’s inevitable anyway due to the changing hardware requirements of the OS, the only question is “when”. I don’t think Windows 10 21H2 supports any CPU which doesn’t have SSE 4.1, it’s only a matter of time when Windows will require support of newer instruction sets.

About LLVM, can’t they compile that thing with an option like -mfma to use hardware FMA3 for constant folding?

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