[Bug tree-optimization/57702] New: Reassoc missed optimizations

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57702

            Bug ID: 57702
           Summary: Reassoc missed optimizations
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org

On:

int f1 (int x, int y) { x += y; return x + 3 * y; }
unsigned int f2 (unsigned int x, unsigned int y) { x += y; return x + 3 * y; }
int f3 (int x, int y) { x += 7 * y; return x + 12 * y; }
unsigned int f4 (unsigned int x, unsigned int y) { x += 7 * y; return x + 12 *
y; }

reassoc optimizes only the f4 function into x += 19 * y; at the tree level, and
at the RTL level combiner happens to optimize it except for f3, which has more
insns than f4.  I don't see why not optimizing this even for signed types would
be problematic, as long as the multiplication is performed in the same signed
type and all terms have the same sign (with different sizes the optimization
could remove undefined overflow, but I don't see how it could introduce it).

When things are vectorized the RTL optimizations will hardly help though.