[Bug tree-optimization/71026] New: Missing division optimizations

["wdijkstr at arm dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71026

            Bug ID: 71026
           Summary: Missing division optimizations
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wdijkstr at arm dot com
  Target Milestone: ---

With -Ofast GCC doesn't reassociate constant multiplies or negates away from
divisors to allow for more reciprocal division optimizations. It is also
possible to avoid divisions (or multiplies) involving immediates in comparisons
that check for a positive/negative result.

float f1(float x, float y) { return x / (y * y); }    // -> x * (1/y) * (1/y)
float f2(float x, float y) { return x / (y * 3.0f); } // -> (x/3) / y
float f3(float x, float y) { return x / -y; }         // -> (-x) / y
int f4(float x) { return (1.0f / x) < 0.0f; }         // -> x < 0.0f
int f5(float x) { return (x / 2.0f) <= 0.0f; }        // -> x <= 0.0f

A quick experiment shows the first transformation could remove almost 100
divisions from SPEC2006, the 2nd 50. The first transformation is only useful if
there is at least one other division by y, so likely best done in the division
reciprocal optimization phase.