[LLVMdev] Impressive performance result for LLVM: complex arithmetic

[Daniel Berlin]

On gcc's side, this is a simple missed opt on the part of builtin lowering.
As a result, the gcc code ends up with a call to muldc3 (complex = 2x2
multiply double) and the llvm code doesn't.
GCC should be fixed in a second, and with that, there is no
appreciable performance difference between the two.

On Thu, Feb 26, 2009 at 4:07 PM, Jon Harrop <jon at ffconsultancy.com> wrote:
>
> Following a discussion about numerical performance on comp.lang.functional
> recently I just tried running a simple C mandelbrot benchmark that uses C99's
> complex arithmetic using gcc and llvm-gcc on a 2.1GHz Opteron 2352 running
> Debian:
>
> gcc:      5.727s
> llvm-gcc: 1.393s
>
> There is still 20% room for improvement but LLVM is >4x faster than gcc here.
> Sweet.
>
> Here's the code:
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <complex.h>
>
> int max_i = 65536;
>
> double sqr(double x) { return x*x; }
>
> double cnorm2(complex z) { return sqr(creal(z)) + sqr(cimag(z)); }
>
> int loop(complex c) {
>    complex z=c;
>    int i=1;
>    while (cnorm2(z) <= 4.0 && i++ < max_i)
>        z = z*z + c;
>    return i;
> }
>
> int main() {
>    for (int j = -39; j < 39; ++j) {
>        for (int i = -39; i < 39; ++i)
>            printf(loop(j/40.0-0.5 + i/40.0*I) > max_i ? "*" : " ");
>        printf("\n");
>    }
>    return 0;
> }
>
> --
> Dr Jon Harrop, Flying Frog Consultancy Ltd.
> http://www.ffconsultancy.com/?e
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