[Bug middle-end/44382] Slow integer multiply

[wschmidt at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44382

William J. Schmidt <wschmidt at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |wschmidt at gcc dot gnu.org

--- Comment #7 from William J. Schmidt <wschmidt at gcc dot gnu.org> 2011-07-12 17:33:06 UTC ---
The test case from bug 45671 is as follows:

int myfunction (int a, int b, int c, int d, int e, int f, int g, int h) {
  int ret;

  ret = a + b + c + d + e + f + g + h;
  return ret;

}

Compiling with -O3 results in a series of dependent add instructions to
accumulate the sum.

        add 4,3,4
        add 4,4,5
        add 4,4,6
        add 4,4,7
        add 4,4,8
        add 4,4,9
        add 4,4,10


If we regrouped to (a+b)+(c+d)+... we can do multiple adds in parallel on
different execution units.